Mailman 3 February 2021 - Python-checkins

GH-42128: Add Pattern Matching to What's New (#24667)
by willingc 28 Feb '21

28 Feb '21

https://github.com/python/cpython/commit/41934b399bf688c8674c386e22c43f080b… commit: 41934b399bf688c8674c386e22c43f080bf10d66 branch: master author: Carol Willing <carolcode(a)willingconsulting.com> committer: willingc <carolcode(a)willingconsulting.com> date: 2021-02-28T15:43:17-08:00 summary: GH-42128: Add Pattern Matching to What's New (#24667) * Add Pattern Matching to What's New * add review suggestions * fix stray indent * Add suggestions from gvr and lr * trim whitespace files: M Doc/whatsnew/3.10.rst diff --git a/Doc/whatsnew/3.10.rst b/Doc/whatsnew/3.10.rst index 12db463008132..26b7076420c87 100644 --- a/Doc/whatsnew/3.10.rst +++ b/Doc/whatsnew/3.10.rst @@ -225,6 +225,281 @@ See :class:`typing.Callable`, :class:`typing.ParamSpec`, (Contributed by Ken Jin in :issue:`41559`.) +PEP 634: Structural Pattern Matching +------------------------------------ + +Structural pattern matching has been added in the form of a *match statement* +and *case statements* of patterns with associated actions. Patterns +consist of sequences, mappings, primitive data types as well as class instances. +Pattern matching enables programs to extract information from complex data types, +branch on the structure of data, and apply specific actions based on different +forms of data. + +Syntax and operations +~~~~~~~~~~~~~~~~~~~~~ + +The generic syntax of pattern matching is:: + + match subject: + case <pattern_1>: + <action_1> + case <pattern_2>: + <action_2> + case <pattern_3>: + <action_3> + case _: + <action_wildcard> + +A match statement takes an expression and compares its value to successive +patterns given as one or more case blocks. Specifically, pattern matching +operates by: + + 1. using data with type and shape (the ``subject``) + 2. evaluating the ``subject`` in the ``match`` statement + 3. comparing the subject with each pattern in a ``case`` statement + from top to bottom until a match is confirmed. + 4. executing the action associated with the pattern of the confirmed + match + 5. If an exact match is not confirmed, the last case, a wildcard ``_``, + if provided, will be used as the matching case. If an exact match is + not confirmed and a wildcard case does not exists, the entire match + block is a no-op. + +Declarative approach +~~~~~~~~~~~~~~~~~~~~ + +Readers may be aware of pattern matching through the simple example of matching +a subject (data object) to a literal (pattern) with the switch statement found +in C, Java or JavaScript (and many other languages). Often the switch statement +is used for comparison of an object/expression with case statements containing +literals. + +More powerful examples of pattern matching can be found in languages, such as +Scala and Elixir. With structural pattern matching, the approach is "declarative" and +explicitly states the conditions (the patterns) for data to match. + +While an "imperative" series of instructions using nested "if" statements +could be used to accomplish something similar to structural pattern matching, +it is less clear than the "declarative" approach. Instead the "declarative" +approach states the conditions to meet for a match and is more readable through +its explicit patterns. While structural pattern matching can be used in its +simplest form comparing a variable to a literal in a case statement, its +true value for Python lies in its handling of the subject's type and shape. + +Simple pattern: match to a literal +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Let's look at this example as pattern matching in its simplest form: a value, +the subject, being matched to several literals, the patterns. In the example +below, ``status`` is the subject of the match statement. The patterns are +each of the case statements, where literals represent request status codes. +The associated action to the case is executed after a match:: + + def http_error(status): + match status: + case 400: + return "Bad request" + case 404: + return "Not found" + case 418: + return "I'm a teapot" + case _: + return "Something's wrong with the Internet" + +If the above function is passed a ``status`` of 418, "I'm a teapot" is returned. +If the above function is passed a ``status`` of 500, the case statement with +``_`` will match as a wildcard, and "Something's wrong with the Internet" is +returned. +Note the last block: the variable name, ``_``, acts as a *wildcard* and insures +the subject will always match. The use of ``_`` is optional. + +You can combine several literals in a single pattern using ``|`` ("or"):: + + case 401 | 403 | 404: + return "Not allowed" + +Behavior without the wildcard +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +If we modify the above example by removing the last case block, the example +becomes:: + + def http_error(status): + match status: + case 400: + return "Bad request" + case 404: + return "Not found" + case 418: + return "I'm a teapot" + +Without the use of ``_`` in a case statement, a match may not exist. If no +match exists, the behavior is a no-op. For example, if ``status`` of 500 is +passed, a no-op occurs. + +Pattterns with a literal and variable +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Patterns can look like unpacking assignments, and a pattern may be used to bind +variables. In this example, a data point can be unpacked to its x-coordinate +and y-coordinate:: + + # point is an (x, y) tuple + match point: + case (0, 0): + print("Origin") + case (0, y): + print(f"Y={y}") + case (x, 0): + print(f"X={x}") + case (x, y): + print(f"X={x}, Y={y}") + case _: + raise ValueError("Not a point") + +The first pattern has two literals, ``(0, 0)``, and may be thought of as an +extension of the literal pattern shown above. The next two patterns combine a +literal and a variable, and the variable *binds* a value from the subject +(``point``). The fourth pattern captures two values, which makes it +conceptually similar to the unpacking assignment ``(x, y) = point``. + +Patterns and classes +~~~~~~~~~~~~~~~~~~~~ + +If you are using classes to structure your data, you can use as a pattern +the class name followed by an argument list resembling a constructor. This +pattern has the ability to capture class attributes into variables:: + + class Point: + x: int + y: int + + def location(point): + match point: + case Point(x=0, y=0): + print("Origin is the point's location.") + case Point(x=0, y=y): + print(f"Y={y} and the point is on the y-axis.") + case Point(x=x, y=0): + print(f"X={x} and the point is on the x-axis.") + case Point(): + print("The point is located somewhere else on the plane.") + case _: + print("Not a point") + +Patterns with positional parameters +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +You can use positional parameters with some builtin classes that provide an +ordering for their attributes (e.g. dataclasses). You can also define a specific +position for attributes in patterns by setting the ``__match_args__`` special +attribute in your classes. If it's set to ("x", "y"), the following patterns +are all equivalent (and all bind the ``y`` attribute to the ``var`` variable):: + + Point(1, var) + Point(1, y=var) + Point(x=1, y=var) + Point(y=var, x=1) + +Nested patterns +~~~~~~~~~~~~~~~ + +Patterns can be arbitrarily nested. For example, if our data is a short +list of points, it could be matched like this:: + + match points: + case []: + print("No points in the list.") + case [Point(0, 0)]: + print("The origin is the only point in the list.") + case [Point(x, y)]: + print(f"A single point {x}, {y} is in the list.") + case [Point(0, y1), Point(0, y2)]: + print(f"Two points on the Y axis at {y1}, {y2} are in the list.") + case _: + print("Something else is found in the list.") + +Complex patterns and the wildcard +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +To this point, the examples have used ``_`` alone in the last case statement. +A wildcard can be used in more complex patterns, such as ``('error', code, _)``. +For example:: + + match test_variable: + case ('warning', code, 40): + print("A warning has been received.") + case ('error', code, _): + print(f"An error {code} occured.") + +In the above case, ``test_variable`` will match for ('error', code, 100) and +('error', code, 800). + +Guard +~~~~~ + +We can add an ``if`` clause to a pattern, known as a "guard". If the +guard is false, ``match`` goes on to try the next case block. Note +that value capture happens before the guard is evaluated:: + + match point: + case Point(x, y) if x == y: + print(f"The point is located on the diagonal Y=X at {x}.") + case Point(x, y): + print(f"Point is not on the diagonal.") + +Other Key Features +~~~~~~~~~~~~~~~~~~ + +Several other key features: + +- Like unpacking assignments, tuple and list patterns have exactly the + same meaning and actually match arbitrary sequences. Technically, + the subject must be an instance of ``collections.abc.Sequence``. + Therefore, an important exception is that patterns don't match iterators. + Also, to prevent a common mistake, sequence patterns don't match strings. + +- Sequence patterns support wildcards: ``[x, y, *rest]`` and ``(x, y, + *rest)`` work similar to wildcards in unpacking assignments. The + name after ``*`` may also be ``_``, so ``(x, y, *_)`` matches a sequence + of at least two items without binding the remaining items. + +- Mapping patterns: ``{"bandwidth": b, "latency": l}`` captures the + ``"bandwidth"`` and ``"latency"`` values from a dict. Unlike sequence + patterns, extra keys are ignored. A wildcard ``**rest`` is also + supported. (But ``**_`` would be redundant, so it not allowed.) + +- Subpatterns may be captured using the ``as`` keyword:: + + case (Point(x1, y1), Point(x2, y2) as p2): ... + + This binds x1, y1, x2, y2 like you would expect without the ``as`` clause, + and p2 to the entire second item of the subject. + +- Most literals are compared by equality. However, the singletons ``True``, + ``False`` and ``None`` are compared by identity. + +- Named constants may be used in patterns. These named constants must be + dotted names to prevent the constant from being interpreted as a capture + variable:: + + from enum import Enum + class Color(Enum): + RED = 0 + GREEN = 1 + BLUE = 2 + + match color: + case Color.RED: + print("I see red!") + case Color.GREEN: + print("Grass is green") + case Color.BLUE: + print("I'm feeling the blues :(") + +For the full specification see :pep:`634`. Motivation and rationale +are in :pep:`635`, and a longer tutorial is in :pep:`636`. + Better error messages in the parser -----------------------------------

