Mailman 3 November 2017 - Python-Dev

3.7.0a3 cutoff extended a week to 12-04
by Ned Deily 28 Nov '17

28 Nov '17

We are extending the cutoff for the next 3.7 alpha preview (3.7.0a3) a week, moving it from today to 12-04 12:00 UTC. The main reason is a selfish one: I have been traveling and mainly offline for the last few weeks and I am still catching up with activity. Since we are getting close to feature code cutoff for the 3.7 cycle, it would be better to get things in sooner than later. Following alpha 3, we will have one more alpha preview, 3.7.0a4 on 2018-01-08, prior to the feature code cutoff with 3.7.0b1 on 2018-01-29. Note that 12-04 is also the scheduled date for the next 3.6.x maintenance release release candidate, 3.6.4rc1. So I hope you can take advantage of the extra days for both release cycles. Thanks again for all your efforts! --Ned -- Ned Deily nad(a)python.org -- []

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Tricky way of of creating a generator via a comprehension expression
by Serhiy Storchaka 27 Nov '17

27 Nov '17

From https://stackoverflow.com/questions/45190729/differences-between-generator-…. g = [(yield i) for i in range(3)] Syntactically this looks like a list comprehension, and g should be a list, right? But actually it is a generator. This code is equivalent to the following code: def _make_list(it): result = [] for i in it: result.append(yield i) return result g = _make_list(iter(range(3))) Due to "yield" in the expression _make_list() is not a function returning a list, but a generator function returning a generator. This change in semantic looks unintentional to me. It looks like leaking an implementation detail. If a list comprehension would be implemented not via creating and calling an intermediate function, but via an inlined loop (like in Python 2) this would be a syntax error if used outside of a function or would make an outer function a generator function. __result = [] __i = None try: for __i in range(3): __result.append(yield __i) g = __result finally: del __result, __i I don't see how the current behavior can be useful.

22 155

generator vs iterator etc. (was: How assignment should work with generators?)
by Koos Zevenhoven 27 Nov '17

27 Nov '17

On Mon, Nov 27, 2017 at 3:55 PM, Steven D'Aprano <steve(a)pearwood.info> wrote: > On Mon, Nov 27, 2017 at 12:17:31PM +0300, Kirill Balunov wrote: > > > > 2. Should this work only for generators or for any iterators? > > I don't understand why you are even considering singling out *only* > generators. A generator is a particular implementation of an iterator. I > can write: > > def gen(): > yield 1; yield 2; yield 3 > > it = gen() > > or I can write: > > it = iter([1, 2, 3]) > > and the behaviour of `it` should be identical. > > > I can see where this is coming from. The thing is that "iterator" and "generator" are mostly synonymous, except two things: (1) Generators are iterators that are produced by a generator function (2) Generator functions are sometimes referred to as just "generators" The concept of "generator" thus overlaps with both "iterator" and "generator function". Then there's also "iterator" and "iterable", which are two different things: (3) If `obj` is an *iterable*, then `it = iter(obj)` is an *iterator* (over the contents of `obj`) ( 4) Iterators yield values, for example on explicit calls to next(it). Personally I have leaned towards keeping a clear distinction between "generator function" and "generator", which leads to the situation that "generator" and "iterator" are mostly synonymous for me. Sometimes, for convenience, I use the term "generator" to refer to "iterators" more generally. This further seems to have a minor benefit that "generators" and "iterables" are less easily confused with each other than "iterators" and "iterables". I thought about this issue some time ago for the `views` package, which has a separation between sequences (seq) and other iterables (gen): https://github.com/k7hoven/views The functionality provided by `views.gen` is not that interesting—it's essentially a subset of itertools functionality, but with an API that parallels `views.seq` which works with sequences (iterable, sliceable, chainable, etc.). I used the name `gen`, because iterator/iterable variants of the functionality can be implemented with generator functions (although also with other kinds of iterators/iterables). Calling the thing `iter` would have conflicted with the builtin `iter`. HOWEVER, this naming can be confusing for those that lean more towards using "generator" to also mean "generator function", and for those that are comfortable with the term "iterator" despite its resemblance to "iterable". Now I'm actually seriously considering to consider renaming `views.gen` to ` views.iter` when I have time. After all, there's already `views.range` which "conflicts" with the builtin range. Anyway, the point is that the naming is suboptimal. SOLUTION: Maybe (a) all iterators should be called iterators or (b) all iterators should be called generators, regardless of whether they are somehow a result of a generator function having been called in the past. (I'm not going into the distinction between things that can receive values via `send` or any other possible distinctions between different types of iterators and iterables.) —Koos (discussion originated from python-ideas, but cross-posted to python-dev in case there's more interest there) -- + Koos Zevenhoven + http://twitter.com/k7hoven +

