Python-ideas July 2020

python-ideas@python.org

93 participants
61 discussions

Make fnmatch.filter accept a tuple of patterns

by Andre Delfino

Frequently, while globbing, one needs to work with multiple extensions. I’d like to propose for fnmatch.filter to handle a tuple of patterns (while preserving the single str argument functionality, alas str.endswith), as a first step for glob.i?glob to accept multiple patterns as well. Here is the implementation I came up with: https://github.com/python/cpython/compare/master...andresdelfino:fnmatch-mu… If this is deemed reasonable, I’ll write tests and documentation updates. Any opinion?

2 weeks, 5 days

Dataclasses, keyword args, and inheritance

by George Leslie-Waksman

The proposed implementation of dataclasses prevents defining fields with defaults before fields without defaults. This can create limitations on logical grouping of fields and on inheritance. Take, for example, the case: @dataclass class Foo: some_default: dict = field(default_factory=dict) @dataclass class Bar(Foo): other_field: int this results in the error: 5 @dataclass ----> 6 class Bar(Foo): 7 other_field: int 8 ~/.pyenv/versions/3.6.2/envs/clover_pipeline/lib/python3.6/site-packages/dataclasses.py in dataclass(_cls, init, repr, eq, order, hash, frozen) 751 752 # We're called as @dataclass, with a class. --> 753 return wrap(_cls) 754 755 ~/.pyenv/versions/3.6.2/envs/clover_pipeline/lib/python3.6/site-packages/dataclasses.py in wrap(cls) 743 744 def wrap(cls): --> 745 return _process_class(cls, repr, eq, order, hash, init, frozen) 746 747 # See if we're being called as @dataclass or @dataclass(). ~/.pyenv/versions/3.6.2/envs/clover_pipeline/lib/python3.6/site-packages/dataclasses.py in _process_class(cls, repr, eq, order, hash, init, frozen) 675 # in __init__. Use "self" if possible. 676 '__dataclass_self__' if 'self' in fields --> 677 else 'self', 678 )) 679 if repr: ~/.pyenv/versions/3.6.2/envs/clover_pipeline/lib/python3.6/site-packages/dataclasses.py in _init_fn(fields, frozen, has_post_init, self_name) 422 seen_default = True 423 elif seen_default: --> 424 raise TypeError(f'non-default argument {f.name!r} ' 425 'follows default argument') 426 TypeError: non-default argument 'other_field' follows default argument I understand that this is a limitation of positional arguments because the effective __init__ signature is: def __init__(self, some_default: dict = <something>, other_field: int): However, keyword only arguments allow an entirely reasonable solution to this problem: def __init__(self, *, some_default: dict = <something>, other_field: int): And have the added benefit of making the fields in the __init__ call entirely explicit. So, I propose the addition of a keyword_only flag to the @dataclass decorator that renders the __init__ method using keyword only arguments: @dataclass(keyword_only=True) class Bar(Foo): other_field: int --George Leslie-Waksman

1 month

Add extend_const action to argparse

by python＠roganartu.com

Forgive me if this idea has been discussed before, I searched the mailing lists, the CPython repo, and the issue tracker and was unable to find anything. I have found myself a few times in a position where I have a repeated argument that uses the `append` action, along with some convenience arguments that append a specific const to that same dest (eg: `--filter-x` being made equivalent to `--filter x` via `append_const`). This is particularly useful in cli apps that expose some kind of powerful-but-verbose filtering capability, while also providing shorter aliases for common invocations. I'm sure there are other use cases, but this is the one I'm most familiar with. The natural extension to this filtering idea are convenience args that set two const values (eg: `--filter x --filter y` being equivalent to `--filter-x-y`), but there is no `extend_const` action to enable this. While this is possible (and rather straight forward) to add via a custom action, I feel like this should be a built-in action instead. `append` has `append_const`, it seems intuitive and reasonable to expect `extend` to have `extend_const` too (my anecdotal experience the first time I came across this need was that I simply tried using `extend_const` without checking the docs, assuming it already existed). Please see this gist for a working example that may help explain the idea and intended use case more clearly: https://gist.github.com/roganartu/7c2ec129d868ecda95acfbd655ef0ab2