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bpo-29753: fix merging packed bitfields in ctypes struct/union (GH-19850)
by miss-islington 28 Feb '21

28 Feb '21

https://github.com/python/cpython/commit/0d7ad9fb38c041c46094087b0cf2c8ce44… commit: 0d7ad9fb38c041c46094087b0cf2c8ce44916b11 branch: master author: Filipe Laíns <lains(a)riseup.net> committer: miss-islington <31488909+miss-islington(a)users.noreply.github.com> date: 2021-02-28T14:43:19-08:00 summary: bpo-29753: fix merging packed bitfields in ctypes struct/union (GH-19850) >From the commit message: > When the structure is packed we should always expand when needed, > otherwise we will add some padding between the fields. This patch makes > sure we always merge bitfields together. It also changes the field merging > algorithm so that it handles bitfields correctly. Automerge-Triggered-By: GH:jaraco files: A Misc/NEWS.d/next/Library/2020-05-02-01-01-30.bpo-29753.n2M-AF.rst M Lib/ctypes/test/test_bitfields.py M Modules/_ctypes/cfield.c diff --git a/Lib/ctypes/test/test_bitfields.py b/Lib/ctypes/test/test_bitfields.py index 992b8c4da3a77..68b618a05f436 100644 --- a/Lib/ctypes/test/test_bitfields.py +++ b/Lib/ctypes/test/test_bitfields.py @@ -233,6 +233,69 @@ class X(Structure): else: self.assertEqual(sizeof(X), sizeof(c_int) * 2) + @unittest.skipIf(os.name == 'nt', reason='Posix only') + def test_packed_posix(self): + test_cases = { + ( + ("a", c_uint8, 4), + ("b", c_uint8, 4), + ): 1, + ( + ("a", c_uint8, 1), + ("b", c_uint16, 1), + ("c", c_uint32, 1), + ("d", c_uint64, 1), + ): 1, + ( + ("a", c_uint8, 8), + ("b", c_uint16, 1), + ("c", c_uint32, 1), + ("d", c_uint64, 1), + ): 2, + ( + ("a", c_uint32, 9), + ("b", c_uint16, 10), + ("c", c_uint32, 25), + ("d", c_uint64, 1), + ): 6, + ( + ("a", c_uint32, 9), + ("b", c_uint16, 10), + ("c", c_uint32, 25), + ("d", c_uint64, 5), + ): 7, + ( + ("a", c_uint16), + ("b", c_uint16, 9), + ("c", c_uint16, 1), + ("d", c_uint16, 1), + ("e", c_uint16, 1), + ("f", c_uint16, 1), + ("g", c_uint16, 3), + ("h", c_uint32, 10), + ("i", c_uint32, 20), + ("j", c_uint32, 2), + ): 8, + ( + ("a", c_uint16, 9), + ("b", c_uint16, 10), + ("d", c_uint16), + ("c", c_uint8, 8), + ): 6, + ( + ("a", c_uint32, 9), + ("b", c_uint32), + ("c", c_uint32, 8), + ): 7, + } + + for fields, size in test_cases.items(): + with self.subTest(fields=fields): + class X(Structure): + _pack_ = 1 + _fields_ = list(fields) + self.assertEqual(sizeof(X), size) + def test_anon_bitfields(self): # anonymous bit-fields gave a strange error message class X(Structure): diff --git a/Misc/NEWS.d/next/Library/2020-05-02-01-01-30.bpo-29753.n2M-AF.rst b/Misc/NEWS.d/next/Library/2020-05-02-01-01-30.bpo-29753.n2M-AF.rst new file mode 100644 index 0000000000000..f2a2842239b9c --- /dev/null +++ b/Misc/NEWS.d/next/Library/2020-05-02-01-01-30.bpo-29753.n2M-AF.rst @@ -0,0 +1 @@ +In ctypes, now packed bitfields are calculated properly and the first item of packed bitfields is now shrank correctly. diff --git a/Modules/_ctypes/cfield.c b/Modules/_ctypes/cfield.c index 5bd96f1eb8c18..d96a1b0651baa 100644 --- a/Modules/_ctypes/cfield.c +++ b/Modules/_ctypes/cfield.c @@ -71,6 +71,18 @@ PyCField_FromDesc(PyObject *desc, Py_ssize_t index, Py_DECREF(self); return NULL; } + +#ifndef MS_WIN32 + /* if we have a packed bitfield, calculate the minimum number of bytes we + need to fit it. otherwise use the specified size. */ + if (pack && bitsize) { + size = (bitsize - 1) / 8 + 1; + } else +#endif + size = dict->size; + + proto = desc; + if (bitsize /* this is a bitfield request */ && *pfield_size /* we have a bitfield open */ #ifdef MS_WIN32 @@ -87,7 +99,9 @@ PyCField_FromDesc(PyObject *desc, Py_ssize_t index, } else if (bitsize /* this is a bitfield request */ && *pfield_size /* we have a bitfield open */ && dict->size * 8 >= *pfield_size - && (*pbitofs + bitsize) <= dict->size * 8) { + /* if this is a packed bitfield, always expand it. + otherwise calculate if we need to expand it. */ + && (((*pbitofs + bitsize) <= dict->size * 8) || pack)) { /* expand bit field */ fieldtype = EXPAND_BITFIELD; #endif @@ -95,7 +109,9 @@ PyCField_FromDesc(PyObject *desc, Py_ssize_t index, /* start new bitfield */ fieldtype = NEW_BITFIELD; *pbitofs = 0; - *pfield_size = dict->size * 8; + /* use our calculated size (size) instead of type size (dict->size), + which can be different for packed bitfields */ + *pfield_size = size * 8; } else { /* not a bit field */ fieldtype = NO_BITFIELD; @@ -103,9 +119,6 @@ PyCField_FromDesc(PyObject *desc, Py_ssize_t index, *pfield_size = 0; } - size = dict->size; - proto = desc; - /* Field descriptors for 'c_char * n' are be scpecial cased to return a Python string instead of an Array object instance... */ @@ -170,10 +183,16 @@ PyCField_FromDesc(PyObject *desc, Py_ssize_t index, break; case EXPAND_BITFIELD: - *poffset += dict->size - *pfield_size/8; - *psize += dict->size - *pfield_size/8; + /* increase the size if it is a packed bitfield. + EXPAND_BITFIELD should not be selected for non-packed fields if the + current size isn't already enough. */ + if (pack) + size = (*pbitofs + bitsize - 1) / 8 + 1; + + *poffset += size - *pfield_size/8; + *psize += size - *pfield_size/8; - *pfield_size = dict->size * 8; + *pfield_size = size * 8; if (big_endian) self->size = (bitsize << 16) + *pfield_size - *pbitofs - bitsize;

1 0

bpo-37146: Deactivate opcode cache only when using huntrleaks in the test suite (GH-24643)
by pablogsal 28 Feb '21