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Re: [Python-Dev] Second post: PEP 557, Data Classes
by Greg Ewing 27 Nov '17

27 Nov '17

Chris Angelico wrote: > I'm not sure there's any distinction between a "point" and a "vector > from the origin to a point". They transform differently. For example, translation affects a point, but makes no difference to a vector. There are two ways of dealing with that. One is to use vectors to represent both and have two different operations, "transform point" and "transform vector". The other is to represent them using different types and have one operation that does different things depending on the type. The advantage of the latter is that you can't accidentally apply the wrong operation, e.g. transform_point on something that's actually a vector. (There's actually a third way -- use homogeneous coordinates and represent points as (x, y, z, 1) and vectors as (x, y, z, 0). But that's really a variation on the "different types" idea.) -- Greg

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Summary of Python tracker Issues
by Python tracker 24 Nov '17

24 Nov '17

ACTIVITY SUMMARY (2017-11-17 - 2017-11-24) Python tracker at https://bugs.python.org/ To view or respond to any of the issues listed below, click on the issue. Do NOT respond to this message. Issues counts and deltas: open 6267 ( +6) closed 37610 (+57) total 43877 (+63) Open issues with patches: 2416 Issues opened (45) ================== #32067: Deprecate accepting unrecognized braces in regular expressions https://bugs.python.org/issue32067 opened by serhiy.storchaka #32068: textpad from curses package isn't handling backspace key https://bugs.python.org/issue32068 opened by bacher09 #32070: Clarify the behavior of the staticmethod builtin https://bugs.python.org/issue32070 opened by haikuginger #32071: Add py.test-like "-k" test selection to unittest https://bugs.python.org/issue32071 opened by jonash #32072: Issues with binary plists https://bugs.python.org/issue32072 opened by serhiy.storchaka #32073: Add copy_directory_metadata parameter to shutil.copytree https://bugs.python.org/issue32073 opened by desbma #32075: Expose ZipImporter Type Object in the include header files. https://bugs.python.org/issue32075 opened by Decorater #32076: Expose LockFile on Windows https://bugs.python.org/issue32076 opened by pitrou #32077: Documentation: Some Unicode object functions don't indicate wh https://bugs.python.org/issue32077 opened by Mathew M. #32079: version install 3.6.3 hangs in test_socket https://bugs.python.org/issue32079 opened by Raúl Alvarez #32080: Error Installing Python 3.6.3 on ubuntu 16.04 https://bugs.python.org/issue32080 opened by sachin #32081: ipaddress should support fast IP lookups https://bugs.python.org/issue32081 opened by Attila Nagy #32082: atexit module: allow getting/setting list of handlers directly https://bugs.python.org/issue32082 opened by erik.bray #32083: sqlite3 Cursor.description can't return column types https://bugs.python.org/issue32083 opened by kyoshidajp #32084: [Security] http.server can be abused to redirect to (almost) a https://bugs.python.org/issue32084 opened by vstinner #32085: [Security] A New Era of SSRF - Exploiting URL Parser in Trendi https://bugs.python.org/issue32085 opened by vstinner #32087: deprecated-removed directive generates overlapping msgids in . https://bugs.python.org/issue32087 opened by cocoatomo #32089: In developer mode (-X dev), ResourceWarning is only emited onc https://bugs.python.org/issue32089 opened by vstinner #32090: test_put() of test_multiprocessing queue tests has a race cond https://bugs.python.org/issue32090 opened by vstinner #32091: test_s_option() of test_site.HelperFunctionsTests failed on x8 https://bugs.python.org/issue32091 opened by vstinner #32092: mock.patch with autospec does not consume self / cls argument https://bugs.python.org/issue32092 opened by cbelu #32093: macOS: implement time.thread_time() using thread_info() https://bugs.python.org/issue32093 opened by vstinner #32096: Py_DecodeLocale() fails if used before the runtime is initiali https://bugs.python.org/issue32096 opened by eric.snow #32097: doctest does not consider \r\n a <BLANKLINE> https://bugs.python.org/issue32097 opened by X-Istence #32098: Hardcoded