Python-ideas January 2020

python-ideas@python.org

59 participants
38 discussions

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?

6 months

New explicit methods to trim strings

by Alex Grigoryev

Following the discussion here (https://link.getmailspring.com/link/7D84D131-65B6-4EF7-9C43-51957F9DFAA9@ge…) I propose to add 3 new string methods: str.trim, str.ltrim, str.rtrim Another option would be to change API for str.split method to work correctly with sequences. In [1]: def ltrim(s, seq): ...: return s[len(seq):] if s.startswith(seq) else s ...: In [2]: def rtrim(s, seq): ...: return s[:-len(seq)] if s.endswith(seq) else s ...: In [3]: def trim(s, seq): ...: return ltrim(rtrim(s, seq), seq) ...: In [4]: s = 'mailto:maria@gmail.com' In [5]: ltrim(s, 'mailto:') Out[5]: 'maria(a)gmail.com' In [6]: rtrim(s, 'com') Out[6]: 'mailto:maria@gmail.' In [7]: trim(s, 'm') Out[7]: 'ailto:maria@gmail.co'

6 months, 1 week

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 >

6 months, 3 weeks

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

7 months

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

pickle.reduce and deconstruct

by Andrew Barnert

Pickling uses an extensible protocol that lets any class determine how its instances can be deconstructed and reconstructed. Both `pickle` and `copy` use this protocol, but it could be useful more generally. Unfortunately, to use it more generally requires relying on undocumented details. I think we should expose a couple of helpers to fix that: # Return the same (shallow) reduction tuple that pickle.py, copy.py, and _pickle.c would use pickle.reduce(obj) -> (callable, args[, state[, litems[, ditem[, statefunc]]]]) # Return a callable and arguments to construct a (shallow) equivalent object # Raise a TypeError when that isn't possible pickle.deconstruct(obj) -> callable, args, kw So, why do you want these? There are many cases where you want to "deconstruct" an object if possible. Pattern matching depends on being able to deconstruct objects like this. Auto-generating a `__repr__` as suggested in Chris's thread. Quick&dirty REPL stuff, and deeper reflection stuff using `inspect.Signature` and friends. Of course not every type tells `pickle` what to do in an appropriate way that we can use, but a pretty broad range of types do, including (I think; I haven't double-checked all of them) `@dataclass`, `namedtuple`, `(a)attr.s`, many builtin and extension types, almost all reasonable types that use `copyreg`, and any class that pickles via the simplest customization hook `__getnewargs[_ex]__`. That's more than enough to be useful. And, just as important, it won't (except in intentionally pathological cases) give us a false positive, where a type is correctly pickleable and we think we can deconstruct it but the deconstruction is wrong. (For some uses, you are going to want to fall back to heuristics that are often right but sometimes misleadingly wrong, but I don't think the `pickle` module should offer anything like that. Maybe `inspect` should.) The way to get the necessary information isn't fully documented, and neither is the way to interpret it. And I don't think it _should_ be documented, because it changes every so often, and for good reasons; we don't want anyone writing third-party code that relies on those details. Plus, a different Python implementation might conceivably do it differently. Public helpers exposed from `pickle` itself won't have those problems. Here's a first take at the code. def reduce(obj, proto=pickle.DEFAULT_PROTOCOL): """reduce(obj) -> (callable, args[, state[, litems[, ditem[, statefunc]]]]) Return the same reduction tuple that the pickle and copy modules use """ cls = type(obj) if reductor := copyreg.dispatch_table.get(cls): return reductor(obj) # Note that this is not a special method call (not looked up on the type) if reductor := getattr(obj, "__reduce_ex__"): return reductor(proto) if reductor := getattr(obj, "__reduce__"): return reductor() raise TypeError(f"{cls.__name__} objects are not reducible") def deconstruct(obj): """deconstruct(obj) -> callable, args, kw callable(*args, **kw) will construct an equivalent object """ reduction = reduce(obj) # If any of the optional members are included, pickle/copy has to # modify the object after construction, so there is no useful single # call we can deconstruct to. if any(reduction[2:]): raise TypeError(f"{type(obj).__name__} objects are not deconstrutable") func, args, *_ = reduction # Most types (including @dataclass, namedtuple, and many builtins) # use copyreg.__newobj__ as the constructor func. The args tuple is # the type (or, when appropriate, some other registered # constructor) followed by the actual args. However, any function # with the same name will be treated the same way (because under the # covers, this is optimized to a special opcode). if func.__name__ == "__newobj__": return args[0], args[1:], {} # Mainly only used by types that implement __getnewargs_ex__ use # copyreg.__newobj_ex__ as the constructor func. The args tuple # holds the type, *args tuple, and **kwargs dict. Again, this is # special-cased by name. if func.__name__ == "__newobj_ex__": return args # If any other special copyreg functions are added in the future, # this code won't know how to handle them, so bail. if func.__module__ == 'copyreg': raise TypeError(f"{type(obj).__name__} objects are not deconstrutable") # Otherwise, the type implements a custom __reduce__ or __reduce_ex__, # and whatever it specifies as the constructor is the real constructor. return func, args, {} Actually looking at that code, I think it makes a better argument for why we don't want to make all the internal details public. :) Here are some quick (completely untested) examples of other things we could build on it. # in inspect def deconstruct(obj): """deconstruct(obj) -> callable, bound_args Calling the callable on the bound_args would construct an equivalent object """ func, args, kw = pickle.deconstruct(obj) sig = inspect.signature(func) return func, sig.bind(*args, **kw) # in reprlib, for your __repr__ to delegate to def auto_repr(obj): func, bound_args = inspect.deconstruct(obj) args = itertools.chain( map(repr, bound_args.args), (f"{key!r}={value!r}" for key, value in bound_args.kwargs.items())) return f"{func.__name__}({', '.join(args)})" # or maybe as a class decorator def auto_repr(cls): def __repr__(self): func, bound_args = inspect.deconstruct(self) args = itertools.chain( map(repr, bound_args.args), (f"{key!r}={value!r}" for key, value in bound_args.kwargs.items())) return f"{func.__name__}({', '.join(args)})" cls.__repr__ = __repr__ return cls