28 Feb '21

https://github.com/python/cpython/commit/af5fa13ef6f648fc7a7a33a7556db13887… commit: af5fa13ef6f648fc7a7a33a7556db13887e7d643 branch: master author: Pablo Galindo <Pablogsal(a)gmail.com> committer: pablogsal <Pablogsal(a)gmail.com> date: 2021-02-28T22:41:09Z summary: bpo-37146: Deactivate opcode cache only when using huntrleaks in the test suite (GH-24643) files: M Include/cpython/ceval.h M Lib/test/libregrtest/setup.py M Python/ceval.c M Python/clinic/sysmodule.c.h M Python/sysmodule.c diff --git a/Include/cpython/ceval.h b/Include/cpython/ceval.h index 06338928f6738..05dcfac2ed634 100644 --- a/Include/cpython/ceval.h +++ b/Include/cpython/ceval.h @@ -28,3 +28,5 @@ PyAPI_FUNC(Py_ssize_t) _PyEval_RequestCodeExtraIndex(freefunc); PyAPI_FUNC(int) _PyEval_SliceIndex(PyObject *, Py_ssize_t *); PyAPI_FUNC(int) _PyEval_SliceIndexNotNone(PyObject *, Py_ssize_t *); + +PyAPI_FUNC(void) _PyEval_DeactivateOpCache(void); diff --git a/Lib/test/libregrtest/setup.py b/Lib/test/libregrtest/setup.py index 1f264c1be49fe..7738d4f28b658 100644 --- a/Lib/test/libregrtest/setup.py +++ b/Lib/test/libregrtest/setup.py @@ -62,6 +62,7 @@ def setup_tests(ns): if ns.huntrleaks: unittest.BaseTestSuite._cleanup = False + sys._deactivate_opcache() if ns.memlimit is not None: support.set_memlimit(ns.memlimit) diff --git a/Python/ceval.c b/Python/ceval.c index 7862643983e08..8ec00bc2400fb 100644 --- a/Python/ceval.c +++ b/Python/ceval.c @@ -107,17 +107,19 @@ static long dxp[256]; #endif /* per opcode cache */ -#ifdef Py_DEBUG -// --with-pydebug is used to find memory leak. opcache makes it harder. -// So we disable opcache when Py_DEBUG is defined. -// See bpo-37146 -#define OPCACHE_MIN_RUNS 0 /* disable opcache */ -#else -#define OPCACHE_MIN_RUNS 1024 /* create opcache when code executed this time */ -#endif +static int opcache_min_runs = 1024; /* create opcache when code executed this many times */ #define OPCODE_CACHE_MAX_TRIES 20 #define OPCACHE_STATS 0 /* Enable stats */ +// This function allows to deactivate the opcode cache. As different cache mechanisms may hold +// references, this can mess with the reference leak detector functionality so the cache needs +// to be deactivated in such scenarios to avoid false positives. See bpo-3714 for more information. +void +_PyEval_DeactivateOpCache(void) +{ + opcache_min_runs = 0; +} + #if OPCACHE_STATS static size_t opcache_code_objects = 0; static size_t opcache_code_objects_extra_mem = 0; @@ -1705,9 +1707,9 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, PyFrameObject *f, int throwflag) f->f_stackdepth = -1; f->f_state = FRAME_EXECUTING; - if (co->co_opcache_flag < OPCACHE_MIN_RUNS) { + if (co->co_opcache_flag < opcache_min_runs) { co->co_opcache_flag++; - if (co->co_opcache_flag == OPCACHE_MIN_RUNS) { + if (co->co_opcache_flag == opcache_min_runs) { if (_PyCode_InitOpcache(co) < 0) { goto exit_eval_frame; } diff --git a/Python/clinic/sysmodule.c.h b/Python/clinic/sysmodule.c.h index addd58922e7f5..04c8481149887 100644 --- a/Python/clinic/sysmodule.c.h +++ b/Python/clinic/sysmodule.c.h @@ -938,6 +938,24 @@ sys_getandroidapilevel(PyObject *module, PyObject *Py_UNUSED(ignored)) #endif /* defined(ANDROID_API_LEVEL) */ +PyDoc_STRVAR(sys__deactivate_opcache__doc__, +"_deactivate_opcache($module, /)\n" +"--\n" +"\n" +"Deactivate the opcode cache permanently"); + +#define SYS__DEACTIVATE_OPCACHE_METHODDEF \ + {"_deactivate_opcache", (PyCFunction)sys__deactivate_opcache, METH_NOARGS, sys__deactivate_opcache__doc__}, + +static PyObject * +sys__deactivate_opcache_impl(PyObject *module); + +static PyObject * +sys__deactivate_opcache(PyObject *module, PyObject *Py_UNUSED(ignored)) +{ + return sys__deactivate_opcache_impl(module); +} + #ifndef SYS_GETWINDOWSVERSION_METHODDEF #define SYS_GETWINDOWSVERSION_METHODDEF #endif /* !defined(SYS_GETWINDOWSVERSION_METHODDEF) */ @@ -965,4 +983,4 @@ sys_getandroidapilevel(PyObject *module, PyObject *Py_UNUSED(ignored)) #ifndef SYS_GETANDROIDAPILEVEL_METHODDEF #define SYS_GETANDROIDAPILEVEL_METHODDEF #endif /* !defined(SYS_GETANDROIDAPILEVEL_METHODDEF) */ -/*[clinic end generated code: output=bbc4963fe86a29d9 input=a9049054013a1b77]*/ +/*[clinic end generated code: output=68c62b9ca317a0c8 input=a9049054013a1b77]*/ diff --git a/Python/sysmodule.c b/Python/sysmodule.c index 13b9034bf7369..686b6cae3b294 100644 --- a/Python/sysmodule.c +++ b/Python/sysmodule.c @@ -1958,6 +1958,20 @@ sys_getandroidapilevel_impl(PyObject *module) #endif /* ANDROID_API_LEVEL */ +/*[clinic input] +sys._deactivate_opcache + +Deactivate the opcode cache permanently +[clinic start generated code]*/ + +static PyObject * +sys__deactivate_opcache_impl(PyObject *module) +/*[clinic end generated code: output=00e20982bd012122 input=501eac146735ccf9]*/ +{ + _PyEval_DeactivateOpCache(); + Py_RETURN_NONE; +} + static PyMethodDef sys_methods[] = { /* Might as well keep this in alphabetic order */ @@ -2011,6 +2025,7 @@ static PyMethodDef sys_methods[] = { SYS_GET_ASYNCGEN_HOOKS_METHODDEF SYS_GETANDROIDAPILEVEL_METHODDEF SYS_UNRAISABLEHOOK_METHODDEF + SYS__DEACTIVATE_OPCACHE_METHODDEF {NULL, NULL} /* sentinel */ };

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Add whatsnew note for GH-22904 (#24672)
by tim-one 28 Feb '21

28 Feb '21

https://github.com/python/cpython/commit/e8f5ddd33e44dae4e0121f87a7bf92d754… commit: e8f5ddd33e44dae4e0121f87a7bf92d754807e49 branch: master author: Dennis Sweeney <36520290+sweeneyde(a)users.noreply.github.com> committer: tim-one <tim.peters(a)gmail.com> date: 2021-02-28T15:32:04-06:00 summary: Add whatsnew note for GH-22904 (#24672) files: M Doc/whatsnew/3.10.rst diff --git a/Doc/whatsnew/3.10.rst b/Doc/whatsnew/3.10.rst index 310554eabe670..12db463008132 100644 --- a/Doc/whatsnew/3.10.rst +++ b/Doc/whatsnew/3.10.rst @@ -615,6 +615,11 @@ Optimizations bytecode level. It is now around 100% faster to create a function with parameter annotations. (Contributed by Yurii Karabas and Inada Naoki in :issue:`42202`) +* Substring search functions such as ``str1 in str2`` and ``str2.find(str1)`` + now sometimes use Crochemore & Perrin's "Two-Way" string searching + algorithm to avoid quadratic behavior on long strings. (Contributed + by Dennis Sweeney in :issue:`41972`) + Deprecated ==========

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bpo-41972: Use the two-way algorithm for string searching (GH-22904)
by tim-one 28 Feb '21

