Mailman 3 December 2014 - Python-ideas

@classproperty, @abc.abstractclasspropery, etc.
by K. Richard Pixley Dec. 16, 2020

Dec. 16, 2020

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 … [View More]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 [View Less]

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Specify number of items to allocate for array.array() constructor
by Sven Rahmann Feb. 22, 2020

Feb. 22, 2020

At the moment, the array module of the standard library allows to create arrays of different numeric types and to initialize them from an iterable (eg, another array). What's missing is the possiblity to specify the final size of the array (number of items), especially for large arrays. I'm thinking of suffix arrays (a text indexing data structure) for large texts, eg the human genome and its reverse complement (about 6 billion characters from the alphabet ACGT). The suffix array is a long int … [View More]

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Implicit string literal concatenation considered harmful?
by Guido van Rossum March 14, 2018

March 14, 2018

I just spent a few minutes staring at a bug caused by a missing comma -- I got a mysterious argument count error because instead of foo('a', 'b') I had written foo('a' 'b'). This is a fairly common mistake, and IIRC at Google we even had a lint rule against this (there was also a Python dialect used for some specific purpose where this was explicitly forbidden). Now, with modern compiler technology, we can (and in fact do) evaluate compile-time string literal concatenation with the '+' … [View More]

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pep 397
by Liam Marsh Feb. 23, 2015

Feb. 23, 2015

hello, *the pep 397 says that any python script is able to choose the language version which will run it, between all the versions installed on the computer, using on windows a launcher in the "C:\windows" folder.* can the idle version be chosen like this too, or can the idle "run" command do it? (except it is already like this and I have a problem) thank you, and have a nice day/evening! (and sorry if this sounds irritating)

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python on mobile
by Fetchinson . Jan. 28, 2015

Jan. 28, 2015

Hi all, maybe the best list would be python-dev but I don't dare waking the sleeping lions there :) So my question is this: is there a coherent mobile strategy among core dev people? I mean you guys release python for linux/macos/windows and the question is if there are any plans to do the same for a mobile platform. It doesn't have to be android or ios just anything that the core dev team chooses and sticks with. I've been developing python apps for smartphones (mostly hobby projects though) … [View More]

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Extending traceback to (optionally) format and show locals.
by Robert Collins Jan. 18, 2015

Jan. 18, 2015

For context please see http://bugs.python.org/issue22937 and http://bugs.python.org/issue22936. I have two questions I'm hoping to get answered through this thread: - does the change in question need a PEP? Antoine seemed to think it didn't, as long as it was opt-in (via the unittest CLI etc) - Is my implementation approach sound (for traceback, unittest I think I have covered :))? Implementation wise, I think its useful to work in the current traceback module layout - that is to alter … [View More]

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Adding `pathlib.Path` method that would send file to recycle bin
by Ram Rachum Jan. 7, 2015

Jan. 7, 2015

Hi guys, I want to have a method of `pathlib.Path` that would send a file to the recycle bin. (i.e. soft delete.) What do you think about adding this? I see there's a PyPI package `Send2Trash` that does this, but it would be nice if this was in the standard library. Thanks, Ram.

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Re: [Python-ideas] weakref.WeakKeyDictionary is (basically) useless
by Kevin Norris Dec. 31, 2014

Dec. 31, 2014

On Wed, Dec 31, 2014 at 6:00 AM, Antoine Pitrou <solipsis(a)pitrou.net> wrote: >> Why would you use it? The official documentation mentions >> implementing a [descriptor][1]. See [this paste][2] for a simple >> example of this sort of thing. > > How about something like this (untested): > A cursory glance suggests that might work for my use-case, but I would want to examine it more thoroughly for race conditions and other hazards. In particular, I'd be … [View More]

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Python hook just before NameError
by Chris Angelico Dec. 31, 2014

Dec. 31, 2014

I mainly did up this patch to see how hard it would be, and now it's turned out to be fairly simple, I'm curious as to whether it would actually be useful to people. At the point where a global/builtin name lookup is about to raise NameError, first try calling a Python function, along the same lines as __getattr__. If that function hasn't been defined, raise NameError as normal; but if it has, let it either raise NameError or return some object, which is then used as if the name had been bound … [View More]

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weakref.WeakKeyDictionary is (basically) useless
by Kevin Norris Dec. 31, 2014

Dec. 31, 2014

Let's talk about weakref.WeakKeyDictionary. First, what is it? It's a dictionary whose keys are referenced weakly. That is, the dictionary takes weak references to its keys. If a key is garbage collected, it magically vanishes from the dictionary. This saves programmers much of the trouble of manually culling dead weak references from data structures. Why would you use it? The official documentation mentions implementing a [descriptor][1]. See [this paste][2] for a simple example of this … [View More]sort of thing. Turning the WeakKeyDictionary into a regular dictionary would be troublesome. Since the descriptor object will (probably) never die, if it used a normal dictionary, it would keep all/most/some/more-than-one instances of Bar() alive forever. While this simple example would be better implemented as a property, or even a bare attribute, it is nice if you have many separate Bar classes and complicated per-property processing. Why is it broken? To go back to our simple example, imagine someone subclasses Bar (let's call the subclass Baz) and implements __eq__() and/or __hash__(). Instances of Baz may not work at all (if __hash__ is None) or they may "work" but behave nonsensically (all values-equal instances of Baz will have the same .foo attribute - imagine trying to track that bug down!). While we could forbid such a subclass in our API docs, this runs afoul of the open/closed principle. What can we do about it? It's possible someone is relying on this behavior, so we probably should not change the existing class. Instead, we can offer a new class called, say, WeakKeyIDDictionary. Said class would use object identity instead of object equality/hashing to uniquify keys. It would also reference keys weakly, just like WeakKeyDictionary does now. Implementing such a class is somewhat tricky, since weak references are difficult to reason about correctly. I've written up a [simplistic prototype][3], but I'm concerned about both performance and correctness. Specifically, I know that I need to call _remove_dead_refs() periodically, but I don't see where I can do that without compromising performance (e.g. making key lookup O(n) is a Bad Idea). I also would note it looks nothing like the stdlib implementations of WeakKey/ValueDictionary, which finagle raw weakrefs directly. These problems are related, of course, since the stdlib uses finalizer callbacks to cull dead references as they die. Frankly, the stdlib's code looks incredibly hairy to me and I doubt I could correctly re-implement it. Finally, I've no idea if it's subject to race conditions or other issues (it is a completely untested example I threw together in 5 minutes). Because this class is simultaneously useful and hard to write correctly, it would be a nice addition to the standard library (probably not in the form I've written it!), or failing that, some other library. I've Googled for such a library and come up empty. Should I submit a bug, a PEP, or is this whole idea just stupid? [1]: https://docs.python.org/3/reference/datamodel.html#implementing-descriptors [2]: http://pastebin.com/90j2ZAqC [3]: http://pastebin.com/cpZh2bTa -- Kevin Norris [View Less]

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