Mailman 3 August 2023 - Python-ideas

factory for sentinel objects
by Tim Hoffmann 31 Aug '23

31 Aug '23

The standard pattern to create a sentinel in Python is >>> Unset = object() While this is often good enough, it has some shortcomings: - repr(Unset) is unhelpful: <object at 0x1ded9911b60> - copy/deepcopy create a copy of the sentinel object, which can lead to surprising results such as: >>> d = {'val': Unset} >>> d['val'] is Unset True >>> d2 = copy.deepcopy(d) >>> d2['val'] is Unset False - The code "Unset = object()" is quite obscure for people not familiar with the sentinel pattern. I propose to provide a sentinel() factory function so that >>> Unset = sentinel("Unset") >>> repr(unset) <Unset> >>> copy.copy(Unset) is Unset True >>> copy.deepcopy(Unset) is Unset True A simple implementation would be something like def sentinel(name): return type(name, (), { '__repr__': lambda self: f"<{self.__class__.__name__}>", '__copy__': lambda self: self, '__deepcopy__': lambda self, memo: self, }) Likely, the implementation should be refined a bit more. While it's not rocket science, getting sentinels correct is cumbersome for end users. Providing such a function in the standard library is only a minor maintainance burden, but a significant help for users working with sentinels. Thanks for your consideration! Tim

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while(tt--)
by Daniil.arashkevich＠gmail.com 21 Aug '23

21 Aug '23

Currently in Python we have construction like this: tt = 5 while t: # do something tt -= 1 It would be great if in Python we have something like this: tt = 5 while (tt--): # do something It is exists in C++. And in my opinion it is very Pythonic

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len(Enum) should raise TypeError
by Richard Hajek 14 Aug '23

14 Aug '23

# Feature or enhancement `len(Enum)` has no useful meaning and should therefore raise the appropriate error # Pitch I cannot figure out any scenario, in which len(Enum) should not raise. Enum is a type and as any other types, such as `len(list)` or `len(dict)`, and should not give any meaningful result on len. Thoughts?

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else without except
by Daniel Walker 07 Aug '23

07 Aug '23

As most of you probably know, you can use else with try blocks: try: do_stuff() except SomeExceptionClass: handle_error() else: no_error_occurred() Here, no_error_occurred will only be called if do_stuff() didn't raise an exception. However, the following is invalid syntax: try: do_stuff() else: no_error_occurred() Now you might say that this isn't needed as you can achieve the same result with do_stuff() no_error_occurred() However, what if I want to use finally as well: try: do_stuff() else: no_error_occurred() finally: cleanup() and I need no_error_occurred to be called before cleanup? For my actual use case, I've done try: do_stuff() except Exception: raise else: no_error_occurred() finally: cleanup() This seems very non-Pythonic. Is there a reason why else without except has to be invalid syntax?

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Conditional 1-line expression in python
by Dom Grigonis 05 Aug '23

05 Aug '23

Hi everyone, I am not very keen on long discussions on such matter as I do not think there is much to discuss: there is no technical complexity in it and it doesn’t really change or introduce anything new. It is only a matter of opinion & style/design preferences with respect to logical order and brevity of a statement. So I thought, if anyone can be bothered on such question and instead of writing 3-minute e-mail, would take few seconds to answer 3-question poll. https://q5yitzu62.supersurvey.com <https://q5yitzu62.supersurvey.com/> Would be interesting to see if my preference is an outlier or not really. Kind regards, D. Grigonis

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