Python-ideas July 2018

python-ideas@python.org

111 participants
70 discussions

Dataclasses, keyword args, and inheritance

by George Leslie-Waksman

The proposed implementation of dataclasses prevents defining fields with defaults before fields without defaults. This can create limitations on logical grouping of fields and on inheritance. Take, for example, the case: @dataclass class Foo: some_default: dict = field(default_factory=dict) @dataclass class Bar(Foo): other_field: int this results in the error: 5 @dataclass ----> 6 class Bar(Foo): 7 other_field: int 8 ~/.pyenv/versions/3.6.2/envs/clover_pipeline/lib/python3.6/site-packages/dataclasses.py in dataclass(_cls, init, repr, eq, order, hash, frozen) 751 752 # We're called as @dataclass, with a class. --> 753 return wrap(_cls) 754 755 ~/.pyenv/versions/3.6.2/envs/clover_pipeline/lib/python3.6/site-packages/dataclasses.py in wrap(cls) 743 744 def wrap(cls): --> 745 return _process_class(cls, repr, eq, order, hash, init, frozen) 746 747 # See if we're being called as @dataclass or @dataclass(). ~/.pyenv/versions/3.6.2/envs/clover_pipeline/lib/python3.6/site-packages/dataclasses.py in _process_class(cls, repr, eq, order, hash, init, frozen) 675 # in __init__. Use "self" if possible. 676 '__dataclass_self__' if 'self' in fields --> 677 else 'self', 678 )) 679 if repr: ~/.pyenv/versions/3.6.2/envs/clover_pipeline/lib/python3.6/site-packages/dataclasses.py in _init_fn(fields, frozen, has_post_init, self_name) 422 seen_default = True 423 elif seen_default: --> 424 raise TypeError(f'non-default argument {f.name!r} ' 425 'follows default argument') 426 TypeError: non-default argument 'other_field' follows default argument I understand that this is a limitation of positional arguments because the effective __init__ signature is: def __init__(self, some_default: dict = <something>, other_field: int): However, keyword only arguments allow an entirely reasonable solution to this problem: def __init__(self, *, some_default: dict = <something>, other_field: int): And have the added benefit of making the fields in the __init__ call entirely explicit. So, I propose the addition of a keyword_only flag to the @dataclass decorator that renders the __init__ method using keyword only arguments: @dataclass(keyword_only=True) class Bar(Foo): other_field: int --George Leslie-Waksman

7 months, 2 weeks

@classproperty, @abc.abstractclasspropery, etc.

by K. Richard Pixley

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

10 months, 1 week

Add the imath module

by Serhiy Storchaka

What are your thoughts about adding a new imath module for integer mathematics? It could contain the following functions: * factorial(n) Is just moved from the math module, but non-integer types are rejected. Currently math.factorial() accepts also integer floats like 3.0. It looks to me, the rationale was that at the time when math.factorial() was added, all function in the math module worked with floats. But now we can revise this decision. * gcd(n, m) Is just moved from the math module. * as_integer_ration(x) Equivalents to: def as_integer_ration(x): if hasattr(x, 'as_integer_ration'): return x.as_integer_ration() else: return (x.numerator, x.denominator) * binom(n, k) Returns factorial(n) // (factorial(k) * factorial(n-k)), but uses more efficient algorithm. * sqrt(n) Returns the largest integer r such that r**2 <= n and (r+1)**2 > n. * isprime(n) Tests if n is a prime number. * primes() Returns an iterator of prime numbers: 2, 3, 5, 7, 11, 13,... Are there more ideas?

1 year, 7 months

Re: [Python-ideas] Allow star unpacking within an slice expression

by Neil Girdhar

I didn't think of this when we were discussing 448. I ran into this today, so I agree with you that it would be nice to have this. Best, Neil On Monday, December 4, 2017 at 1:02:09 AM UTC-5, Eric Wieser wrote: > > Hi, > > I've been thinking about the * unpacking operator while writing some numpy > code. PEP 448 allows the following: > > values = 1, *some_tuple, 2 > object[(1, *some_tuple, 2)] > > It seems reasonable to me that it should be extended to allow > > item = object[1, *some_tuple, 2] > item = object[1, *some_tuple, :] > > Was this overlooked in the original proposal, or deliberately rejected? > > Eric >

