add overload to tuple.__new__ to better express an empty tuple

[xavdid]

This PR adds an overload to the tuple() constructor to specify that an empty tuple is returned, not Tuple[Any]. It covers the following code snippet (using the Pylance type checker):

from typing import Tuple
Point3D = Tuple[int, int, int]

t1: Point3D = 1, 2, 3  # correct no error
t2: Point3D = 1, 2, 3, 4  # correct error; Element size mismatch; expected 3 but received 4
t3: Point3D = () # correct error; Element size mismatch; expected 3 but received 0
t4: Point3D = tuple() # expected error, none thrown

The docs specify that:

If no argument is given, the constructor creates a new empty tuple, ().

This is my first typeshed PR, so please feel free to correct any mistakes I've made!

---

For more context, see: microsoft/pylance-release#2445 (comment)

[xavdid]

Ah, looks like CI is unhappy:

stdlib/builtins.pyi:788: error: Overloaded function signatures 1 and 2 overlap with incompatible return types [misc]

I see that most __new__ functions take cls: type[Self] as their first arg, but Self isn't subscriptable, so I wasn't sure how to specify that the return was Tuple[()] and not the default Tuple[Any]. Guidance here is appreciated 😅

[srittau]

It's unclear to me way mypy primer explodes, but I think it's related to isinstance checks, i.e. mypy thinks that isinstance(x, tuple) means that x is an empty tuple. Could you try reordering the overloads (and add a comment)?

[xavdid]

Sure thing! added in d556d17; hope that comment is descriptive enough- I don't quite follow the issue.

[xavdid]

Ah, I think I'm following now. Using tuple as a class not the result of an instantiation is the issue.

tuple should return Self, but tuple() should return Tuple[()]. I'm not sure how that's expressed in the interfaces correctly.

[srittau]

I'm not totally sure what's going on here myself. primer output looks much better, although it's weird that the remaining problems are in mypy itself. Maybe a mypy developer could look over this? Cc @JukkaL @hauntsaninja

[hauntsaninja]

I'm not sure either, will look into it soon.

I think the first error on config_files = tuple(map(os.path.expanduser, defaults.CONFIG_FILES)) is not specific to mypy's codebase and is a genuine false positive. The other two errors involve a custom mypy plugin that mypy uses on the mypy codebase; it's possible there's a bug in that plugin, but I'd need to investigate more to know what's going on.

[xavdid]

Thanks for taking a look! Let me know if there's anything else I can do to be helpful.

[hauntsaninja]

Here, python/mypy#12319 will fix the errors from the plugin.

I unfortunately don't have a fix for the errors from config_files = tuple(map(os.path.expanduser, defaults.CONFIG_FILES)), but that's life.

I also filed python/mypy#12320 since it looks like a similar issue could crop up in other parts of mypy's codebase.

[JelleZijlstra]

I'm not sure this change is worth it if we lose correct typing for subclasses of tuple. After class X(tuple), what does reveal_type(X()) do?

[AlexWaygood]

I'm not sure this change is worth it if we lose correct typing for subclasses of tuple. After class X(tuple), what does reveal_type(X()) do?

Empty tuples and tuple subclasses are both reasonably common, but I can't say I've ever seen an example of an empty tuple subclass instance. And for non-empty tuple subclass instances, we'll still have correct type inference, as that overload still returns Self. So I think it will probably be okay?

[AlexWaygood]

Seems like this change might also fix

• x = tuple() # type: Tuple[()] Incompatible types in assignment mypy#6283

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

mypy (https://github.com/python/mypy)
+ mypy/config_parser.py:182: error: Argument 1 to "map" has incompatible type overloaded function; expected "Callable[[str], AnyStr]"  [arg-type]

[AlexWaygood]

I'm not sure this change is worth it if we lose correct typing for subclasses of tuple. After class X(tuple), what does reveal_type(X()) do?

For both master and this PR, if I do mypy --custom-typeshed-dir on the following snippet, mypy yields the following result:

# test.py
class X(tuple): ...
reveal_type(X())  # Revealed type is "test.X"

So this doesn't seem to cause any regressions in that regard.