28 Feb '21

https://github.com/python/cpython/commit/73a85c4e1da42db28e3de57c868d24a089… commit: 73a85c4e1da42db28e3de57c868d24a089b8d277 branch: master author: Dennis Sweeney <36520290+sweeneyde(a)users.noreply.github.com> committer: tim-one <tim.peters(a)gmail.com> date: 2021-02-28T12:20:50-06:00 summary: bpo-41972: Use the two-way algorithm for string searching (GH-22904) Implement an enhanced variant of Crochemore and Perrin's Two-Way string searching algorithm, which reduces worst-case time from quadratic (the product of the string and pattern lengths) to linear. This applies to forward searches (like``find``, ``index``, ``replace``); the algorithm for reverse searches (like ``rfind``) is not changed. Co-authored-by: Tim Peters <tim.peters(a)gmail.com> files: A Misc/NEWS.d/next/Core and Builtins/2020-10-23-23-17-23.bpo-41972.kbAwg4.rst A Objects/stringlib/stringlib_find_two_way_notes.txt M Lib/test/string_tests.py M Objects/stringlib/fastsearch.h diff --git a/Lib/test/string_tests.py b/Lib/test/string_tests.py index 527f505c0169b..840d7bb7550f7 100644 --- a/Lib/test/string_tests.py +++ b/Lib/test/string_tests.py @@ -5,6 +5,7 @@ import unittest, string, sys, struct from test import support from collections import UserList +import random class Sequence: def __init__(self, seq='wxyz'): self.seq = seq @@ -317,6 +318,44 @@ def test_rindex(self): else: self.checkraises(TypeError, 'hello', 'rindex', 42) + def test_find_periodic_pattern(self): + """Cover the special path for periodic patterns.""" + def reference_find(p, s): + for i in range(len(s)): + if s.startswith(p, i): + return i + return -1 + + rr = random.randrange + choices = random.choices + for _ in range(1000): + p0 = ''.join(choices('abcde', k=rr(10))) * rr(10, 20) + p = p0[:len(p0) - rr(10)] # pop off some characters + left = ''.join(choices('abcdef', k=rr(2000))) + right = ''.join(choices('abcdef', k=rr(2000))) + text = left + p + right + with self.subTest(p=p, text=text): + self.checkequal(reference_find(p, text), + text, 'find', p) + + def test_find_shift_table_overflow(self): + """When the table of 8-bit shifts overflows.""" + N = 2**8 + 100 + + # first check the periodic case + # here, the shift for 'b' is N + 1. + pattern1 = 'a' * N + 'b' + 'a' * N + text1 = 'babbaa' * N + pattern1 + self.checkequal(len(text1)-len(pattern1), + text1, 'find', pattern1) + + # now check the non-periodic case + # here, the shift for 'd' is 3*(N+1)+1 + pattern2 = 'ddd' + 'abc' * N + "eee" + text2 = pattern2[:-1] + "ddeede" * 2 * N + pattern2 + "de" * N + self.checkequal(len(text2) - N*len("de") - len(pattern2), + text2, 'find', pattern2) + def test_lower(self): self.checkequal('hello', 'HeLLo', 'lower') self.checkequal('hello', 'hello', 'lower') diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-10-23-23-17-23.bpo-41972.kbAwg4.rst b/Misc/NEWS.d/next/Core and Builtins/2020-10-23-23-17-23.bpo-41972.kbAwg4.rst new file mode 100644 index 0000000000000..609a0ff0be253 --- /dev/null +++ b/Misc/NEWS.d/next/Core and Builtins/2020-10-23-23-17-23.bpo-41972.kbAwg4.rst @@ -0,0 +1 @@ +Substring search functions such as ``str1 in str2`` and ``str2.find(str1)`` now sometimes use the "Two-Way" string comparison algorithm to avoid quadratic behavior on long strings. \ No newline at end of file diff --git a/Objects/stringlib/fastsearch.h b/Objects/stringlib/fastsearch.h index 56a4467d35381..6574720b609f4 100644 --- a/Objects/stringlib/fastsearch.h +++ b/Objects/stringlib/fastsearch.h @@ -9,10 +9,16 @@ /* note: fastsearch may access s[n], which isn't a problem when using Python's ordinary string types, but may cause problems if you're using this code in other contexts. also, the count mode returns -1 - if there cannot possible be a match in the target string, and 0 if + if there cannot possibly be a match in the target string, and 0 if it has actually checked for matches, but didn't find any. callers beware! */ +/* If the strings are long enough, use Crochemore and Perrin's Two-Way + algorithm, which has worst-case O(n) runtime and best-case O(n/k). + Also compute a table of shifts to achieve O(n/k) in more cases, + and often (data dependent) deduce larger shifts than pure C&P can + deduce. */ + #define FAST_COUNT 0 #define FAST_SEARCH 1 #define FAST_RSEARCH 2 @@ -160,6 +166,353 @@ STRINGLIB(rfind_char)(const STRINGLIB_CHAR* s, Py_ssize_t n, STRINGLIB_CHAR ch) #undef MEMCHR_CUT_OFF +/* Change to a 1 to see logging comments walk through the algorithm. */ +#if 0 && STRINGLIB_SIZEOF_CHAR == 1 +# define LOG(...) printf(__VA_ARGS__) +# define LOG_STRING(s, n) printf("\"%.*s\"", n, s) +#else +# define LOG(...) +# define LOG_STRING(s, n) +#endif + +Py_LOCAL_INLINE(Py_ssize_t) +STRINGLIB(_lex_search)(const STRINGLIB_CHAR *needle, Py_ssize_t len_needle, + Py_ssize_t *return_period, int invert_alphabet) +{ + /* Do a lexicographic search. Essentially this: + >>> max(needle[i:] for i in range(len(needle)+1)) + Also find the period of the right half. */ + Py_ssize_t max_suffix = 0; + Py_ssize_t candidate = 1; + Py_ssize_t k = 0; + // The period of the right half. + Py_ssize_t period = 1; + + while (candidate + k < len_needle) { + // each loop increases candidate + k + max_suffix + STRINGLIB_CHAR a = needle[candidate + k]; + STRINGLIB_CHAR b = needle[max_suffix + k]; + // check if the suffix at candidate is better than max_suffix + if (invert_alphabet ? (b < a) : (a < b)) { + // Fell short of max_suffix. + // The next k + 1 characters are non-increasing + // from candidate, so they won't start a maximal suffix. + candidate += k + 1; + k = 0; + // We've ruled out any period smaller than what's + // been scanned since max_suffix. + period = candidate - max_suffix; + } + else if (a == b) { + if (k + 1 != period) { + // Keep scanning the equal strings + k++; + } + else { + // Matched a whole period. + // Start matching the next period. + candidate += period; + k = 0; + } + } + else { + // Did better than max_suffix, so replace it. + max_suffix = candidate; + candidate++; + k = 0; + period = 1; + } + } + *return_period = period; + return max_suffix; +} + +Py_LOCAL_INLINE(Py_ssize_t) +STRINGLIB(_factorize)(const STRINGLIB_CHAR *needle, + Py_ssize_t len_needle, + Py_ssize_t *return_period) +{ + /* Do a "critical factorization", making it so that: + >>> needle = (left := needle[:cut]) + (right := needle[cut:]) + where the "local period" of the cut is maximal. + + The local period of the cut is the minimal length of a string w + such that (left endswith w or w endswith left) + and (right startswith w or w startswith left). + + The Critical Factorization Theorem says that this maximal local + period is the global period of the string. + + Crochemore and Perrin (1991) show that this cut can be computed + as the later of two cuts: one that gives a lexicographically + maximal right half, and one that gives the same with the + with respect to a reversed alphabet-ordering. + + This is what we want to happen: + >>> x = "GCAGAGAG" + >>> cut, period = factorize(x) + >>> x[:cut], (right := x[cut:]) + ('GC', 'AGAGAG') + >>> period # right half period + 2 + >>> right[period:] == right[:-period] + True + + This is how the local period lines up in the above example: + GC | AGAGAG + AGAGAGC = AGAGAGC + The length of this minimal repetition is 7, which is indeed the + period of the original string. */ + + Py_ssize_t cut1, period1, cut2, period2, cut, period; + cut1 = STRINGLIB(_lex_search)(needle, len_needle, &period1, 0); + cut2 = STRINGLIB(_lex_search)(needle, len_needle, &period2, 1); + + // Take the later cut. + if (cut1 > cut2) { + period = period1; + cut = cut1; + } + else { + period = period2; + cut = cut2; + } + + LOG("split: "); LOG_STRING(needle, cut); + LOG(" + "); LOG_STRING(needle + cut, len_needle - cut); + LOG("\n"); + + *return_period = period; + return cut; +} + +#define SHIFT_TYPE uint8_t +#define NOT_FOUND ((1U<<(8*sizeof(SHIFT_TYPE))) - 1U) +#define SHIFT_OVERFLOW (NOT_FOUND - 1U) + +#define TABLE_SIZE_BITS 6 +#define TABLE_SIZE (1U << TABLE_SIZE_BITS) +#define TABLE_MASK (TABLE_SIZE - 1U) + +typedef struct STRINGLIB(_pre) { + const STRINGLIB_CHAR *needle; + Py_ssize_t len_needle; + Py_ssize_t cut; + Py_ssize_t period; + int is_periodic; + SHIFT_TYPE table[TABLE_SIZE]; +} STRINGLIB(prework); + + +Py_LOCAL_INLINE(void) +STRINGLIB(_preprocess)(const STRINGLIB_CHAR *needle, Py_ssize_t len_needle, + STRINGLIB(prework) *p) +{ + p->needle = needle; + p->len_needle = len_needle; + p->cut = STRINGLIB(_factorize)(needle, len_needle, &(p->period)); + assert(p->period + p->cut <= len_needle); + p->is_periodic = (0 == memcmp(needle, + needle + p->period, + p->cut * STRINGLIB_SIZEOF_CHAR)); + if (p->is_periodic) { + assert(p->cut <= len_needle/2); + assert(p->cut < p->period); + } + else { + // A lower bound on the period + p->period = Py_MAX(p->cut, len_needle - p->cut) + 1; + } + // Now fill up a table + memset(&(p->table[0]), 0xff, TABLE_SIZE*sizeof(SHIFT_TYPE)); + assert(p->table[0] == NOT_FOUND); + assert(p->table[TABLE_MASK] == NOT_FOUND); + for (Py_ssize_t i = 0; i < len_needle; i++) { + Py_ssize_t shift = len_needle - i; + if (shift > SHIFT_OVERFLOW) { + shift = SHIFT_OVERFLOW; + } + p->table[needle[i] & TABLE_MASK] = Py_SAFE_DOWNCAST(shift, + Py_ssize_t, + SHIFT_TYPE); + } +} + +Py_LOCAL_INLINE(Py_ssize_t) +STRINGLIB(_two_way)(const STRINGLIB_CHAR *haystack, Py_ssize_t len_haystack, + STRINGLIB(prework) *p) +{ + // Crochemore and Perrin's (1991) Two-Way algorithm. + // See http://www-igm.univ-mlv.fr/~lecroq/string/node26.html#SECTION00260 + Py_ssize_t len_needle = p->len_needle; + Py_ssize_t cut = p->cut; + Py_ssize_t period = p->period; + const STRINGLIB_CHAR *needle = p->needle; + const STRINGLIB_CHAR *window = haystack; + const STRINGLIB_CHAR *last_window = haystack + len_haystack - len_needle; + SHIFT_TYPE *table = p->table; + LOG("===== Two-way: \"%s\" in \"%s\". =====\n", needle, haystack); + + if (p->is_periodic) { + LOG("Needle is periodic.\n"); + Py_ssize_t memory = 0; + periodicwindowloop: + while (window <= last_window) { + Py_ssize_t i = Py_MAX(cut, memory); + + // Visualize the line-up: + LOG("> "); LOG_STRING(haystack, len_haystack); + LOG("\n> "); LOG("%*s", window - haystack, ""); + LOG_STRING(needle, len_needle); + LOG("\n> "); LOG("%*s", window - haystack + i, ""); + LOG(" ^ <-- cut\n"); + + if (window[i] != needle[i]) { + // Sunday's trick: if we're going to jump, we might + // as well jump to line up the character *after* the + // current window. + STRINGLIB_CHAR first_outside = window[len_needle]; + SHIFT_TYPE shift = table[first_outside & TABLE_MASK]; + if (shift == NOT_FOUND) { + LOG("\"%c\" not found. Skipping entirely.\n", + first_outside); + window += len_needle + 1; + } + else { + LOG("Shifting to line up \"%c\".\n", first_outside); + Py_ssize_t memory_shift = i - cut + 1; + window += Py_MAX(shift, memory_shift); + } + memory = 0; + goto periodicwindowloop; + } + for (i = i + 1; i < len_needle; i++) { + if (needle[i] != window[i]) { + LOG("Right half does not match. Jump ahead by %d.\n", + i - cut + 1); + window += i - cut + 1; + memory = 0; + goto periodicwindowloop; + } + } + for (i = memory; i < cut; i++) { + if (needle[i] != window[i]) { + LOG("Left half does not match. Jump ahead by period %d.\n", + period); + window += period; + memory = len_needle - period; + goto periodicwindowloop; + } + } + LOG("Left half matches. Returning %d.\n", + window - haystack); + return window - haystack; + } + } + else { + LOG("Needle is not periodic.