value in Lib/test/test_os.py:L1324:URandomTests.get_ https://bugs.python.org/issue32098 opened by hackan #32101: Add PYTHONDEVMODE=1 to enable the developer mode https://bugs.python.org/issue32101 opened by vstinner #32102: Add "capture_output=True" option to subprocess.run https://bugs.python.org/issue32102 opened by ncoghlan #32104: add method throw() to asyncio.Task https://bugs.python.org/issue32104 opened by Oleg K2 #32107: test_uuid uses the incorrect bitmask https://bugs.python.org/issue32107 opened by barry #32108: configparser bug: section is emptied if you assign a section t https://bugs.python.org/issue32108 opened by simonltwick #32110: Make codecs.StreamReader.read() more compatible with read() of https://bugs.python.org/issue32110 opened by serhiy.storchaka #32112: Should uuid.UUID() accept another UUID() instance? https://bugs.python.org/issue32112 opened by mjpieters #32113: Strange behavior with await in a generator expression https://bugs.python.org/issue32113 opened by levkivskyi #32114: The get_event_loop change in bpo28613 did not update the docum https://bugs.python.org/issue32114 opened by r.david.murray #32115: Ignored SIGCHLD causes asyncio.Process.wait to hang forever https://bugs.python.org/issue32115 opened by rogpeppe #32116: CSV import and export simplified https://bugs.python.org/issue32116 opened by paullongnet #32117: Tuple unpacking in return and yield statements https://bugs.python.org/issue32117 opened by dacut #32118: Docs: add note about sequence comparisons containing non-order https://bugs.python.org/issue32118 opened by Dubslow #32119: test_notify_all() of test_multiprocessing_forkserver failed on https://bugs.python.org/issue32119 opened by vstinner #32121: tracemalloc.Traceback.format() should have an option to revers https://bugs.python.org/issue32121 opened by pitrou #32122: Improve -x option documentation https://bugs.python.org/issue32122 opened by serhiy.storchaka #32123: Make the API of argparse.HelpFormatter public https://bugs.python.org/issue32123 opened by Bernhard10 #32124: Document functions safe to be called before Py_Initialize() https://bugs.python.org/issue32124 opened by vstinner #32125: Remove global configuration variable Py_UseClassExceptionsFlag https://bugs.python.org/issue32125 opened by vstinner #32126: [asyncio] test failure when the platform lacks a functional s https://bugs.python.org/issue32126 opened by xdegaye Most recent 15 issues with no replies (15) ========================================== #32125: Remove global configuration variable Py_UseClassExceptionsFlag https://bugs.python.org/issue32125 #32123: Make the API of argparse.HelpFormatter public https://bugs.python.org/issue32123 #32122: Improve -x option documentation https://bugs.python.org/issue32122 #32115: Ignored SIGCHLD causes asyncio.Process.wait to hang forever https://bugs.python.org/issue32115 #32114: The get_event_loop change in bpo28613 did not update the docum https://bugs.python.org/issue32114 #32098: Hardcoded value in Lib/test/test_os.py:L1324:URandomTests.get_ https://bugs.python.org/issue32098 #32090: test_put() of test_multiprocessing queue tests has a race cond https://bugs.python.org/issue32090 #32087: deprecated-removed directive generates overlapping msgids in . https://bugs.python.org/issue32087 #32085: [Security] A New Era of SSRF - Exploiting URL Parser in Trendi https://bugs.python.org/issue32085 #32082: atexit module: allow getting/setting list of handlers directly https://bugs.python.org/issue32082 #32081: ipaddress should support fast IP lookups https://bugs.python.org/issue32081 #32080: Error Installing Python 3.6.3 on ubuntu 16.04 https://bugs.python.org/issue32080 #32079: version install 3.6.3 hangs in test_socket https://bugs.python.org/issue32079 #32076: Expose LockFile on Windows https://bugs.python.org/issue32076 #32073: Add copy_directory_metadata parameter to shutil.copytree https://bugs.python.org/issue32073 Most recent 15 issues waiting for review (15) ============================================= #32124: Document functions safe to be called before Py_Initialize() https://bugs.python.org/issue32124 #32121: tracemalloc.Traceback.format() should have an option to revers https://bugs.python.org/issue32121 #32118: Docs: add note about sequence