1 year, 7 months

Specify number of items to allocate for array.array() constructor

by Sven Rahmann

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

1 year, 8 months

Please consider adding of functions file system operations to pathlib

by George Fischhof

Good day all, as a continuation of thread "OS related file operations (copy, move, delete, rename...) should be placed into one module" https://mail.python.org/pipermail/python-ideas/2017-January/044217.html please consider making pathlib to a central file system module with putting file operations (copy, move, delete, rmtree etc) into pathlib. BR, George

1 year, 11 months

Start argument for itertools.accumulate() [Was: Proposal: A Reduce-Map Comprehension and a "last" builtin]

by Raymond Hettinger

> On Friday, April 6, 2018 at 8:14:30 AM UTC-7, Guido van Rossum wrote: > On Fri, Apr 6, 2018 at 7:47 AM, Peter O'Connor <peter.ed...(a)gmail.com> wrote: >> So some more humble proposals would be: >> >> 1) An initializer to itertools.accumulate >> functools.reduce already has an initializer, I can't see any controversy to adding an initializer to itertools.accumulate > > See if that's accepted in the bug tracker. It did come-up once but was closed for a number reasons including lack of use cases. However, Peter's signal processing example does sound interesting, so we could re-open the discussion. For those who want to think through the pluses and minuses, I've put together a Q&A as food for thought (see below). Everybody's design instincts are different -- I'm curious what you all think think about the proposal. Raymond --------------------------------------------- Q. Can it be done? A. Yes, it wouldn't be hard. _sentinel = object() def accumulate(iterable, func=operator.add, start=_sentinel): it = iter(iterable) if start is _sentinel: try: total = next(it) except StopIteration: return else: total = start yield total for element in it: total = func(total, element) yield total Q. Do other languages do it? A. Numpy, no. R, no. APL, no. Mathematica, no. Haskell, yes. * http://docs.scipy.org/doc/numpy/reference/generated/numpy.ufunc.accumulate.… * https://stat.ethz.ch/R-manual/R-devel/library/base/html/cumsum.html * http://microapl.com/apl/apl_concepts_chapter5.html \+ 1 2 3 4 5 1 3 6 10 15 * https://reference.wolfram.com/language/ref/Accumulate.html * https://www.haskell.org/hoogle/?hoogle=mapAccumL Q. How much work for a person to do it currently? A. Almost zero effort to write a simple helper function: myaccum = lambda it, func, start: accumulate(chain([start], it), func) Q. How common is the need? A. Rare. Q. Which would be better, a simple for-loop or a customized itertool? A. The itertool is shorter but more opaque (especially with respect to the argument order for the function call): result = [start] for x in iterable: y = func(result[-1], x) result.append(y) versus: result = list(accumulate(iterable, func, start=start)) Q. How readable is the proposed code? A. Look at the following code and ask yourself what it does: accumulate(range(4, 6), operator.mul, start=6) Now test your understanding: How many values are emitted? What is the first value emitted? Are the two sixes related? What is this code trying to accomplish? Q. Are there potential surprises or oddities? A. Is it readily apparent which of assertions will succeed? a1 = sum(range(10)) a2 = sum(range(10), 0) assert a1 == a2 a3 = functools.reduce(operator.add, range(10)) a4 = functools.reduce(operator.add, range(10), 0) assert a3 == a4 a4 = list(accumulate(range(10), operator.add)) a5 = list(accumulate(range(10), operator.add, start=0)) assert a5 == a6 Q. What did the Python 3.0 Whatsnew document have to say about reduce()? A. "Removed reduce(). Use functools.reduce() if you really need it; however, 99 percent of the time an explicit for loop is more readable." Q. What would this look like in real code? A. We have almost no real-world examples, but here is one from a StackExchange post: def wsieve(): # wheel-sieve, by Will Ness. ideone.com/mqO25A->0hIE89 wh11 = [ 2,4,2,4,6,2,6,4,2,4,6,6, 2,6,4,2,6,4,6,8,4,2,4,2, 4,8,6,4,6,2,4,6,2,6,6,4, 2,4,6,2,6,4,2,4,2,10,2,10] cs = accumulate(cycle(wh11), start=11) yield( next( cs)) # cf. ideone.com/WFv4f ps = wsieve() # codereview.stackexchange.com/q/92365/9064 p = next(ps) # 11 psq = p*p # 121 D = dict( zip( accumulate(wh11, start=0), count(0))) # start from sieve = {} for c in cs: if c in sieve: wheel = sieve.pop(c) for m in wheel: if not m in sieve: break sieve[m] = wheel # sieve[143] = wheel@187 elif c < psq: yield c else: # (c==psq) # map (p*) (roll wh from p) = roll (wh*p) from (p*p) x = [p*d for d in wh11] i = D[ (p-11) % 210] wheel = accumulate(cycle(x[i:] + x[:i]), start=psq) p = next(ps) ; psq = p*p next(wheel) ; m = next(wheel) sieve[m] = wheel

