Python-ideas February 2020

python-ideas@python.org

95 participants
66 discussions

Adding a thin wrapper class around the functions in stdlib.heapq
by bunslow 17 Feb '22

17 Feb '22

Nothing so bombastic this time. The heapq functions are basically all named "heapsomething", and basically all take a "heap" for their first argument, with supplementary args coming after. It's a textbook example of the (hypothetical) Object Oriented Manifesto™ where defining a class increases type safety and programmers' conceptual clarity. There're practically no drawbacks, and the code to be added would be very simple. Updating the tests and docs would probably be harder. In pure Python, such a class would look like this: class Heap(list): def __init__(self, iterable=None): if iterable: super().__init__(iterable) else: super().__init__() self.heapify() push = heapq.heappush pop = heapq.heappop pushpop = heapq.heappushpop replace = heapq.heapreplace heapify = heapq.heapify # This could be a simple wrapper as well, but I had the following thoughts anyways, so here they are def nsmallest(self, n, key=None): # heapq.nsmallest makes a *max* heap of the first n elements, # while we know that self is already a min heap, so we can # make the max heap construction faster self[:n] = reversed(self[:n]) return heapq.nsmallest(n, self, key) # do we define nlargest on a minheap?? Wrapping around the C builtin functions (which aren't descriptors) would be a bit harder, but not much so: from functools import partial class Heap(list): def __init__(self, iterable=None): if iterable: super().__init__(iterable) else: super().__init__() self.heapify = partial(heapq.heapify, self) self.push = partial(heapq.heappush, self) ... self.heapify() Thoughts?

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Make fnmatch.filter accept a tuple of patterns
by Andre Delfino 26 Mar '21

26 Mar '21

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?

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Dataclasses, keyword args, and inheritance
by George Leslie-Waksman 11 Mar '21

11 Mar '21

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

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Add command-line option to unittest for enabling post-mortem debugging
by Dominik Vilsmeier 09 Jan '21

09 Jan '21

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.

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@classproperty, @abc.abstractclasspropery, etc.
by K. Richard Pixley 16 Dec '20

16 Dec '20

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

10 15

PYTHONLOGGING env variable
by Bar Harel 06 Jun '20

06 Jun '20

Another idea I've had that may be of use: PYTHONLOGGING environment variable. Setting PYTHONLOGGING to any log level or level name will initialize logging.basicConfig() with that appropriate level. Another option would be that -x dev or a different -x logging will initialize basic config. Will be useful mostly for debugging purposes instead of temporarily modifying the code. Kinda surprised it doesn't exist tbh. Bar Harel

9 22

SerialExecutor for concurrent.futures + Convenience constructor
by Jonathan Crall 10 Apr '20