comparisons containing non-order https://bugs.python.org/issue32118 #32117: Tuple unpacking in return and yield statements https://bugs.python.org/issue32117 #32114: The get_event_loop change in bpo28613 did not update the docum https://bugs.python.org/issue32114 #32110: Make codecs.StreamReader.read() more compatible with read() of https://bugs.python.org/issue32110 #32107: test_uuid uses the incorrect bitmask https://bugs.python.org/issue32107 #32101: Add PYTHONDEVMODE=1 to enable the developer mode https://bugs.python.org/issue32101 #32096: Py_DecodeLocale() fails if used before the runtime is initiali https://bugs.python.org/issue32096 #32092: mock.patch with autospec does not consume self / cls argument https://bugs.python.org/issue32092 #32089: In developer mode (-X dev), ResourceWarning is only emited onc https://bugs.python.org/issue32089 #32087: deprecated-removed directive generates overlapping msgids in . https://bugs.python.org/issue32087 #32077: Documentation: Some Unicode object functions don't indicate wh https://bugs.python.org/issue32077 #32075: Expose ZipImporter Type Object in the include header files. https://bugs.python.org/issue32075 #32073: Add copy_directory_metadata parameter to shutil.copytree https://bugs.python.org/issue32073 Top 10 most discussed issues (10) ================================= #10544: yield expression inside generator expression does nothing https://bugs.python.org/issue10544 26 msgs #32096: Py_DecodeLocale() fails if used before the runtime is initiali https://bugs.python.org/issue32096 25 msgs #27535: Ignored ResourceWarning warnings leak memory in warnings regis https://bugs.python.org/issue27535 18 msgs #32089: In developer mode (-X dev), ResourceWarning is only emited onc https://bugs.python.org/issue32089 10 msgs #32030: PEP 432: Rewrite Py_Main() https://bugs.python.org/issue32030 9 msgs #32071: Add py.test-like "-k" test selection to unittest https://bugs.python.org/issue32071 9 msgs #32107: test_uuid uses the incorrect bitmask https://bugs.python.org/issue32107 9 msgs #30811: A venv created and activated from within a virtualenv uses the https://bugs.python.org/issue30811 8 msgs #32075: Expose ZipImporter Type Object in the include header files. https://bugs.python.org/issue32075 6 msgs #32121: tracemalloc.Traceback.format() should have an option to revers https://bugs.python.org/issue32121 6 msgs Issues closed (54) ================== #1102: Add support for _msi.Record.GetString() and _msi.Record.GetInt https://bugs.python.org/issue1102 closed by berker.peksag #12239: msilib VT_EMPTY SummaryInformation properties raise an error ( https://bugs.python.org/issue12239 closed by berker.peksag #12276: 3.x ignores sys.tracebacklimit=0 https://bugs.python.org/issue12276 closed by serhiy.storchaka #12382: [msilib] Obscure exception message when trying to open a non-e https://bugs.python.org/issue12382 closed by berker.peksag #19610: Give clear error messages for invalid types used for setup.py https://bugs.python.org/issue19610 closed by berker.peksag #23719: PEP 475: port test_eintr to Windows https://bugs.python.org/issue23719 closed by vstinner #26606: logging.baseConfig is missing the encoding parameter https://bugs.python.org/issue26606 closed by vinay.sajip #27068: Add a detach() method to subprocess.Popen https://bugs.python.org/issue27068 closed by vstinner #28684: [asyncio] bind() on a unix socket raises PermissionError on An https://bugs.python.org/issue28684 closed by xdegaye #29040: building Android with android-ndk-r14 https://bugs.python.org/issue29040 closed by xdegaye #29184: skip tests of test_socketserver when bind() raises PermissionE https://bugs.python.org/issue29184 closed by xdegaye #29185: test_distutils fails on Android API level 24 https://bugs.python.org/issue29185 closed by xdegaye #29556: Remove unused #include <langinfo.h> https://bugs.python.org/issue29556 closed by Chi Hsuan Yen #30456: 2to3 docs: example of fix for duplicates in second argument of https://bugs.python.org/issue30456 closed by berker.peksag #30904: Python 3 logging HTTPHandler sends duplicate Host header https://bugs.python.org/issue30904 closed by vinay.sajip #30987: Support for ISO-TP protocol in SocketCAN https://bugs.python.org/issue30987 closed by berker.peksag #30989: Sort only when needed in TimedRotatingFileHandler's getFilesTo https://bugs.python.org/issue30989 closed by vinay.sajip #31324: support._match_test() used by test.bisect is very inefficient https://bugs.python.org/issue31324 closed by vstinner #31325: req_rate is a namedtuple type rather than instance https://bugs.python.org/issue31325 closed by rhettinger #31520: ResourceWarning: unclosed <socket.socket [closed] fd=3, ...