2 years

Re: [Python-ideas] Trigonometry in degrees

by Hugh Fisher

> Date: Thu, 7 Jun 2018 12:33:29 +0000 > From: Robert Vanden Eynde <robertvandeneynde(a)hotmail.com> > To: python-ideas <python-ideas(a)python.org> > Subject: [Python-ideas] Trigonometry in degrees > Message-ID: > > > I suggest adding degrees version of the trigonometric functions in the math module. > > - Useful in Teaching and replacing calculators by python, importing something is seen by the young students much more easy than to define a function. I agree that degrees are useful for teaching. They are also very useful for graphics programming, especially with my favourite OpenGL API. But I think that the use of radians in programming language APIs is more prevalent, so the initial advantage of easy learning will be outweighed by the long term inconvenience of adjusting to what everyone else is doing. Writing degrees(x) and radians(x) is a little inconvenient, but it does make it clear what units are being used. And even if your proposal is adopted, there is still going to be a lot of code around that uses the older math routines. With the current API it is a least safe to assume that angles are radians unless stated otherwise. > - Special values could be treated, aka when the angle is a multiple of 90, young students are often surprise to see that cos(pi/2) != 0 > > Testing for a special value Isn't very costly (x % 90 == 0) but it could be pointed out that there is a small overhead using the "degrees" equivalent of trig function because of the radians to degrees conversion And the special values testing. Not just young students :-) I agree with this, but I would prefer the check to be in the implementation of the existing functions as well. Any sin/cos very close to 0 becomes 0, any close to 1 becomes 1. > - Standard names will be chosen so that everyone will use the same name convention. I suggest adding a "d" like sind, cosd, tand, acosd, asind, atand, atan2d. Not "d". In the OpenGL 3D API, and many 3D languages/APIs since, appending "d" means "double precision". It's even sort of implied by the C math library which has sinf and friends for single precision. > > Creating a new package like 'from math.degrees import cos' however I would not recommend that because "cos" in the source code would mean to lookup the import to know if it's in degrees or radians (and that leads to very filthy bugs). Also "degrees" is already so the name would have to change the name of the package. Agree, not a good idea. -- cheers, Hugh Fisher

2 years, 5 months

Support localization of unicode descriptions

by Pander

Dear all, Who might also be interested in setting up a project that supports localization for Unicode block description and character description. Translations are available from https://github.com/unicode-table/unicode-table-data/tree/master/loc <https://github.com/unicode-table/unicode-table-data/tree/master/loc> If possible, use a gettext approach similar to https://pypi.org/project/pycountry/ <https://pypi.org/project/pycountry/> Implementing this feature will allow users to read Unicode descriptions in their own language, other than English. For example, now is possible only in English: from unicodedata import name print(name('ß')) LATIN SMALL LETTER SHARP S So unicodedata could provide a way to translate LATIN SMALL LETTER SHARP S to e.g. German with: from unicodedata import name from unicodedata_l10n import LOCALED_DIR from gettext import translation german = translation('UnicodeData' LOCALED_DIR, languages=['de']) german.install() print(_(name('ß'))) LATEINISCHER KLEINBUCHSTABE SCHARFES S and something similar for unicodedata.category Best, Pander

2 years, 8 months

Dart like multi line strings identation

by Marius Räsener

Hey List, this is my very first approach to suggest a Python improvement I'd think worth discussing. At some point, maybe with Dart 2.0 or a little earlier, Dart is now supporting multiline strings with "proper" identation (tried, but I can't find the according docs at the moment. probably due to the rather large changes related to dart 2.0 and outdated docs.) What I have in mind is probably best described with an Example: print(""" I am a multiline String. """) the closing quote defines the "margin indentation" - so in this example all lines would get reduces by their leading 4 spaces, resulting in a "clean" and unintended string. anyways, if dart or not, doesn't matter - I like the Idea and I think python3.x could benefit from it. If that's possible at all :) I could also imagine that this "indentation cleanup" only is applied if the last quotes are on their own line? Might be too complicated though, I can't estimated or understand this... thx for reading, Marius

2 years, 8 months

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Python-ideas July 2018