7 months, 2 weeks

addition of "nameof" operator

by Johan Vergeer

I have worked with both C# and Python for a while now and there is one feature of C# I'm missing in the Python language. This feature is the "nameof" operator. (https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators…). The place I use this the most in C# is in `ToString()` methods or logging messages. This makes sure the developer cannot forget to update the name of a member. As an example I created this `Person` class. ``` class Person: def __init__(self, name, age): self.name = name self.age = age def __repr__(self): return f"Person(name: {self.name}, age: {self.age})" ``` With the `nameof` operator this would look like the following: ``` class Person: def __init__(self, name, age): self.name = name self.age = age def __repr__(self): return f"{nameof(Person)}({nameof(self.name)}: {self.name}, {nameof(self.age)}: {self.age})" ``` What do you think about this?

7 months, 3 weeks

concurrent.futures cancel pending at Executor.shutdown

by Miguel Ángel Prosper

Hi, I think it would be very helpful to have an additional argument (cancel for example) added to Executor.shutdown that cancels all pending futures submitted to the executor. Then context manager would gain the ability to abort all futures incase of a exception, additionally this would also implement the missing cuterpart of the multiprocessing module terminate, currently we only have close.

7 months, 3 weeks

Re: addition of "nameof" operator

by Alex Hall

Here's a simple runtime implementation that doesn't require any dependencies or source code access: import dis import inspect from functools import lru_cache def nameof(_): frame = inspect.currentframe().f_back return _nameof(frame.f_code, frame.f_lasti) @lru_cache def _nameof(code, offset): instructions = list(dis.get_instructions(code)) (current_instruction_index, current_instruction), = ( (index, instruction) for index, instruction in enumerate(instructions) if instruction.offset == offset ) assert current_instruction.opname == "CALL_FUNCTION" name_instruction = instructions[current_instruction_index - 1] assert name_instruction.opname.startswith("LOAD_") return name_instruction.argrepr def test(): print(nameof(dis)) print(nameof(dis.get_instructions)) x = 1 print(nameof(x)) if __name__ == '__main__': test()

7 months, 3 weeks

Proposal for an additional type system

by guenter-hofer＠aon.at

The types in most programming languages are only technical types. If a variable has a type, you may know that it is of type integer or double, but you don't know if it's a length or an area. The types required are semantic types. stypes: length area time speed scalar allowed expressions: length = length length + length : length time = time time + time : time scalar * length : length scalar * time : time sqrt(area) : length speed = speed length / time : speed foo : time bar : length baz : length bar = 23 baz = 42.3 foo = 17.4 speed'variable = (bar + baz) / foo baz = length'5 bar = length'foo Python's dynamic type checking remains. These type names are all user defined. A type name with a tick before an expression means that this expression has the semantic type of the type name. The function names in the allowed expressions statement don't need to be names of existing functions.

7 months, 3 weeks

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Python-ideas January 2020