> w https://bugs.python.org/issue31520 closed by vstinner #31543: Optimize wrapper descriptors using FASTCALL https://bugs.python.org/issue31543 closed by vstinner #31626: Writing in freed memory in _PyMem_DebugRawRealloc() after shri https://bugs.python.org/issue31626 closed by vstinner #31672: string.Template should use re.ASCII flag https://bugs.python.org/issue31672 closed by barry #31701: faulthandler dumps 'Windows fatal exception: code 0xe06d7363' https://bugs.python.org/issue31701 closed by vstinner #31805: support._is_gui_available() hangs on x86-64 Sierra 3.6/3.x bui https://bugs.python.org/issue31805 closed by vstinner #31897: Unexpected exceptions in plistlib.loads https://bugs.python.org/issue31897 closed by serhiy.storchaka #31911: Use malloc_usable_size() in pymalloc for realloc https://bugs.python.org/issue31911 closed by vstinner #31943: Add asyncio.Handle.cancelled() method https://bugs.python.org/issue31943 closed by asvetlov #32022: Python crashes with mutually recursive code https://bugs.python.org/issue32022 closed by vstinner #32025: Add time.thread_time() https://bugs.python.org/issue32025 closed by vstinner #32031: Do not use the canonical path in pydoc test_mixed_case_module_ https://bugs.python.org/issue32031 closed by xdegaye #32042: Option for comparing values instead of reprs in doctest https://bugs.python.org/issue32042 closed by rhettinger #32043: Add a new -X dev option: "developer mode" https://bugs.python.org/issue32043 closed by vstinner #32047: asyncio: enable debug mode when -X dev is used https://bugs.python.org/issue32047 closed by vstinner #32050: Fix -x option documentation https://bugs.python.org/issue32050 closed by vstinner #32060: Should an ABC fail if no abstract methods are defined? https://bugs.python.org/issue32060 closed by rhettinger #32064: python msilib view.fetch is not returning none https://bugs.python.org/issue32064 closed by berker.peksag #32065: range_iterator doesn't have length, leads to surprised result https://bugs.python.org/issue32065 closed by r.david.murray #32066: asyncio: Support pathlib.Path in create_unix_connection; sock https://bugs.python.org/issue32066 closed by yselivanov #32069: Drop loop._make_legacy_ssl_transport https://bugs.python.org/issue32069 closed by asvetlov #32074: Might be a wrong implementation https://bugs.python.org/issue32074 closed by serhiy.storchaka #32078: string result of str(bytes()) in Python3 https://bugs.python.org/issue32078 closed by vstinner #32086: C API: Clarify which C functions are safe to be called before https://bugs.python.org/issue32086 closed by vstinner #32088: Display DeprecationWarning, PendingDeprecationWarning and Impo https://bugs.python.org/issue32088 closed by lukasz.langa #32094: _args_from_interpreter_flags() doesn't keep -X options https://bugs.python.org/issue32094 closed by vstinner #32095: AIX: ModuleNotFoundError: No module named '_ctypes' - make ins https://bugs.python.org/issue32095 closed by zach.ware #32099: Use range in itertools roundrobin recipe https://bugs.python.org/issue32099 closed by rhettinger #32100: IDLE: PathBrowser isn't working https://bugs.python.org/issue32100 closed by terry.reedy #32103: Inconsistent text at TypeError in concatenation https://bugs.python.org/issue32103 closed by serhiy.storchaka #32105: Please add asyncio.BaseEventLoop.connect_accepted_socket Versi https://bugs.python.org/issue32105 closed by yselivanov #32106: Move sysmodule.c to Modules? https://bugs.python.org/issue32106 closed by serhiy.storchaka #32109: Separated square brackets will generate a tuple instead of a l https://bugs.python.org/issue32109 closed by berker.peksag #32111: ObjectListView crashes on python3.5 https://bugs.python.org/issue32111 closed by serhiy.storchaka #32120: python 3.6.0 is not importing sqlite3 https://bugs.python.org/issue32120 closed by berker.peksag