\n"); + assert(cut < len_needle); + STRINGLIB_CHAR needle_cut = needle[cut]; + windowloop: + while (window <= last_window) { + + // Visualize the line-up: + LOG("> "); LOG_STRING(haystack, len_haystack); + LOG("\n> "); LOG("%*s", window - haystack, ""); + LOG_STRING(needle, len_needle); + LOG("\n> "); LOG("%*s", window - haystack + cut, ""); + LOG(" ^ <-- cut\n"); + + if (window[cut] != needle_cut) { + // Sunday's trick: if we're going to jump, we might + // as well jump to line up the character *after* the + // current window. + STRINGLIB_CHAR first_outside = window[len_needle]; + SHIFT_TYPE shift = table[first_outside & TABLE_MASK]; + if (shift == NOT_FOUND) { + LOG("\"%c\" not found. Skipping entirely.\n", + first_outside); + window += len_needle + 1; + } + else { + LOG("Shifting to line up \"%c\".\n", first_outside); + window += shift; + } + goto windowloop; + } + for (Py_ssize_t i = cut + 1; i < len_needle; i++) { + if (needle[i] != window[i]) { + LOG("Right half does not match. Advance by %d.\n", + i - cut + 1); + window += i - cut + 1; + goto windowloop; + } + } + for (Py_ssize_t i = 0; i < cut; i++) { + if (needle[i] != window[i]) { + LOG("Left half does not match. Advance by period %d.\n", + period); + window += period; + goto windowloop; + } + } + LOG("Left half matches. Returning %d.\n", window - haystack); + return window - haystack; + } + } + LOG("Not found. Returning -1.\n"); + return -1; +} + +Py_LOCAL_INLINE(Py_ssize_t) +STRINGLIB(_two_way_find)(const STRINGLIB_CHAR *haystack, + Py_ssize_t len_haystack, + const STRINGLIB_CHAR *needle, + Py_ssize_t len_needle) +{ + LOG("###### Finding \"%s\" in \"%s\".\n", needle, haystack); + STRINGLIB(prework) p; + STRINGLIB(_preprocess)(needle, len_needle, &p); + return STRINGLIB(_two_way)(haystack, len_haystack, &p); +} + +Py_LOCAL_INLINE(Py_ssize_t) +STRINGLIB(_two_way_count)(const STRINGLIB_CHAR *haystack, + Py_ssize_t len_haystack, + const STRINGLIB_CHAR *needle, + Py_ssize_t len_needle, + Py_ssize_t maxcount) +{ + LOG("###### Counting \"%s\" in \"%s\".\n", needle, haystack); + STRINGLIB(prework) p; + STRINGLIB(_preprocess)(needle, len_needle, &p); + Py_ssize_t index = 0, count = 0; + while (1) { + Py_ssize_t result; + result = STRINGLIB(_two_way)(haystack + index, + len_haystack - index, &p); + if (result == -1) { + return count; + } + count++; + if (count == maxcount) { + return maxcount; + } + index += result + len_needle; + } + return count; +} + +#undef SHIFT_TYPE +#undef NOT_FOUND +#undef SHIFT_OVERFLOW +#undef TABLE_SIZE_BITS +#undef TABLE_SIZE +#undef TABLE_MASK + +#undef LOG +#undef LOG_STRING + Py_LOCAL_INLINE(Py_ssize_t) FASTSEARCH(const STRINGLIB_CHAR* s, Py_ssize_t n, const STRINGLIB_CHAR* p, Py_ssize_t m, @@ -195,10 +548,22 @@ FASTSEARCH(const STRINGLIB_CHAR* s, Py_ssize_t n, } mlast = m - 1; - skip = mlast - 1; + skip = mlast; mask = 0; if (mode != FAST_RSEARCH) { + if (m >= 100 && w >= 2000 && w / m >= 5) { + /* For larger problems where the needle isn't a huge + percentage of the size of the haystack, the relatively + expensive O(m) startup cost of the two-way algorithm + will surely pay off. */ + if (mode == FAST_SEARCH) { + return STRINGLIB(_two_way_find)(s, n, p, m); + } + else { + return STRINGLIB(_two_way_count)(s, n, p, m, maxcount); + } + } const STRINGLIB_CHAR *ss = s + m - 1; const STRINGLIB_CHAR *pp = p + m - 1; @@ -207,41 +572,118 @@ FASTSEARCH(const STRINGLIB_CHAR* s, Py_ssize_t n, /* process pattern[:-1] */ for (i = 0; i < mlast; i++) { STRINGLIB_BLOOM_ADD(mask, p[i]); - if (p[i] == p[mlast]) + if (p[i] == p[mlast]) { skip = mlast - i - 1; + } } /* process pattern[-1] outside the loop */ STRINGLIB_BLOOM_ADD(mask, p[mlast]); + if (m >= 100 && w >= 8000) { + /* To ensure that we have good worst-case behavior, + here's an adaptive version of the algorithm, where if + we match O(m) characters without any matches of the + entire needle, then we predict that the startup cost of + the two-way algorithm will probably be worth it. */ + Py_ssize_t hits = 0; + for (i = 0; i <= w; i++) { + if (ss[i] == pp[0]) { + /* candidate match */ + for (j = 0; j < mlast; j++) { + if (s[i+j] != p[j]) { + break; + } + } + if (j == mlast) { + /* got a match! */ + if (mode != FAST_COUNT) { + return i; + } + count++; + if (count == maxcount) { + return maxcount; + } + i = i + mlast; + continue; + } + /* miss: check if next character is part of pattern */ + if (!STRINGLIB_BLOOM(mask, ss[i+1])) { + i = i + m; + } + else { + i = i + skip; + } + hits += j + 1; + if (hits >= m / 4 && i < w - 1000) { + /* We've done O(m) fruitless comparisons + anyway, so spend the O(m) cost on the + setup for the two-way algorithm. */ + Py_ssize_t res; + if (mode == FAST_COUNT) { + res = STRINGLIB(_two_way_count)( + s+i, n-i, p, m, maxcount-count); + return count + res; + } + else { + res = STRINGLIB(_two_way_find)(s+i, n-i, p, m); + if (res == -1) { + return -1; + } + return i + res; + } + } + } + else { + /* skip: check if next character is part of pattern */ + if (!STRINGLIB_BLOOM(mask, ss[i+1])) { + i = i + m; + } + } + } + if (mode != FAST_COUNT) { + return -1; + } + return count; + } + /* The standard, non-adaptive version of the algorithm. */ for (i = 0; i <= w; i++) { /* note: using mlast in the skip path slows things down on x86 */ if (ss[i] == pp[0]) { /* candidate match */ - for (j = 0; j < mlast; j++) - if (s[i+j] != p[j]) + for (j = 0; j < mlast; j++) { + if (s[i+j] != p[j]) { break; + } + } if (j == mlast) { /* got a match! */ - if (mode != FAST_COUNT) + if (mode != FAST_COUNT) { return i; + } count++; - if (count == maxcount) + if (count == maxcount) { return maxcount; + } i = i + mlast; continue; } /* miss: check if next character is part of pattern */ - if (!STRINGLIB_BLOOM(mask, ss[i+1])) + if (!STRINGLIB_BLOOM(mask, ss[i+1])) { i = i + m; - else + } + else { i = i + skip; - } else { + } + } + else { /* skip: check if next character is part of pattern */ - if (!STRINGLIB_BLOOM(mask, ss[i+1])) + if (!STRINGLIB_BLOOM(mask, ss[i+1])) { i = i + m; + } } } - } else { /* FAST_RSEARCH */ + } + else { /* FAST_RSEARCH */ /* create compressed boyer-moore delta 1 table */ @@ -250,28 +692,36 @@ FASTSEARCH(const STRINGLIB_CHAR* s, Py_ssize_t n, /* process pattern[:0:-1] */ for (i = mlast; i > 0; i--) { STRINGLIB_BLOOM_ADD(mask, p[i]); - if (p[i] == p[0]) + if (p[i] == p[0]) { skip = i - 1; + } } for (i = w; i >= 0; i--) { if (s[i] == p[0]) { /* candidate match */ - for (j = mlast; j > 0; j--) - if (s[i+j] != p[j]) + for (j = mlast; j > 0; j--) { + if (s[i+j] != p[j]) { break; - if (j == 0) + } + } + if (j == 0) { /* got a match! */ return i; + } /* miss: check if previous character is part of pattern */ - if (i > 0 && !STRINGLIB_BLOOM(mask, s[i-1])) + if (i > 0 && !STRINGLIB_BLOOM(mask, s[i-1])) { i = i - m; - else + } + else { i = i - skip; - } else { + } + } + else { /* skip: check if previous character is part of pattern */ - if (i > 0 && !STRINGLIB_BLOOM(mask, s[i-1])) + if (i > 0 && !STRINGLIB_BLOOM(mask, s[i-1])) { i = i - m; + } } } } diff --git a/Objects/stringlib/stringlib_find_two_way_notes.txt b/Objects/stringlib/stringlib_find_two_way_notes.txt new file mode 100644 index 0000000000000..afe45431a75ac --- /dev/null +++ b/Objects/stringlib/stringlib_find_two_way_notes.txt @@ -0,0 +1,431 @@ +This document explains Crochemore and Perrin's Two-Way string matching +algorithm, in which a smaller string (the "pattern" or "needle") +is searched for in a longer string (the "text" or "haystack"), +determining whether the needle is a substring of the haystack, and if +so, at what index(es). It is to be used by Python's string +(and bytes-like) objects when calling `find`, `index`, `__contains__`, +or implicitly in methods like `replace` or `partition`. + +This is essentially a re-telling of the paper + + Crochemore M., Perrin D., 1991, Two-way string-matching, + Journal of the ACM 38(3):651-675. + +focused more on understanding and examples than on rigor. See also +the code sample here: + + http://www-igm.univ-mlv.fr/~lecroq/string/node26.html#SECTION00260 + +The algorithm runs in O(len(needle) + len(haystack)) time and with +O(1) space. However, since there is a larger preprocessing cost than +simpler algorithms, this Two-Way algorithm is to be used only when the +needle and haystack lengths meet certain thresholds. + + +These are the basic steps of the algorithm: + + * "Very carefully" cut the needle in two. + * For each alignment attempted: + 1. match the right part + * On failure, jump by the amount matched + 1 + 2. then match the left part. + * On failure jump by max(len(left), len(right)) + 1 + * If the needle is periodic, don't re-do comparisons; maintain + a "memory" of how many characters you already know match. + + +-------- Matching the right part -------- + +We first scan the right part of the needle to check if it matches the +the aligned characters in the haystack. We scan left-to-right, +and if a mismatch occurs, we jump ahead by the amount matched plus 1. + +Example: + + text: ........EFGX................... + pattern: ....abcdEFGH.... + cut: <<<<>>>> + +Matched 3, so jump ahead by 4: + + text: ........EFGX................... + pattern: ....abcdEFGH.... + cut: <<<<>>>> + +Why are we allowed to do this? Because we cut the needle very +carefully, in such a way that if the cut is ...abcd + EFGH... then +we have + + d != E + cd != EF + bcd != EFG + abcd != EFGH + ... and so on. + +If this is true for every pair of equal-length substrings around the +cut, then the following alignments do not work, so we can skip them: + + text: ........EFG.................... + pattern: ....abcdEFGH.... + ^ (Bad because d != E) + text: ........EFG.................... + pattern: ....abcdEFGH.... + ^^ (Bad because cd != EF) + text: ........EFG.................... + pattern: ....abcdEFGH.... + ^^^ (Bad because bcd != EFG) + +Skip 3 alignments => increment alignment by 4. + + +-------- If len(left_part) < len(right_part) -------- + +Above is the core idea, and it begins to suggest how the algorithm can +be linear-time. There is one bit of subtlety involving what to do +around the end of the needle: if the left half is shorter than the +right, then we could run into something like this: + + text: .....EFG...... + pattern: cdEFGH + +The same argument holds that we can skip ahead by 4, so long as + + d != E + cd != EF + ?cd != EFG + ??cd != EFGH + etc. + +The question marks represent "wildcards" that always match; they're +outside the limits of the needle, so there's no way for them to +invalidate a match. To ensure that the inequalities above are always +true, we need them to be true for all possible '?' values. We thus +need cd != FG and cd != GH, etc. + + +-------- Matching the left part -------- + +Once we have ensured the right part matches, we scan the left part +(order doesn't matter, but traditionally right-to-left), and if we +find a mismatch, we jump ahead by +max(len(left_part), len(right_part)) + 1. That we can jump by +at least len(right_part) + 1 we have already seen: + + text: .....EFG..... + pattern: abcdEFG + Matched 3, so jump by 4, + using the fact that d != E, cd != EF, and bcd != EFG. + +But we can also jump by at least len(left_part) + 1: + + text: ....cdEF..... + pattern: abcdEF + Jump by len('abcd') + 1 = 5. + + Skip the alignments: + text: ....cdEF..... + pattern: abcdEF + text: ....cdEF..... + pattern: abcdEF + text: ....cdEF..... + pattern: abcdEF + text: ....cdEF..... + pattern: abcdEF + +This requires the following facts: + d != E + cd != EF + bcd != EF? + abcd != EF?? + etc., for all values of ?s, as above. + +If we have both sets of inequalities, then we can indeed jump by +max(len(left_part), len(right_part)) + 1. Under the assumption of such +a nice splitting of the needle, we now have enough to prove linear +time for the search: consider the forward-progress/comparisons ratio +at each alignment position. If a mismatch occurs in the right part, +the ratio is 1 position forward per comparison. On the other hand, +if a mismatch occurs in the left half, we advance by more than +len(needle)//2 positions for at most len(needle) comparisons, +so this ratio is more than 1/2. This average "movement speed" is +bounded below by the constant "1 position per 2 comparisons", so we +have linear time. + + +-------- The periodic case -------- + +The sets of inequalities listed so far seem too good to be true in +the general case. Indeed, they fail when a needle is periodic: +there's no way to split 'AAbAAbAAbA' in two such that + + (the stuff n characters to the left of the split) + cannot equal + (the stuff n characters to the right of the split) + for all n. + +This is because no matter how you cut it, you'll get +s[cut-3:cut] == s[cut:cut+3]. So what do we do? We still cut the +needle in two so that n can be as big as possible. If we were to +split it as + + AAbA + AbAAbA + +then A == A at the split, so this is bad (we failed at length 1), but +if we split it as + + AA + bAAbAAbA + +we at least have A != b and AA != bA, and we fail at length 3 +since ?AA == bAA. We already knew that a cut to make length-3 +mismatch was impossible due to the period, but we now see that the +bound is sharp; we can get length-1 and length-2 to mismatch. + +This is exactly the content of the *critical factorization theorem*: +that no matter the period of the original needle, you can cut it in +such a way that (with the appropriate question marks), +needle[cut-k:cut] mismatches needle[cut:cut+k] for all k < the period. + +Even "non-periodic" strings are periodic with a period equal to +their length, so for such needles, the CFT already guarantees that +the algorithm described so far will work, since we can cut the needle +so that the length-k chunks on either side of the cut mismatch for all +k < len(needle). Looking closer at the algorithm, we only actually +require that k go up to max(len(left_part), len(right_part)). +So long as the period exceeds that, we're good. + +The more general shorter-period case is a bit harder. The essentials +are the same, except we use the periodicity to our advantage by +"remembering" periods that we've already compared. In our running +example, say we're computing + + "AAbAAbAAbA" in "bbbAbbAAbAAbAAbbbAAbAAbAAbAA". + +We cut as AA + bAAbAAbA, and then the algorithm runs as follows: + + First alignment: + bbbAbbAAbAAbAAbbbAAbAAbAAbAA + AAbAAbAAbA + ^^X + - Mismatch at third position, so jump by 3. + - This requires that A!=b and AA != bA. + + Second alignment: + bbbAbbAAbAAbAAbbbAAbAAbAAbAA + AAbAAbAAbA + ^^^^^^^^ + X + - Matched entire right part + - Mismatch at left part. + - Jump forward a period, remembering the existing comparisons + + Third alignment: + bbbAbbAAbAAbAAbbbAAbAAbAAbAA + AAbAAbAAbA + mmmmmmm^^X + - There's "memory": a bunch of characters were already matched. + - Two more characters match beyond that. + - The 8th character of the right part mismatched, so jump by 8 + - The above rule is more complicated than usual: we don't have + the right inequalities for lengths 1 through 7, but we do have + shifted copies of the length-1 and length-2 inequalities, + along with knowledge of the mismatch. We can skip all of these + alignments at once: + + bbbAbbAAbAAbAAbbbAAbAAbAAbAA + AAbAAbAAbA + ~ A != b at the cut + bbbAbbAAbAAbAAbbbAAbAAbAAbAA + AAbAAbAAbA + ~~ AA != bA at the cut + bbbAbbAAbAAbAAbbbAAbAAbAAbAA + AAbAAbAAbA + ^^^^X 7-3=4 match, and the 5th misses. + bbbAbbAAbAAbAAbbbAAbAAbAAbAA + AAbAAbAAbA + ~ A != b at the cut + bbbAbbAAbAAbAAbbbAAbAAbAAbAA + AAbAAbAAbA + ~~ AA != bA at the cut + bbbAbbAAbAAbAAbbbAAbAAbAAbAA + AAbAAbAAbA + ^X 7-3-3=1 match and the 2nd misses. + bbbAbbAAbAAbAAbbbAAbAAbAAbAA + AAbAAbAAbA + ~ A != b at the cut + + Fourth alignment: + bbbAbbAAbAAbAAbbbAAbAAbAAbAA + AAbAAbAAbA + ^X + - Second character mismatches, so jump by 2. + + Fifth alignment: + bbbAbbAAbAAbAAbbbAAbAAbAAbAA + AAbAAbAAbA + ^^^^^^^^ + X + - Right half matches, so use memory and skip ahead by period=3 + + Sixth alignment: + bbbAbbAAbAAbAAbbbAAbAAbAAbAA + AAbAAbAAbA + mmmmmmmm^^ + - Right part matches, left part is remembered, found a match! + +The one tricky skip by 8 here generalizes: if we have a period of p, +then the CFT says we can ensure the cut has the inequality property +for lengths 1 through p-1, and jumping by p would line up the +matching characters and mismatched character one period earlier. +Inductively, this proves that we can skip by the number of characters +matched in the right half, plus 1, just as in the original algorithm. + +To make it explicit, the memory is set whenever the entire right part +is matched and is then used as a starting point in the next alignment. +In such a case, the alignment jumps forward one period, and the right +half matches all except possibly the last period. Additionally, +if we cut so that the left part has a length strictly less than the +period (we always can!), then we can know that the left part already +matches. The memory is reset to 0 whenever there is a mismatch in the +right part. + +To prove linearity for the periodic case, note that if a right-part +character mismatches, then we advance forward 1 unit per comparison. +On the other hand, if the entire right part matches, then the skipping +forward by one period "defers" some of the comparisons to the next +alignment, where they will then be spent at the usual rate of +one comparison per step forward. Even if left-half comparisons +are always "wasted", they constitute less than half of all +comparisons, so the average rate is certainly at least 1 move forward +per 2 comparisons. + + +-------- When to choose the periodic algorithm --------- + +The periodic algorithm is always valid but has an overhead of one +more "memory" register and some memory computation steps, so the +here-described-first non-periodic/long-period algorithm -- skipping by +max(len(left_part), len(right_part)) + 1 rather than the period -- +should be preferred when possible. + +Interestingly, the long-period algorithm does not require an exact +computation of the period; it works even with some long-period, but +undeniably "periodic" needles: + + Cut: AbcdefAbc == Abcde + fAbc + +This cut gives these inequalities: + + e != f + de != fA + cde != fAb + bcde != fAbc + Abcde != fAbc? + The first failure is a period long, per the CFT: + ?Abcde == fAbc?? + +A sufficient condition for using the long-period algorithm is having +the period of the needle be greater than +max(len(left_part), len(right_part)). This way, after choosing a good +split, we get all of the max(len(left_part), len(right_part)) +inequalities around the cut that were required in the long-period +version of the algorithm. + +With all of this in mind, here's how we choose: + + (1) Choose a "critical factorization" of the needle -- a cut + where we have period minus 1 inequalities in a row. + More specifically, choose a cut so that the left_part + is less than one period long. + (2) Determine the period P_r of the right_part. + (3) Check if the left part is just an extension of the pattern of + the right part, so that the whole needle has period P_r. + Explicitly, check if + needle[0:cut] == needle[0+P_r:cut+P_r] + If so, we use the periodic algorithm. If not equal, we use the + long-period algorithm. + +Note that if equality holds in (3), then the period of the whole +string is P_r. On the other hand, suppose equality does not hold. +The period of the needle is then strictly greater than P_r. Here's +a general fact: + + If p is a substring of s and p has period r, then the period + of s is either equal to r or greater than len(p). + +We know that needle_period != P_r, +and therefore needle_period > len(right_part). +Additionally, we'll choose the cut (see below) +so that len(left_part) < needle_period. + +Thus, in the case where equality does not hold, we have that +needle_period >= max(len(left_part), len(right_part)) + 1, +so the long-period algorithm works, but otherwise, we know the period +of the needle. + +Note that this decision process doesn't always require an exact +computation of the period -- we can get away with only computing P_r! + + +-------- Computing the cut -------- + +Our remaining tasks are now to compute a cut of the needle with as +many inequalities as possible, ensuring that cut < needle_period. +Meanwhile, we must also compute the period P_r of the right_part. + +The computation is relatively simple, essentially doing this: + + suffix1 = max(needle[i:] for i in range(len(needle))) + suffix2 = ... # the same as above, but invert the alphabet + cut1 = len(needle) - len(suffix1) + cut2 = len(needle) - len(suffix2) + cut = max(cut1, cut2) # the later cut + +For cut2, "invert the alphabet" is different than saying min(...), +since in lexicographic order, we still put "py" < "python", even +if the alphabet is inverted. Computing these, along with the method +of computing the period of the right half, is easiest to read directly +from the source code in fastsearch.h, in which these are computed +in linear time. + +Crochemore & Perrin's Theorem 3.1 give that "cut" above is a +critical factorization less than the period, but a very brief sketch +of their proof goes something like this (this is far from complete): + + * If this cut splits the needle as some + needle == (a + w) + (w + b), meaning there's a bad equality + w == w, it's impossible for w + b to be bigger than both + b and w + w + b, so this can't happen. We thus have all of + the ineuqalities with no question marks. + * By maximality, the right part is not a substring of the left + part. Thus, we have all of the inequalities involving no + left-side question marks. + * If you have all of the inequalities without right-side question + marks, we have a critical factorization. + * If one such inequality fails, then there's a smaller period, + but the factorization is nonetheless critical. Here's where + you need the redundancy coming from computing both cuts and + choosing the later one. + + +-------- Some more Bells and Whistles -------- + +Beyond Crochemore & Perrin's original algorithm, we can use a couple +more tricks for speed in fastsearch.h: + + 1. Even though C&P has a best-case O(n/m) time, this doesn't occur + very often, so we add a Boyer-Moore bad character table to + achieve sublinear time in more cases. + + 2. The prework of computing the cut/period is expensive per + needle character, so we shouldn't do it if it won't pay off. + For this reason, if the needle and haystack are long enough, + only automatically start with two-way if the needle's length + is a small percentage of the length of the haystack. + + 3. In cases where the needle and haystack are large but the needle + makes up a significant percentage of the length of the + haystack, don't pay the expensive two-way preprocessing cost + if you don't need to. Instead, keep track of how many + character comparisons are equal, and if that exceeds + O(len(needle)), then pay that cost, since the simpler algorithm + isn't doing very well.

