isinstance(x, Tuple) fails when equivalent expressions don't

[lbenezriravin]

reproducer

from typing import Any, Dict, Tuple

x = (1,2)
isinstance(x, tuple)
isinstance(x, Tuple[Any, ...])
isinstance(x, Tuple)

observed behavior

$ pipenv run mypy test.py 
test.py:6: error: Argument 2 to "isinstance" has incompatible type "_SpecialForm"; expected "Union[type, Tuple[Union[type, Tuple[Any, ...]], ...]]"

expected behavior
The python documentation states

A plain Tuple is equivalent to Tuple[Any, ...], and in turn to tuple.

If the above is true, IMO either all of the examples should raise errors, or none of them should. Only the third example is raising an error right now. I'm not quite sure what the error means in any case.

environment
master as of 2019-04-15

$ pipenv graph
mypy==0.710+dev.2c8a809693cf5be0b29d272d7f0711521bd4a55a
  - mypy-extensions [required: >=0.4.0,<0.5.0, installed: 0.4.1]
  - typed-ast [required: >=1.3.1,<1.4.0, installed: 1.3.1]

As always, thanks for all the work you do here!

[gvanrossum]

I am guessing the sentence you quote from the docs applies to its use in type annotations, not in isinstance(). In that case IMO only tuple should be allowed, because at runtime the other use of Tuple raises a TypeError. IMO it's a mypy bug that it doesn't report both uses of Tuple as errors. (I'm too lazy to look up what the mypy docs say about this -- they tend to be more correct than the Python docs about type checking matters.)

[lbenezriravin]

Ah, I see that now. If you don't mind me asking, why does Tuple behave differently from other superficially similar generics? For example,

y = {1: 1}
isinstance(y, Dict)
isinstance(y, Sequence)

don't raise TypeErrors unless I parameterize the generics.

[gvanrossum]

Because Tuple is not a generic class, it's a special form. This is so that it can take a variable number of parameters.

[lbenezriravin]

That makes sense. Thanks for the guidance!

[ilevkivskyi]

Essentially, the mypy behavior is opposite to what happens at runtime:

isinstance((), Tuple)  # Works at runtime, error in mypy
isinstance((), Tuple[Any, ...])  # Error at runtime, no error in mypy

I think however this is a low priority, you should really just use tuple.

[AlexWaygood]

On 0.941, mypy now correctly emits an error for isinstance(x, Tuple[Any, ...]). It still gives a false-positive error for isinstance(x, Tuple), though:

from typing import Any, Tuple

x = (1,2)
isinstance(x, tuple)
isinstance(x, Tuple[Any, ...])  # error: Argument 2 to "isinstance" has incompatible type "object"; expected "Union[type, UnionType, Tuple[Union[type, UnionType, Tuple[Any, ...]], ...]]"
isinstance(x, Tuple)  # error: Argument 2 to "isinstance" has incompatible type "_SpecialForm"; expected "Union[type, UnionType, Tuple[Union[type, UnionType, Tuple[Any, ...]], ...]]"

[AlexWaygood]

(Closed by accident)

[gvanrossum]

So what's to do here? The OP is about runtime behavior, which should not be discussed in the mypy tracker.

[AlexWaygood]

I read the OP as being about the false-positive error mypy emits on this snippet of code:

from typing import Tuple
isinstance(x, Tuple)

This works fine at runtime, but mypy 0.942 reports:

error: Argument 2 to "isinstance" has incompatible type "_SpecialForm"; expected "Union[type, UnionType, Tuple[Union[type, UnionType, Tuple[Any, ...]], ...]]"

It's hardly a priority to fix, but it does seem to me to be a legitimate bug?

[AlexWaygood]

I've just closed #6864 as a duplicate -- #6864 is about isinstance(x, collections.abc.Callable) causing mypy to error out, with exactly the same error message. (That also works fine at runtime.)

[randolf-scholz]

#14014 is likely also a duplicate. This issue persists when checking against Callable class pattern in match-case statements

match x:
    case collections.abc.Callable() as func:
        ...

Literally all other classes in collections.abc seem to work without problem.