1 0

Python initialization and embedded Python
by Victor Stinner 24 Nov '17

24 Nov '17

Hi, The CPython internals evolved during Python 3.7 cycle. I would like to know if we broke the C API or not. Nick Coghlan and Eric Snow are working on cleaning up the Python initialization with the "on going" PEP 432: https://www.python.org/dev/peps/pep-0432/ Many global variables used by the "Python runtime" were move to a new single "_PyRuntime" variable (big structure made of sub-structures). See Include/internal/pystate.h. A side effect of moving variables from random files into header files is that it's not more possible to fully initialize _PyRuntime at "compilation time". For example, previously, it was possible to refer to local C function (functions declared with "static", so only visible in the current file). Now a new "initialization function" is required to must be called. In short, it means that using the "Python runtime" before it's initialized by _PyRuntime_Initialize() is now likely to crash. For example, calling PyMem_RawMalloc(), before calling _PyRuntime_Initialize(), now calls the function NULL: dereference a NULL pointer, and so immediately crash with a segmentation fault. I'm writing this email to ask if this change is an issue or not to embedded Python and the Python C API. Is it still possible to call "all" functions of the C API before calling Py_Initialize()? I was bitten by the bug while reworking the Py_Main() function to split it into subfunctions and cleanup the code to handle the command line arguments and environment variables. I fixed the issue in main() by calling _PyRuntime_Initialize() as soon as possible: it's now the first instruction of main() :-) (See Programs/python.c) To give a more concrete example: Py_DecodeLocale() is the recommanded function to decode bytes from the operating system, but this function calls PyMem_RawMalloc() which does crash before _PyRuntime_Initialize() is called. Is Py_DecodeLocale() used to initialize Python? For example, "void Py_SetProgramName(wchar_t *);" expects a text string, whereas main() gives argv as bytes. Calling Py_SetProgramName() from argv requires to decode bytes... So use Py_DecodeLocale()... Should we do something in Py_DecodeLocale()? Maybe crash if _PyRuntime_Initialize() wasn't called yet? Maybe, the minimum change is to expose _PyRuntime_Initialize() in the public C API? Victor

8 29

PEP 559 - built-in noop()
by Barry Warsaw 24 Nov '17

24 Nov '17

I couldn’t resist one more PEP from the Core sprint. I won’t reveal where or how this one came to me. -Barry PEP: 559 Title: Built-in noop() Author: Barry Warsaw <barry(a)python.org> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 2017-09-08 Python-Version: 3.7 Post-History: 2017-09-09 Abstract ======== This PEP proposes adding a new built-in function called ``noop()`` which does nothing but return ``None``. Rationale ========= It is trivial to implement a no-op function in Python. It's so easy in fact that many people do it many times over and over again. It would be useful in many cases to have a common built-in function that does nothing. One use case would be for PEP 553, where you could set the breakpoint environment variable to the following in order to effectively disable it:: $ setenv PYTHONBREAKPOINT=noop Implementation ============== The Python equivalent of the ``noop()`` function is exactly:: def noop(*args, **kws): return None The C built-in implementation is available as a pull request. Rejected alternatives ===================== ``noop()`` returns something ---------------------------- YAGNI. This is rejected because it complicates the semantics. For example, if you always return both ``*args`` and ``**kws``, what do you return when none of those are given? Returning a tuple of ``((), {})`` is kind of ugly, but provides consistency. But you might also want to just return ``None`` since that's also conceptually what the function was passed. Or, what if you pass in exactly one positional argument, e.g. ``noop(7)``. Do you return ``7`` or ``((7,), {})``? And so on. The author claims that you won't ever need the return value of ``noop()`` so it will always return ``None``. Coghlin's Dialogs (edited for formatting): My counterargument to this would be ``map(noop, iterable)``, ``sorted(iterable, key=noop)``, etc. (``filter``, ``max``, and ``min`` all accept callables that accept a single argument, as do many of the itertools operations). Making ``noop()`` a useful default function in those cases just needs the definition to be:: def noop(*args, **kwds): return args[0] if args else None The counterargument to the counterargument is that using ``None`` as the default in all these cases is going to be faster, since it lets the algorithm skip the callback entirely, rather than calling it and having it do nothing useful. Copyright ========= This document has been placed in the public domain. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End:

