Incorrect signature for tempfile.mkstemp

[tgdane]

The dir argument to tempfile.mkstemp appears to be incorrectly typed.

Steps for reproduction

Create file:

# file: demo.py
import tempfile 
f = tempfile.mkstemp(dir="abc") 

then run:

$ mypy demo.py

which outputs:

a.py:2: error: Argument "dir" to "mkstemp" has incompatible type "str"; expected "None"
Found 1 error in 1 file (checked 1 source file)

System information

• Windows 10
• Conda 4.7.11
• Python 3.7.4
• mypy 0.740
• mypy-extensions 0.4.3

[JelleZijlstra]

I can't reproduce this issue in mypy 0.740 on Python 3.7.0 or 3.8.0.

For reference, the current stub for mkstemp in the Python 3 stubs is

def mkstemp(suffix: Optional[AnyStr] = ..., prefix: Optional[AnyStr] = ..., dir: Optional[_DirT[AnyStr]] = ...,
            text: bool = ...) -> Tuple[int, AnyStr]: ...

I'm not sure how that definition could produce the mypy error you cite, so maybe there's a mypy bug involved.

[andersk]

I can reproduce with mypy 0.740 on Python 3.5 (but not 3.6, 3.7, or 3.8).

$ cat Dockerfile
FROM python:3.5-alpine
RUN apk add --no-cache gcc musl-dev
RUN pip install mypy==0.740
RUN echo 'import tempfile; tempfile.mkstemp(dir="/var/tmp")' | mypy /dev/stdin
$ docker build .
…
Step 4/4 : RUN echo 'import tempfile; tempfile.mkstemp(dir="/var/tmp")' | mypy /dev/stdin
 ---> Running in f2643c17e40c
dev/stdin:1: error: Argument "dir" to "mkstemp" has incompatible type "str"; expected "None"
Found 1 error in 1 file (checked 1 source file)
The command '/bin/sh -c echo 'import tempfile; tempfile.mkstemp(dir="/var/tmp")' | mypy /dev/stdin' returned a non-zero code: 1

git bisect blames python/mypy@d30c0d2, which points to #3226 as a potential trigger.

[srittau]

Interesting. Minimal test case, independent from Python version or current tempfile hints:

from typing import AnyStr, Optional, TypeVar, Union
  
_T = TypeVar("_T")
_DirT = Union[_T]

def foo(x: Optional[_DirT[AnyStr]]) -> AnyStr: ...
foo("")  # error: Argument 1 to "foo" has incompatible type "str"; expected "None"

I wonder whether that is a mypy issue or if we are doing anything wrong in the type hints. @JelleZijlstra

[JelleZijlstra]

Does having a separate overload for x: None help? This does look like a mypy bug to me.

[srittau]

Using overloads results in a different, but still interesting error:

from typing import AnyStr, Optional, TypeVar, Union, overload
  
_T = TypeVar("_T")
_DirT = Union[_T]

@overload
def foo(x: None) -> AnyStr: ...
@overload
def foo(x: _DirT[AnyStr]) -> AnyStr: ...  # error: Overloaded function signature 2 will never be matched: signature 1's parameter type(s) are the same or broader
def foo(x): ...

[JelleZijlstra]

Now that seems clearly wrong.

[andersk]

The typing module also doesn’t let you do that at runtime:

Python 3.8.0 (default, Oct 21 2019, 20:02:18) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing import AnyStr, TypeVar, Union
>>> _T = TypeVar("_T")
>>> _DirT = Union[_T]
>>> _DirT[AnyStr]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'TypeVar' object is not subscriptable

[JelleZijlstra]

This only reproed on Python 3.5, which is no longer supported by mypy or typeshed.