Mailman 3 September 2014 - Python-ideas

@classproperty, @abc.abstractclasspropery, etc.
by K. Richard Pixley 17 Dec '20

17 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 15 Mar '18

15 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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including psutil in the standard library?
by Stefano Borini 12 Nov '14

12 Nov '14

I am wondering if it would be possible to include psutil (https://pypi.python.org/pypi/psutil ) in the standard library, and if not, what would be needed. I am not a developer of it, but I am using psutil at work with good success. it provides a good deal of services for querying and managing processes in a cross platform way. Any thoughts?

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Negative limit for traceback.*
by Thomas Allen 12 Oct '14

12 Oct '14

I'd like to propose adding the ability to extract last n stack trace entries from a traceback using functions from traceback module. Simply put, passing limit=-n would make functions to return (or print or yield) last abs(n) entries. No-brainer implementation for extract_tb: extracted = list(_extract_tb_iter(tb, limit=(limit, None)[limit < 0])) if limit is not None and limit < 0: return extracted[limit:] return extracted The motivation: http://stackoverflow.com/q/25472412/2301450

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Better integration of multiprocessing with asyncio
by Dan O'Reilly 09 Oct '14

09 Oct '14

I think it would be helpful for folks using the asyncio module to be able to make non-blocking calls to objects in the multiprocessing module more easily. While some use-cases for using multiprocessing can be replaced with ProcessPoolExecutor/run_in_executor, there are others that cannot; more advanced usages of multiprocessing.Pool aren't supported by ProcessPoolExecutor (initializer/initargs, contexts, etc.), and other multiprocessing classes like Lock and Queue have blocking methods that could be made into coroutines. Consider this (extremely contrived, but use your imagination) example of a asyncio-friendly Queue: import asyncio import time def do_proc_work(q, val, val2): time.sleep(3) # Imagine this is some expensive CPU work. ok = val + val2 print("Passing {} to parent".format(ok)) q.put(ok) # The Queue can be used with the normal blocking API, too. item = q.get() print("got {} back from parent".format(item)) def do_some_async_io_task(): # Imagine there's some kind of asynchronous I/O # going on here that utilizes asyncio. asyncio.sleep(5) @asyncio.coroutine def do_work(q): loop.run_in_executor(ProcessPoolExecutor(), do_proc_work, q, 1, 2) do_some_async_io_task() item = yield from q.coro_get() # Non-blocking get that won't affect our io_task print("Got {} from worker".format(item)) item = item + 25 yield from q.coro_put(item) if __name__ == "__main__": q = AsyncProcessQueue() # This is our new asyncio-friendly version of multiprocessing.Queue loop = asyncio.get_event_loop() loop.run_until_complete(do_work(q)) I have seen some rumblings about a desire to do this kind of integration on the bug tracker (http://bugs.python.org/issue10037#msg162497 and http://bugs.python.org/issue9248#msg221963) though that discussion is specifically tied to merging the enhancements from the Billiard library into multiprocessing.Pool. Are there still plans to do that? If so, should asyncio integration with multiprocessing be rolled into those plans, or does it make sense to pursue it separately? Even more generally, do people think this kind of integration is a good idea to begin with? I know using asyncio is primarily about *avoiding* the headaches of concurrent threads/processes, but there are always going to be cases where CPU-intensive work is going to be required in a primarily I/O-bound application. The easier it is to for developers to handle those use-cases, the better, IMO. Note that the same sort of integration could be done with the threading module, though I think there's a fairly limited use-case for that; most times you'd want to use threads over processes, you could probably just use non-blocking I/O instead. Thanks, Dan

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Python 2's re module should take longs
by Ryan Gonzalez 01 Oct '14

01 Oct '14

This works: re.search('(abc)', 'abc').group(1) but this doesn't: re.search('(abc)', 'abc').group(1L) The latter raises "IndexError: no such group". Shouldn't that technically work? -- Ryan If anybody ever asks me why I prefer C++ to C, my answer will be simple: "It's becauseslejfp23(@#Q*(E*EIdc-SEGFAULT. Wait, I don't think that was nul-terminated." Personal reality distortion fields are immune to contradictory evidence. - srean Check out my website: http://kirbyfan64.github.io/

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Implicit submodule imports
by Thomas Gläßle 30 Sep '14