14 22

Script bootstrapping executables on Windows
by Thomas Mansencal 22 Nov '17

22 Nov '17

Hi, This is a Windows specific question, to give a bit of context I'm working in a studio where depending on the project we use different Python interpreters installed in different locations, e.g. Python 2.7.13, Python 2.7.14, Python 3.6. We set PATH, PYTHONHOME and PYTHONPATH accordingly depending the interpreter in use. Our Python packages are living atomically on the network and are added to the environment on a per project basis by extending PYTHONPATH. This is in contrast to using a monolithic virtual environment built with virtualenv or conda. Assuming it is compatible, a Python package might be used with any of the 3 aforementioned interpreters, e.g. yapf (a code formatter). Now, on Windows, if you for example *pip install yapf*, a yapf.exe boostrapping executable and a yapf-script.py file are being generated. The boostrapping executable seems to look for the yapf-script.py file and launch it using the absolute hardcoded interpreter path of the yapf-script.py shebang. Given the above we run into issues if for example yapf was deployed using Python 2.7.13 but the Python 2.7.14 interpreter is being used in the environment instead. We get a "failed to create process." error in that case. What we would like to do is not being tied to the absolute interpreter path but have it defined with a variable or just use #!python. I have tried to search for the above error in cpython source code and the installation directory without luck. I would like to know what module/package is responsible for generating the boostrapping executables to understand how it works and see if we can doctor it for our usage. Bests, Thomas

1 1

Comments on PEP 560 (Core support for typing module and generic types)
by Mark Shannon 21 Nov '17

21 Nov '17

Hi, I am very concerned by this PEP. By far and away the largest change in PEP 560 is the change to the behaviour of object.__getitem__. This is not mentioned in the PEP at all, but is explicit in the draft implementation. The implementation could implement `type.__getitem__` instead of changing `object.__getitem__`, but that is still a major change to the language. In fact, the addition of `__mro_entries__` makes `__class_getitem__` unnecessary. The addition of `__mro_entries__` allows instances of classes that do not subclass `type` to act as classes in some circumstances. That means that any class can implement `__getitem__` to provide a generic type. For example, here is a minimal working implementation of `List`: class Generic: def __init__(self, concrete): self.concrete = concrete def __getitem__(self, index): return self.concrete def __mro_entries__(self): return self.concrete List = Generic(list) class MyList(List): pass # Works perfectly class MyIntList(List[int]): pass # Also works. The name `__mro_entries__` suggests that this method is solely related method resolution order, but it is really about providing an instance of `type` where one is expected. This is analogous to `__int__`, `__float__` and `__index__` which provide an int, float and int respectively. This rather suggests (to me at least) the name `__type__` instead of `__mro_entries__` Also, why return a tuple of classes, not just a single class? The PEP should include the justification for this decision. Should `isinstance` and `issubclass` call `__mro_entries__` before raising an error if the second argument is not a class? In other words, if `List` implements `__mro_entries__` to return `list` then should `issubclass(x, List)` act like `issubclass(x, list)`? (IMO, it shouldn't) The reasoning behind this decision should be made explicit in the PEP. Cheers, Mark.

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Comment on PEP 562 (Module __getattr__ and __dir__)
by Mark Shannon 20 Nov '17

20 Nov '17

Hi, Just one comment. Could the new behaviour of attribute lookup on a module be spelled out more explicitly please? I'm guessing it is now something like: `module.__getattribute__` is now equivalent to: def __getattribute__(mod, name): try: return object.__getattribute__(mod, name) except AttributeError: try: getter = mod.__dict__["__getattr__"] except KeyError: raise AttributeError(f"module has no attribute '{name}'") return getter(name) Cheers, Mark.

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