10 Apr '20

I'd like to propose an improvement to `concurrent.futures`. The library's ThreadPoolExecutor and ProcessPoolExecutor are excellent tools, but there is currently no mechanism for configuring which type of executor you want. Also, there is no duck-typed class that behaves like an executor, but does its processing in serial. Often times a develop will want to run a task in parallel, but depending on the environment they may want to disable threading or process execution. To address this I use a utility called a `SerialExecutor` which shares an API with ThreadPoolExecutor/ProcessPoolExecutor but executes processes sequentially in the same python thread: ```python import concurrent.futures class SerialFuture( concurrent.futures.Future): """ Non-threading / multiprocessing version of future for drop in compatibility with concurrent.futures. """ def __init__(self, func, *args, **kw): super(SerialFuture, self).__init__() self.func = func self.args = args self.kw = kw # self._condition = FakeCondition() self._run_count = 0 # fake being finished to cause __get_result to be called self._state = concurrent.futures._base.FINISHED def _run(self): result = self.func(*self.args, **self.kw) self.set_result(result) self._run_count += 1 def set_result(self, result): """ Overrides the implementation to revert to pre python3.8 behavior """ with self._condition: self._result = result self._state = concurrent.futures._base.FINISHED for waiter in self._waiters: waiter.add_result(self) self._condition.notify_all() self._invoke_callbacks() def _Future__get_result(self): # overrides private __getresult method if not self._run_count: self._run() return self._result class SerialExecutor(object): """ Implements the concurrent.futures API around a single-threaded backend Example: >>> with SerialExecutor() as executor: >>> futures = [] >>> for i in range(100): >>> f = executor.submit(lambda x: x + 1, i) >>> futures.append(f) >>> for f in concurrent.futures.as_completed(futures): >>> assert f.result() > 0 >>> for i, f in enumerate(futures): >>> assert i + 1 == f.result() """ def __enter__(self): return self def __exit__(self, ex_type, ex_value, tb): pass def submit(self, func, *args, **kw): return SerialFuture(func, *args, **kw) def shutdown(self): pass ``` In order to make it easy to choose the type of parallel (or serial) backend with minimal code changes I use the following "Executor" wrapper class (although if this was integrated into concurrent.futures the name would need to change to something better): ```python class Executor(object): """ Wrapper around a specific executor. Abstracts Serial, Thread, and Process Executor via arguments. Args: mode (str, default='thread'): either thread, serial, or process max_workers (int, default=0): number of workers. If 0, serial is forced. """ def __init__(self, mode='thread', max_workers=0): from concurrent import futures if mode == 'serial' or max_workers == 0: backend = SerialExecutor() elif mode == 'thread': backend = futures.ThreadPoolExecutor(max_workers=max_workers) elif mode == 'process': backend = futures.ProcessPoolExecutor(max_workers=max_workers) else: raise KeyError(mode) self.backend = backend def __enter__(self): return self.backend.__enter__() def __exit__(self, ex_type, ex_value, tb): return self.backend.__exit__(ex_type, ex_value, tb) def submit(self, func, *args, **kw): return self.backend.submit(func, *args, **kw) def shutdown(self): return self.backend.shutdown() ``` So in summary, I'm proposing to add a SerialExecutor and SerialFuture class as an alternative to the ThreadPool / ProcessPool executors, and I'm also advocating for some sort of "ParamatrizedExecutor", where the user can construct it in "thread", "process", or "serial" model. -- -Jon

6 26

Add the imath module
by Serhiy Storchaka 22 Mar '20

22 Mar '20

What are your thoughts about adding a new imath module for integer mathematics? It could contain the following functions: * factorial(n) Is just moved from the math module, but non-integer types are rejected. Currently math.factorial() accepts also integer floats like 3.0. It looks to me, the rationale was that at the time when math.factorial() was added, all function in the math module worked with floats. But now we can revise this decision. * gcd(n, m) Is just moved from the math module. * as_integer_ration(x) Equivalents to: def as_integer_ration(x): if hasattr(x, 'as_integer_ration'): return x.as_integer_ration() else: return (x.numerator, x.denominator) * binom(n, k) Returns factorial(n) // (factorial(k) * factorial(n-k)), but uses more efficient algorithm. * sqrt(n) Returns the largest integer r such that r**2 <= n and (r+1)**2 > n. * isprime(n) Tests if n is a prime number. * primes() Returns an iterator of prime numbers: 2, 3, 5, 7, 11, 13,... Are there more ideas?

14 38

New explicit methods to trim strings
by Alex Grigoryev 21 Mar '20

21 Mar '20

Following the discussion here (https://link.getmailspring.com/link/7D84D131-65B6-4EF7-9C43-51957F9DFAA9@ge…) I propose to add 3 new string methods: str.trim, str.ltrim, str.rtrim Another option would be to change API for str.split method to work correctly with sequences. In [1]: def ltrim(s, seq): ...: return s[len(seq):] if s.startswith(seq) else s ...: In [2]: def rtrim(s, seq): ...: return s[:-len(seq)] if s.endswith(seq) else s ...: In [3]: def trim(s, seq): ...: return ltrim(rtrim(s, seq), seq) ...: In [4]: s = 'mailto:maria@gmail.com' In [5]: ltrim(s, 'mailto:') Out[5]: 'maria(a)gmail.com' In [6]: rtrim(s, 'com') Out[6]: 'mailto:maria@gmail.' In [7]: trim(s, 'm') Out[7]: 'ailto:maria@gmail.co'

28 162

SQL string prefix idea
by minecraft2048＠gmail.com 16 Mar '20

16 Mar '20

The idea is to add a new string prefix 's' for SQL string. This string doesn't do anything in Python, unlike b"" or f"" strings, but interactive Python shells like IPython or Jupyter can parse the following characters as SQL syntax instead of Python syntax and give SQL syntax highlighting and autocompletion, and if they are configured correctly, they can do column name autocompletion. Unfortunately when I try to type s"select * from table" it gave me syntax error instead, so I think this need to be implemented in Python language itself instead of module

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Python-ideas February 2020