2 months

Re: [Python-Dev] reversed enumerate

by Steven D'Aprano

Hi Ilya, I'm not sure that this mailing list (Python-Dev) is the right place for this discussion, I think that Python-Ideas (CCed) is the correct place. For the benefit of Python-Ideas, I have left your entire post below, to establish context. [Ilya] > I needed reversed(enumerate(x: list)) in my code, and have discovered > that it wound't work. This is disappointing because operation is well > defined. It isn't really well-defined, since enumerate can operate on infinite iterators, and you cannot reverse an infinite stream. Consider: def values(): while True: yield random.random() a, b = reversed(enumerate(values()) What should the first pair of (a, b) be? However, having said that, I think that your idea is not unreasonable. `enumerate(it)` in the most general case isn't reversable, but if `it` is reversable and sized, there's no reason why `enumerate(it)` shouldn't be too. My personal opinion is that this is a fairly obvious and straightforward enhancement, one which (hopefully!) shouldn't require much, if any, debate. I don't think we need a new class for this, I think enhancing enumerate to be reversable if its underlying iterator is reversable makes good sense. But if you can show some concrete use-cases, especially one or two from the standard library, that would help your case. Or some other languages which offer this functionality as standard. On the other hand, I think that there is a fairly lightweight work around. Define a helper function: def countdown(n): while True: yield n n -= 1 then call it like this: # reversed(enumerate(seq)) zip(countdown(len(seq)-1), reversed(seq))) So it isn't terribly hard to work around this. But I agree that it would be nice if enumerate encapsulated this for the caller. One potentially serious question: what should `enumerate.__reversed__` do when given a starting value? reversed(enumerate('abc', 1)) Should that yield...? # treat the start value as a start value (1, 'c'), (0, 'b'), (-1, 'a') # treat the start value as an end value (3, 'c'), (2, 'b'), (1, 'a') Something else? My preference would be to treat the starting value as an ending value. Steven On Wed, Apr 01, 2020 at 08:45:34PM +0200, Ilya Kamenshchikov wrote: > Hi, > > I needed reversed(enumerate(x: list)) in my code, and have discovered that > it wound't work. This is disappointing because operation is well defined. > It is also well defined for str type, range, and - in principle, but not > yet in practice - on dictionary iterators - keys(), values(), items() as > dictionaries are ordered now. > It would also be well defined on any user type implementing __iter__, > __len__, __reversed__ - think numpy arrays, some pandas dataframes, tensors. > > That's plenty of usecases, therefore I guess it would be quite useful to > avoid hacky / inefficient solutions like described here: > https://code.activestate.com/lists/python-list/706205/. > > If deemed useful, I would be interested in implementing this, maybe > together with __reversed__ on dict keys, values, items. > > Best Regards, > -- > Ilya Kamen > > ----------- > p.s. > > *Sketch* of what I am proposing: > > class reversible_enumerate: > > def __init__(self, iterable): > self.iterable = iterable > self.ctr = 0 > > def __iter__(self): > for e in self.iterable: > yield self.ctr, e > self.ctr += 1 > > def __reversed__(self): > try: > ri = reversed(self.iterable) > except Exception as e: > raise Exception( > "enumerate can only be reversed if iterable to > enumerate can be reversed and has defined length." > ) from e > > try: > l = len(self.iterable) > except Exception as e: > raise Exception( > "enumerate can only be reversed if iterable to > enumerate can be reversed and has defined length." > ) from e > > indexes = range(l-1, -1, -1) > for i, e in zip(indexes, ri): > yield i, e > > for i, c in reversed(reversible_enumerate("Hello World")): > print(i, c) > > for i, c in reversed(reversible_enumerate([11, 22, 33])): > > print(i, c) > _______________________________________________ > Python-Dev mailing list -- python-dev(a)python.org > To unsubscribe send an email to python-dev-leave(a)python.org > https://mail.python.org/mailman3/lists/python-dev.python.org/ > Message archived at https://mail.python.org/archives/list/python-dev@python.org/message/NDDKDUD… > Code of Conduct: http://python.org/psf/codeofconduct/

2 months, 3 weeks

Add command-line option to unittest for enabling post-mortem debugging

by Dominik Vilsmeier

Consider the following example: import unittest def foo(): for x in [1, 2, 'oops', 4]: print(x + 100) class TestFoo(unittest.TestCase): def test_foo(self): self.assertIs(foo(), None) if __name__ == '__main__': unittest.main() If we were calling `foo` directly we could enter post-mortem debugging via `python -m pdb test.py`. However since `foo` is wrapped in a test case, `unittest` eats the exception and thus prevents post-mortem debugging. `--failfast` doesn't help, the exception is still swallowed. Since I am not aware of a solution that enables post-mortem debugging in such a case (without modifying the test scripts, please correct me if one exists), I propose adding a command-line option to `unittest` for [running test cases in debug mode](https://docs.python.org/3/library/unittest.html#unittest.TestCase.deb… so that post-mortem debugging can be used. P.S.: There is also [this SO question](https://stackoverflow.com/q/4398967/3767239) on a similar topic.