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bpo-43251: sqlite3_column_name() failures now raise MemoryError (GH-24609)
by berkerpeksag 28 Feb '21

28 Feb '21

https://github.com/python/cpython/commit/2183d06bc8a481098d62a4ebed8d6982b3… commit: 2183d06bc8a481098d62a4ebed8d6982b3d1602a branch: master author: Erlend Egeberg Aasland <erlend.aasland(a)innova.no> committer: berkerpeksag <berker.peksag(a)gmail.com> date: 2021-02-28T19:01:06+02:00 summary: bpo-43251: sqlite3_column_name() failures now raise MemoryError (GH-24609) files: A Misc/NEWS.d/next/Library/2021-02-18-14-24-42.bpo-43251.n6WZDw.rst M Modules/_sqlite/cursor.c diff --git a/Misc/NEWS.d/next/Library/2021-02-18-14-24-42.bpo-43251.n6WZDw.rst b/Misc/NEWS.d/next/Library/2021-02-18-14-24-42.bpo-43251.n6WZDw.rst new file mode 100644 index 0000000000000..52ce9d93af84c --- /dev/null +++ b/Misc/NEWS.d/next/Library/2021-02-18-14-24-42.bpo-43251.n6WZDw.rst @@ -0,0 +1,2 @@ +Improve :mod:`sqlite3` error handling: ``sqlite3_column_name()`` failures +now result in :exc:`MemoryError`. Patch by Erlend E. Aasland. diff --git a/Modules/_sqlite/cursor.c b/Modules/_sqlite/cursor.c index 74860731d1c7b..ddacb2745d34d 100644 --- a/Modules/_sqlite/cursor.c +++ b/Modules/_sqlite/cursor.c @@ -135,7 +135,6 @@ pysqlite_build_row_cast_map(pysqlite_Cursor* self) { int i; const char* pos; - const char* colname; const char* decltype; PyObject* converter; @@ -152,21 +151,24 @@ pysqlite_build_row_cast_map(pysqlite_Cursor* self) converter = NULL; if (self->connection->detect_types & PARSE_COLNAMES) { - colname = sqlite3_column_name(self->statement->st, i); - if (colname) { - const char *type_start = NULL; - for (pos = colname; *pos != 0; pos++) { - if (*pos == '[') { - type_start = pos + 1; - } - else if (*pos == ']' && type_start != NULL) { - converter = _pysqlite_get_converter(type_start, pos - type_start); - if (!converter && PyErr_Occurred()) { - Py_CLEAR(self->row_cast_map); - return -1; - } - break; + const char *colname = sqlite3_column_name(self->statement->st, i); + if (colname == NULL) { + PyErr_NoMemory(); + Py_CLEAR(self->row_cast_map); + return -1; + } + const char *type_start = NULL; + for (pos = colname; *pos != 0; pos++) { + if (*pos == '[') { + type_start = pos + 1; + } + else if (*pos == ']' && type_start != NULL) { + converter = _pysqlite_get_converter(type_start, pos - type_start); + if (!converter && PyErr_Occurred()) { + Py_CLEAR(self->row_cast_map); + return -1; } + break; } } } @@ -210,10 +212,6 @@ _pysqlite_build_column_name(pysqlite_Cursor *self, const char *colname) const char* pos; Py_ssize_t len; - if (!colname) { - Py_RETURN_NONE; - } - if (self->connection->detect_types & PARSE_COLNAMES) { for (pos = colname; *pos; pos++) { if (*pos == '[') { @@ -311,8 +309,9 @@ _pysqlite_fetch_one_row(pysqlite_Cursor* self) if (!converted && PyErr_ExceptionMatches(PyExc_UnicodeDecodeError)) { PyErr_Clear(); colname = sqlite3_column_name(self->statement->st, i); - if (!colname) { - colname = "<unknown column name>"; + if (colname == NULL) { + PyErr_NoMemory(); + goto error; } PyOS_snprintf(buf, sizeof(buf) - 1, "Could not decode to UTF-8 column '%s' with text '%s'", colname , text); @@ -550,9 +549,15 @@ _pysqlite_query_execute(pysqlite_Cursor* self, int multiple, PyObject* operation if (!descriptor) { goto error; } - column_name = _pysqlite_build_column_name(self, - sqlite3_column_name(self->statement->st, i)); - if (!column_name) { + const char *colname; + colname = sqlite3_column_name(self->statement->st, i); + if (colname == NULL) { + PyErr_NoMemory(); + Py_DECREF(descriptor); + goto error; + } + column_name = _pysqlite_build_column_name(self, colname); + if (column_name == NULL) { Py_DECREF(descriptor); goto error; }