30 Sep '14

Hey folks, What do you think about making it easier to use packages by automatically importing submodules on attribute access. Consider this example: >>> import matplotlib >>> figure = matplotlib.figure.Figure() AttributeError: 'module' object has no attribute 'figure' For the newcomer (like me some months ago) it's not obvious that the solution is to import matplotlib.figure. Worse even: it may sometimes/later on work, if the submodule has been imported from another place. How, I'd like it to behave instead (in pseudo code, since `package` is not a python class right now): class package: def __getattr__(self, name): try: return self.__dict__[name] except KeyError: # either try to import `name` or raise a nicer error message The automatic import feature could also play nicely when porting a package with submodules to or from a simple module with namespaces (as suggested in [1]), making this transition seemless to any user. I'm not sure about potential problems from auto-importing. I currently see the following issues: - harmless looking attribute access can lead to significant code execution including side effects. On the other hand, that could always be the case. - you can't use attribute access anymore to test whether a submodule is imported (must use sys.modules instead, I guess) In principle one can already make this feature happen today, by replacing the object in sys.modules - which is kind of ugly and has probably more flaws. This would also be made easier if there were a module.__getattr__ ([2]) or "metaclass" like feature for modules (which would be just a class then, I guess). Sorry, if this has come up before and I missed it. Anyhow, just interested if anyone else considers this a nice feature. Best regards, Thomas [1] https://mail.python.org/pipermail/python-ideas/2014-September/029341.html [2] https://mail.python.org/pipermail/python-ideas/2012-April/014957.html

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`numbers.Natural`
by Ram Rachum 28 Sep '14

28 Sep '14

I wish the `numbers` module would include a `Natural` class that would simply check whether the number is integral and positive.

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use gmtime(0) epoch in functions that use mktime()
by Akira Li 25 Sep '14

25 Sep '14

Python uses "seconds since the epoch" term to describe time.time() return value. POSIX term is "seconds since the Epoch" (notice the capitalization) where Epoch is 1970-01-01 00:00:00+00:00. C99 term is "calendar time" -- the encoding of the calendar time returned by the time() function is unspecified. Python documentation defines `epoch` as: The :dfn:`epoch` is the point where the time starts. On January 1st of that year, at 0 hours, the "time since the epoch" is zero. For Unix, the epoch is 1970. To find out what the epoch is, look at ``gmtime(0)``. time module documentation specifies calendar.timegm() as the inverse of time.gmtime() while timegm() uses the fixed 1970 Epoch instead of gmtime(0) epoch. datetime.astimezone() (local_timezone()) passes Unix timestamp [1970] to time.localtime() that may expect timestamp with different epoch [gmtime(0)]. email.util.mktime_tz() uses both mktime() [gmtime(0)] and timegm() [1970]. mailbox.py uses both time.time() [gmtime(0)] and timegm() [1970]. http.cookiejar uses both EPOCH_YEAR=1970 and datetime.utcfromtimestamp() [gmtime(0) epoch] for "seconds since epoch". It seems 1970 Epoch is used for file times on Windows (os.stat()) but os.path.getatime() refers to "seconds since epoch" [gmtime(0) epoch]. date{,time}.{,utc}fromtimestamp(), datetime.timestamp() docs equates "POSIX timestamp" [1970 Epoch] and time.time()'s returned value [gmtime(0) epoch]. datetime.timestamp() is inconsistent if gmtime(0) is not 1970. It uses mktime() for naive datetime objects [gmtime(0) epoch]. But it uses POSIX Epoch for aware datetime objects. Correct me if I'm wrong here. --- Possible fixes: Say in the `epoch` definition that stdlib doesn't support gmtime(0).tm_year != 1970. OR don't use mktime() if 1970 Epoch is used e.g., create an aware datetime object in the local timezone instead and use it to compute the result with 1970 Epoch. OR add *analog* of TZ=UTC time.mktime() and use it in stdlib where necessary. Looking at previous attempts (e.g., [1], [2]) to implement timegm(), the problem seems over-constrained. A different name could be used, to avoid wrong expectations e.g., datetime could use `(aware_datetime_object - gmtime0_epoch) // sec` [1] http://bugs.python.org/issue6280, [2] http://bugs.python.org/issue1667546 OR set EPOCH_YEAR=gmtime(0).tm_year instead of 1970 in calendar.timegm(). It may break backward compatibility if there is a system with non-1970 epoch. Deal on a case-by-case basis with other places where 1970 Epoch is used. And drop "POSIX timestamp" [1970 Epoch] and use "seconds since the epoch" [gmtime(0) epoch] in the datetime documentation. Change internal EPOCH year accordingly. What is Python-ideas opinion about it? -- Akira

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