3 months

Python JIT Compilation Thoughts

by redradist＠gmail.com

Hi all, I do not know maybe it was already discussed ... but the toolchain like LLVM is very mature and it can provide the simpler JIT compilation to machine code functionality and it will improve performance of the Python a lot !!

3 months, 1 week

@classproperty, @abc.abstractclasspropery, etc.

by K. Richard Pixley

There's a whole matrix of these and I'm wondering why the matrix is currently sparse rather than implementing them all. Or rather, why we can't stack them as: class foo(object): @classmethod @property def bar(cls, ...): ... Essentially the permutation are, I think: {'unadorned'|abc.abstract}{'normal'|static|class}{method|property|non-callable attribute}. concreteness implicit first arg type name comments {unadorned} {unadorned} method def foo(): exists now {unadorned} {unadorned} property @property exists now {unadorned} {unadorned} non-callable attribute x = 2 exists now {unadorned} static method @staticmethod exists now {unadorned} static property @staticproperty proposing {unadorned} static non-callable attribute {degenerate case - variables don't have arguments} unnecessary {unadorned} class method @classmethod exists now {unadorned} class property @classproperty or @classmethod;@property proposing {unadorned} class non-callable attribute {degenerate case - variables don't have arguments} unnecessary abc.abstract {unadorned} method @abc.abstractmethod exists now abc.abstract {unadorned} property @abc.abstractproperty exists now abc.abstract {unadorned} non-callable attribute @abc.abstractattribute or @abc.abstract;@attribute proposing abc.abstract static method @abc.abstractstaticmethod exists now abc.abstract static property @abc.staticproperty proposing abc.abstract static non-callable attribute {degenerate case - variables don't have arguments} unnecessary abc.abstract class method @abc.abstractclassmethod exists now abc.abstract class property @abc.abstractclassproperty proposing abc.abstract class non-callable attribute {degenerate case - variables don't have arguments} unnecessary I think the meanings of the new ones are pretty straightforward, but in case they are not... @staticproperty - like @property only without an implicit first argument. Allows the property to be called directly from the class without requiring a throw-away instance. @classproperty - like @property, only the implicit first argument to the method is the class. Allows the property to be called directly from the class without requiring a throw-away instance. @abc.abstractattribute - a simple, non-callable variable that must be overridden in subclasses @abc.abstractstaticproperty - like @abc.abstractproperty only for @staticproperty @abc.abstractclassproperty - like @abc.abstractproperty only for @classproperty --rich

3 months, 4 weeks

Bringing the print statement back

by Guido van Rossum

In Python 3.10 we will no longer be burdened by the old parser (though 3rd party tooling needs to catch up). One thing that the PEG parser makes possible in about 20 lines of code is something not entirely different from the old print statement. I have a prototype: Python 3.10.0a0 (heads/print-statement-dirty:5ed19fcc1a, Jun 9 2020, 16:31:17) [Clang 11.0.0 (clang-1100.0.33.8)] on darwin Type "help", "copyright", "credits" or "license" for more information. Cannot read termcap database; using dumb terminal settings. >>> print 2+2 4 >>> print "hello world" hello world >>> print "hello", input("Name:") Name:Guido hello Guido >>> print 1, 2, 3, sep=", " 1, 2, 3 >>> But wait, there's more! The same syntax will make it possible to call *any* function: >>> len "abc" 3 >>> Or any method: >>> import sys >>> sys.getrefcount "abc" 24 >>> Really, *any* method: >>> class C: ... def foo(self, arg): print arg ... >>> C().foo 2+2 4 >>> There are downsides too, though. For example, you can't call a method without arguments: >>> print <built-in function print> >>> Worse, the first argument cannot start with a parenthesis or bracket: >>> print (1, 2, 3) 1 2 3 >>> C().foo (1, 2, 3) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: C.foo() takes 2 positional arguments but 4 were given >>> print (2+2), 42 4 (None, 42) >>> C().foo [0] Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'method' object is not subscriptable >>> No, it's not April 1st. I am seriously proposing this (but I'll withdraw it if the response is a resounding "boo, hiss"). After all, we currently have a bunch of complexity in the parser just to give a helpful error message to people used to Python 2's print statement: >>> print 1, 2, 3 File "<stdin>", line 1 print 1, 2, 3 ^ SyntaxError: Missing parentheses in call to 'print'. Did you mean print(1, 2, 3)? >>> And IIRC there have been a number of aborted attempts at syntactic hacks to allow people to call functions (like print) without parentheses, although (I think) none of them made it into a PEP. The PEG parser makes this much simpler, because it can simply backtrack -- by placing the grammar rule for this syntax (tentatively called "call statement") last in the list of alternatives for "small statement" we ensure that everything that's a valid expression statement (including print() calls) is still an expression statement with exactly the same meaning, while still allowing parameter-less function calls, without lexical hacks. (There is no code in my prototype that checks for a space after 'print' -- it just checks that there's a name, number or string following a name, which is never legal syntax.) One possible extension I didn't pursue (yet -- dare me!) is to allow parameter-less calls inside other expressions. For example, my prototype does not support things like this: >>> a = (len "abc") File "<stdin>", line 1 a = (len "abc") ^ SyntaxError: invalid syntax >>> I think that strikes a reasonable balance between usability and reduced detection of common errors. I could also dial it back a bit, e.g. maybe it's too much to allow 'C().foo x' and we should only allow dotted names (sufficient to access functions in imported modules and method calls on variables). Or maybe we should only allow simple names (allowing 'len x' but disallowing 'sys.getrefcount x'. Or maybe we should really only bring back print statements. I believe there are some other languages that support a similar grammar (Ruby? R? Raku?) but I haven't investigated. Thoughts? -- --Guido van Rossum (python.org/~guido) *Pronouns: he/him **(why is my pronoun here?)* <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-…>

