Mailman 3 June 2014 - 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 21 Feb '20

21 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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Re: [Python-ideas] Empty set, Empty dict
by Wichert Akkerman 01 Jul '14

01 Jul '14

Victor Stinner wrote: 2014-06-10 8:15 GMT+02:00 Neil Girdhar <mistersheik at gmail.com>: > > I've seen this proposed before, and I personally would love this, but my > > guess is that it breaks too much code for too little gain. > > > > On Wednesday, May 21, 2014 12:33:30 PM UTC-4, Frédéric Legembre wrote: > >> > >> > >> Now | Future | > >> ---------------------------------------------------- > >> () | () | empty tuple ( 1, 2, 3 ) > >> [] | [] | empty list [ 1, 2, 3 ] > >> set() | {} | empty set { 1, 2, 3 } > >> {} | {:} | empty dict { 1:a, 2:b, 3:c } > > > Your guess is right. It will break all Python 2 and Python 3 in the world. > > Technically, set((1, 2)) is different than {1, 2}: the first creates a > tuple and loads the global name "set" (which can be replaced at > runtime!), whereas the later uses bytecode and only store values > (numbers 1 and 2). > > It would be nice to have a syntax for empty set, but {} is a no-no. Perhaps {,} would be a possible spelling. For consistency you might want to allow (,) to create an empty tuple as well; personally I would find that more intuitive that (()). Wichert.

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Special keyword denoting an infinite loop
by Thomas Allen 30 Jun '14

30 Jun '14

Rust language defines a special way to make an infinite loop ( http://doc.rust-lang.org/tutorial.html#loops). I propose adding the same keyword to Python. It will be very useful for WSGI servers and will suit as a more convenient replacement for recursion (hence Python doesn't do TRE). I personally find it much prettier than *while True* or *while 1*. It won't cause any problems with existing programs, because *loop* is very rarely used as a variable name. For instance while True: > do_something() > do_something_else() would turn to loop: > do_something() > do_something_else()

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Re: [Python-ideas] Special keyword denoting an infinite loop
by Nick Coghlan 29 Jun '14

29 Jun '14

On 29 June 2014 00:56, Benny Khoo <benny_khoo_99(a)yahoo.com> wrote: > rather than a special keyword in Python, how about having Python to support > the concept of passing block (a group of statements) as argument? I thought > that can be quite elegant solution. So a loop statement can be interpreted > simply as a function that accept a block e.g. loop [block]? > > Supporting block has a lot of practical applications. I remember seeing some > special purpose flow control functions as early as Tcl. We also see it in > Ruby and the more recently the new Swift language. This is a well worn path, and it's difficult to retrofit to an existing language. Ruby, at least, relies heavily on a convention of taking blocks as the last argument to a function to make things work, which is a poor fit to Python's keyword arguments and far more varied positional signatures for higher order functions. PEP 403 and PEP 3150 are a couple of different explorations of the idea a more block-like feature. http://python-notes.curiousefficiency.org/en/latest/pep_ideas/suite_expr.ht… is one that goes even further to consider a delineated subsyntax for Python that would allow entire suites as expressions. However, the stumbling block all these proposals tend to hit is that proponents really, really, struggle to come up with compelling use cases where "just define a named function" isn't a clearer and easier to understand answer. Cheers, Nick. -- Nick Coghlan | ncoghlan(a)gmail.com | Brisbane, Australia

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problems with import
by Neal Becker 28 Jun '14

28 Jun '14

One problem I often encounter is with import search. An example of the problem is with the package mercurial. It has extensions in a subdirectory called 'hgext'. On fedora, I install mercurial using the vendor package. This creates /usr/lib64/python2.7/site-packages/hgext/ Later, I want to try out an extension as a non-privileged user. cd <extension> python setup.py install --user Now I also have ~/.local/lib/python2.7/site-packages/hgext but python won't search there for extensions. Once if finds the system hgext directory, it won't look also in the local one. Any thoughts?

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Accepting keyword arguments for __getitem__
by Stefano Borini 23 Jun '14

23 Jun '14

Dear all, At work we use a notation like LDA[Z=5] to define a specific level of accuracy for our evaluation. This notation is used just for textual labels, but it would be nice if it actually worked at the scripting level, which led me to think to the following: at the moment, we have the following >>> class A: ... def __getitem__(self, y): ... print(y) ... >>> a=A() >>> a[2] 2 >>> a[2,3] (2, 3) >>> a[1:3] slice(1, 3, None) >>> a[1:3, 4] (slice(1, 3, None), 4) >>> I would propose to add the possibility for a[Z=3], where y would then be a dictionary {"Z": 3}. In the case of a[1:3, 4, Z=3, R=5], the value of y would be a tuple containing (slice(1,3,None), 4, {"Z": 3}, {"R": 5}). This allows to preserve the ordering as specified (e.g. a[Z=3, R=4] vs a[R=4, Z=3]). Do you think it would be a good/useful idea? Was this already discussed or proposed in a PEP? Google did not help on this regard. Thank you, Stefano Borini

10 15

as_completed
by Ram Rachum 23 Jun '14

23 Jun '14

What do you think about an argument as_completed=False to Executor.map ? Personally I'd find it really handy.

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Another pathlib suggestion
by Antony Lee 22 Jun '14

22 Jun '14

Handling of Paths with multiple extensions is currently not so easy with pathlib. Specifically, I don't think there is an easy way to go from "foo.tar.gz" to "foo.ext", because Path.with_suffix only replaces the last suffix. I would therefore like to suggest either 1/ add Path.replace_suffix, such that Path("foo.tar.gz").replace_suffix(".tar.gz", ".ext") == Path("foo.ext") (this would also provide extension-checking capabilities, raising ValueError if the first argument is not a valid suffix of the initial path); or 2/ add a second argument to Path.with_suffix, "n_to_strip" (although perhaps with a better name), defaulting to 1, such that Path("foo.tar.gz").with_suffix(".ext", 0) == Path("foo.tar.gz.ext") Path("foo.tar.gz").with_suffix(".ext", 1) == Path("foo.tar.ext") Path("foo.tar.gz").with_suffix(".ext", 2) == Path("foo.ext") # set n_to_strip to len(path.suffixes) for stripping all of them. Path("foo.tar.gz").with_suffix(".ext", 3) raises a ValueError. Best, Antony

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