Mailman 3 March 2015 - Python-ideas

@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

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Specify number of items to allocate for array.array() constructor
by Sven Rahmann 22 Feb '20

22 Feb '20

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 array of the same size (8 bytes per number, so it occupies about 48 GB memory). At the moment I am extending an array in chunks of several million items at a time at a time, which is slow and not elegant. The function below also initializes each item in the array to a given value (0 by default). Is there a reason why there the array.array constructor does not allow to simply specify the number of items that should be allocated? (I do not really care about the contents.) Would this be a worthwhile addition to / modification of the array module? My suggestions is to modify array generation in such a way that you could pass an iterator (as now) as second argument, but if you pass a single integer value, it should be treated as the number of items to allocate. Here is my current workaround (which is slow): def filled_array(typecode, n, value=0, bsize=(1<<22)): """returns a new array with given typecode (eg, "l" for long int, as in the array module) with n entries, initialized to the given value (default 0) """ a = array.array(typecode, [value]*bsize) x = array.array(typecode) r = n while r >= bsize: x.extend(a) r -= bsize x.extend([value]*r) return x

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Implicit string literal concatenation considered harmful?
by Guido van Rossum 14 Mar '18

14 Mar '18

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 '+' operator, so there's really no reason to support 'a' 'b' any more. (The reason was always rather flimsy; I copied it from C but the reason why it's needed there doesn't really apply to Python, as it is mostly useful inside macros.) Would it be reasonable to start deprecating this and eventually remove it from the language? -- --Guido van Rossum (python.org/~guido)

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Reprs of classes and functions
by Serhiy Storchaka 05 Aug '15

05 Aug '15

This idea is already casually mentioned, but sank deep into the threads of the discussion. Raise it up. Currently reprs of classes and functions look as: >>> int <class 'int'> >>> int.from_bytes <built-in method from_bytes of type object at 0x826cf60> >>> open <built-in function open> >>> import collections >>> collections.Counter <class 'collections.Counter'> >>> collections.Counter.fromkeys <bound method Counter.fromkeys of <class 'collections.Counter'>> >>> collections.namedtuple <function namedtuple at 0xb6fc4adc> What if change default reprs of classes and functions to just full qualified name __module__ + '.' + __qualname__ (or just __qualname__ if __module__ is builtins)? This will look more neatly. And such reprs are evaluable.

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Re: [Python-ideas] Top 10 Python modules that need a redesign Was: Geo coordinates conversion in stdlib
by Alexander Belopolsky 01 May '15

01 May '15

On Mon, Mar 23, 2015 at 2:08 AM, anatoly techtonik <techtonik(a)gmail.com> wrote: > > That's nice to know, but IIRC datetime is from the top 10 Python > modules that need a redesign. Things contained therein doesn't pass > human usability check, and are not used as a result. Where have you been when PEP 3108 was discussed? I have not seen any other list of Python modules that needed a redesign, so I cannot tell what's on your top ten list. Speaking of the datetime module, in what sense does it not "pass human usability check"? It does have a few quirks, for example I would rather see date accept a single argument in the constructor which may be a string, another date or a tuple, but I am not even sure this desire is shared by many other humans. It would be nice if datetime classes were named in CamelCase according to PEP 8 conventions, but again this is a very minor quirk. In my view, if anyone is to blame for the "human usability" of the datetime module, it would be Pope Gregory XIII, Benjamin Franklin and scores of unnamed astronomers who made modern timekeeping such a mess.

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Simpler Customization of Class Creation, next round
by Martin Teichmann 09 Apr '15

09 Apr '15

Hello everybody, recently I posted PEP 487, a simpler customization of class creation. For newcomers: I propose the introduction of a __init_subclass__ classmethod which initializes subclasses of a class, simplifying what metaclasses can already do. It took me a while to digest all the ideas from the list here, but well, we're not in a hurry. So, I updated PEP 487, and pushed the changes to github at https://github.com/tecki/peps/commits/master I applied the following changes: PEP 487 contained the possibility to set a namespace for subclasses. The most important usecase for this feature would be to have an OrderedDict as the class definition namespace. As Eric Snow pointed out, that will be soon standard anyways, so I took out this feature from the PEP. The implementation on PyPI now just uses an OrderedDict as a namespace, anticipating the expected changes to CPython. I also did some reading on possible usecases for PEP 487, so that it actually may be used by someone. Some Traits-like thing is a standard usecase, so I looked especially at IPython's traitlets, which are a simple example of that usecase. Currently traitlets use both __new__ and __init__ of a metaclass. So I tried to also introduce a __new_subclass__ the same way I introduced __init_subclass__. This turned out much harder than I thought, actually impossible, because it is type.__new__ that sets the method resolution order, so making super() work in a __new_subclass__ hook is a chicken-egg problem: we need the MRO to find the next base class to call, but the most basic base class is the one creating the MRO. Nick, how did you solve that problem in PEP 422? Anyhow, I think that traitlets can also be written just using __init_subclass__. There is just this weird hint in the docs that you should use __new__ for metaclasses, not __init__, a hint I never understood as the reasons when to use __new__ or __init__ are precisely the same for normal classes and metaclasses. So I think we don't miss out much when not having __new_subclass__. I also updated the implementation of PEP 487, it's still at https://pypi.python.org/pypi/metaclass Greetings Martin

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Adding rule about trailing comma.
by INADA Naoki 02 Apr '15

02 Apr '15

What about adding rule about trailing comma? Yes: foo(aaa, bbb) or foo(aaa, bbb, ) No: foo(aaa, bbb,) # except one-value tuple. or foo(aaa, bbb ) -- INADA Naoki <songofacandy(a)gmail.com>

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History on proposals for Macros?
by Matthew Rocklin 02 Apr '15

02 Apr '15

I want Python to have macros. This is obviously a hard sell. I'm willing to do some legwork to demonstrate value. What would a good proposal include? Are there good examples of failed proposals on this topic? Is the core team open to this topic? Thank you for your time, - Mathew Rocklin

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A mutable alternative to namedtuple
by Luciano Ramalho 01 Apr '15

01 Apr '15

Sometimes we need a simple class to hold some mutable attributes, provide a nice repr, support == for testing, and support iterable unpacking, so you can write: >>> p = Point(3, 4) >>> x, y = p That's very much like the classes built by namedtuple, but mutable. I propose we add to the collections module another class factory. I am calling it plainclass, but perhaps we can think of a better name. Here is how it would be used: >>> import collections >>> Point = collections.plainclass('Point', 'x y') The signature of the plainclass function would be exactly the same as namedtuple, supporting the same alternative ways of naming the attributes. The semantics of the generated Point class would be like this code: https://gist.github.com/ramalho/fd3d367e9d3b2a659faf What do you think? Cheers, Luciano PS. I am aware that there are "Namespace" classes in the standard library (e.g. [2]). They solve a different problem. [2] https://docs.python.org/3/library/argparse.html#argparse.Namespace -- Luciano Ramalho | Author of Fluent Python (O'Reilly, 2015) | http://shop.oreilly.com/product/0636920032519.do | Professor em: http://python.pro.br | Twitter: @ramalhoorg

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infinite dates: psycopg2 would support them
by Thomas Güttler 31 Mar '15

31 Mar '15

Hi, any progress on infinite datetimes? I just got the commitment from psycopg2 (lib to connect to PostgreSQL) developers, that they will support them, if python would: https://github.com/psycopg/psycopg2/issues/283#issuecomment-88016783 I can't help you with code in this context. I have never started a crowd funding campaign. Do you think it would be successful? Regards, Thomas Güttler -- http://www.thomas-guettler.de/

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Python-ideas March 2015