5 months, 2 weeks

Fwd: Re: Experimenting with dict performance, and an immutable dict

by Marco Sulla

On Wed, 22 Jul 2020 at 15:35, Antoine Pitrou <solipsis(a)pitrou.net> wrote: > The deltas also look small for a micro-benchmark. I certainly don't > think this is a sufficient reason to add a new datatype to Python. > I think some of the optimizations can be experimented with dict itself. Deltas seem to be small, because I divide them by the number of timeit runs. I think that a 30-45% gain in constructor speed is not something small. The benchmarks in question have the Name that starts with "constructor". PS: sorry for double mail, I sent this message to only you by mistake. On Wed, 22 Jul 2020 at 18:26, Guido van Rossum <guido(a)python.org> wrote: > Did you study PEP 416 (frozendict) and PEP 603 (frozenmap)? > Yes. About frozenmap, at the very start I added to the benchmarks also immutables.Map, but I removed it when I realized that it was slower than frozendict in every bench. Maybe this is because immutables.Map is a C extension and not a builtin type. About PEP 416, yes, I tried to follow it. Indeed hash is calculated using the strategy described in the PEP. I also take a look to frozenset and tuple code. Frankly I must admit that the rejection of PEP 416 was a good idea. Indeed I started to implement an immutable dict only for a symmetry reason, and because I am somewhat fascinated by functional programming and immutables, without a rational reason. I suppose I would have given up and admitted that a frozendict is quite useless in Python, if I did not see that the constructor speed can be faster. I'm not sure 100% of the result. This is why I'm suggesting to try to implement the speed optimizations to the constructor of dict first: 1. the optimizations could be not safe. Indeed recently I got a segfault that I have to investigate 2. maybe dict can be really optimized further. After that, the difference between dict and frozendict performance could be minimal That said, maybe there are four good use cases for a frozendict: 1. they could be used for __annotations__ of function objects, and similar cases 2. they could be used to implement "immutable" modules or classes 3. frozendict can be easily cached, like tuples. 4. as I said, as an alternative to MappingProxyType, since it's very slow and it's used also in the C code of CPython and in the Python stdlib. This maybe is not useful because: a. the speed of MappingProxyType can be improved b. MappingProxyType can't be replaced because you *really* want and need a proxy

6 months

Access (ordered) dict by index; insert slice

by Hans Ginzel

Date: Fri, 26 Jun 2020 18:47:44 +0200 From: Hans Ginzel <hans(a)matfyz.cz> To: Hans Ginzel <hans(a)artax.karlin.mff.cuni.cz> Subject: Access (ordered) dict by index; insert slice Hello, thank you for making dict ordered. Is it planned to access key,value pair(s) by index? See https://stackoverflow.com/a/44687752/2556118 for example. Both for reading and (re)writing? Is it planned to insert pair(s) on exact index? Or generally to slice? See splice() in Perl, https://perldoc.perl.org/functions/splice.html. Use case: Represent database table metadata (columns). It is useful as to access columns both by name and by index as to insert column on specific position, https://dev.mysql.com/doc/refman/8.0/en/alter-table.html, “ALTER TABLE ADD COLUMN [FIRST |AFTER col]” (consider default order or table storage size optimisation by aligning). Thank you in advance, Hans PS1: Named tuples cannot be used, are immutable. PS2: See https://metacpan.org/pod/perlref#Pseudo-hashes:-Using-an-array-as-a-hash

7 months

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Python-ideas July 2020