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bpo-43345: Enhance TypedDict documentation. (#24668)
by gvanrossum 28 Feb '21

28 Feb '21

https://github.com/python/cpython/commit/1e3c68246ee738b5ec5450b1eb39af2fca… commit: 1e3c68246ee738b5ec5450b1eb39af2fca300cb9 branch: master author: Paul Bryan <pbryan(a)mperativ.io> committer: gvanrossum <gvanrossum(a)gmail.com> date: 2021-02-27T23:02:14-08:00 summary: bpo-43345: Enhance TypedDict documentation. (#24668) files: M Doc/library/typing.rst diff --git a/Doc/library/typing.rst b/Doc/library/typing.rst index 7ef5b3971a91e..9696fd4ef99e6 100644 --- a/Doc/library/typing.rst +++ b/Doc/library/typing.rst @@ -1247,26 +1247,28 @@ These are not used in annotations. They are building blocks for declaring types. assert Point2D(x=1, y=2, label='first') == dict(x=1, y=2, label='first') - The type info for introspection can be accessed via ``Point2D.__annotations__`` - and ``Point2D.__total__``. To allow using this feature with older versions - of Python that do not support :pep:`526`, ``TypedDict`` supports two additional - equivalent syntactic forms:: + The type info for introspection can be accessed via ``Point2D.__annotations__``, + ``Point2D.__total__``, ``Point2D.__required_keys__``, and + ``Point2D.__optional_keys__``. + To allow using this feature with older versions of Python that do not + support :pep:`526`, ``TypedDict`` supports two additional equivalent + syntactic forms:: Point2D = TypedDict('Point2D', x=int, y=int, label=str) Point2D = TypedDict('Point2D', {'x': int, 'y': int, 'label': str}) - By default, all keys must be present in a TypedDict. It is possible - to override this by specifying totality. + By default, all keys must be present in a ``TypedDict``. It is possible to + override this by specifying totality. Usage:: - class point2D(TypedDict, total=False): + class Point2D(TypedDict, total=False): x: int y: int - This means that a point2D TypedDict can have any of the keys omitted. A type - checker is only expected to support a literal False or True as the value of - the total argument. True is the default, and makes all items defined in the - class body be required. + This means that a ``Point2D`` ``TypedDict`` can have any of the keys + omitted. A type checker is only expected to support a literal ``False`` or + ``True`` as the value of the ``total`` argument. ``True`` is the default, + and makes all items defined in the class body required. See :pep:`589` for more examples and detailed rules of using ``TypedDict``. @@ -1980,4 +1982,3 @@ Constant (see :pep:`563`). .. versionadded:: 3.5.2 -

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bpo-43335: Update macro to check gcc version (GH-24662)
by miss-islington 27 Feb '21

27 Feb '21

https://github.com/python/cpython/commit/132e31f4bf20818a469cbcdba75068f26c… commit: 132e31f4bf20818a469cbcdba75068f26cb19a65 branch: 3.9 author: Miss Islington (bot) <31488909+miss-islington(a)users.noreply.github.com> committer: miss-islington <31488909+miss-islington(a)users.noreply.github.com> date: 2021-02-27T16:51:36-08:00 summary: bpo-43335: Update macro to check gcc version (GH-24662) (cherry picked from commit bf9de7ab24d9d7068645b202bc47146b9a4f2726) Co-authored-by: Dong-hee Na <donghee.na(a)python.org> files: M Modules/_ctypes/callbacks.c diff --git a/Modules/_ctypes/callbacks.c b/Modules/_ctypes/callbacks.c index 19c77f4f57ef5..e6e101829a699 100644 --- a/Modules/_ctypes/callbacks.c +++ b/Modules/_ctypes/callbacks.c @@ -430,7 +430,7 @@ CThunkObject *_ctypes_alloc_callback(PyObject *callable, #pragma clang diagnostic push #pragma clang diagnostic ignored "-Wdeprecated-declarations" #endif -#if defined(__GNUC__) +#if defined(__GNUC__) && ((__GNUC__ > 4) || ((__GNUC__ == 4) && (__GNUC_MINOR__ > 5))) #pragma GCC diagnostic push #pragma GCC diagnostic ignored "-Wdeprecated-declarations" #endif @@ -439,7 +439,7 @@ CThunkObject *_ctypes_alloc_callback(PyObject *callable, #if defined(__clang__) || defined(MACOSX) #pragma clang diagnostic pop #endif -#if defined(__GNUC__) +#if defined(__GNUC__) && ((__GNUC__ > 4) || ((__GNUC__ == 4) && (__GNUC_MINOR__ > 5))) #pragma GCC diagnostic pop #endif

1 0

bpo-43335: Update macro to check gcc version (GH-24662)
by corona10 27 Feb '21

27 Feb '21

https://github.com/python/cpython/commit/bf9de7ab24d9d7068645b202bc47146b9a… commit: bf9de7ab24d9d7068645b202bc47146b9a4f2726 branch: master author: Dong-hee Na <donghee.na(a)python.org> committer: corona10 <donghee.na92(a)gmail.com> date: 2021-02-28T09:16:24+09:00 summary: bpo-43335: Update macro to check gcc version (GH-24662) files: M Modules/_ctypes/callbacks.c diff --git a/Modules/_ctypes/callbacks.c b/Modules/_ctypes/callbacks.c index 654cb93dcf777..5a4d1c543f100 100644 --- a/Modules/_ctypes/callbacks.c +++ b/Modules/_ctypes/callbacks.c @@ -431,7 +431,7 @@ CThunkObject *_ctypes_alloc_callback(PyObject *callable, #pragma clang diagnostic push #pragma clang diagnostic ignored "-Wdeprecated-declarations" #endif -#if defined(__GNUC__) +#if defined(__GNUC__) && ((__GNUC__ > 4) || ((__GNUC__ == 4) && (__GNUC_MINOR__ > 5))) #pragma GCC diagnostic push #pragma GCC diagnostic ignored "-Wdeprecated-declarations" #endif @@ -440,7 +440,7 @@ CThunkObject *_ctypes_alloc_callback(PyObject *callable, #if defined(__clang__) || defined(MACOSX) #pragma clang diagnostic pop #endif -#if defined(__GNUC__) +#if defined(__GNUC__) && ((__GNUC__ > 4) || ((__GNUC__ == 4) && (__GNUC_MINOR__ > 5))) #pragma GCC diagnostic pop #endif

1 0

bpo-43321: Fix SystemError in getargs.c (GH-24656)
by methane 27 Feb '21

27 Feb '21

https://github.com/python/cpython/commit/c71d24f55828e7f0f2c8750d2e1b04d045… commit: c71d24f55828e7f0f2c8750d2e1b04d04539beff branch: master author: Inada Naoki <songofacandy(a)gmail.com> committer: methane <songofacandy(a)gmail.com> date: 2021-02-27T20:31:03+09:00 summary: bpo-43321: Fix SystemError in getargs.c (GH-24656) files: A Misc/NEWS.d/next/Core and Builtins/2021-02-26-13-17-52.bpo-43321.TCS3ph.rst M Python/getargs.c diff --git a/Misc/NEWS.d/next/Core and Builtins/2021-02-26-13-17-52.bpo-43321.TCS3ph.rst b/Misc/NEWS.d/next/Core and Builtins/2021-02-26-13-17-52.bpo-43321.TCS3ph.rst new file mode 100644 index 0000000000000..32c5ce14de51c --- /dev/null +++ b/Misc/NEWS.d/next/Core and Builtins/2021-02-26-13-17-52.bpo-43321.TCS3ph.rst @@ -0,0 +1,2 @@ +Fix ``SystemError`` raised when ``PyArg_Parse*()`` is used with ``#`` but +without ``PY_SSIZE_T_CLEAN`` defined. diff --git a/Python/getargs.c b/Python/getargs.c index 936eb6a89a392..b85b575a147fe 100644 --- a/Python/getargs.c +++ b/Python/getargs.c @@ -655,14 +655,14 @@ static const char * convertsimple(PyObject *arg, const char **p_format, va_list *p_va, int flags, char *msgbuf, size_t bufsize, freelist_t *freelist) { +#define RETURN_ERR_OCCURRED return msgbuf /* For # codes */ #define REQUIRE_PY_SSIZE_T_CLEAN \ if (!(flags & FLAG_SIZE_T)) { \ PyErr_SetString(PyExc_SystemError, \ "PY_SSIZE_T_CLEAN macro must be defined for '#' formats"); \ - return NULL; \ + RETURN_ERR_OCCURRED; \ } -#define RETURN_ERR_OCCURRED return msgbuf const char *format = *p_format; char c = *format++;

1 0