Unclear error message: incompatible type "type[str]"; expected "Callable[[_T], _T]"

[Jacobfaib]

To Reproduce

from collections.abc import Sequence
from typing import TypeVar

_T = TypeVar("_T")

def foo(seq: Sequence[_T]) -> None:
    seq = tuple(map(str, seq))

$ mypy ./mypy_bug.py
mypy_bug.py: note: In function "foo":
mypy_bug.py:7:19: error: Argument 1 to "map" has incompatible type "type[str]"; expected "Callable[[_T], _T]"
[arg-type]
      seq = tuple(map(str, seq))
                      ^~~
Found 1 error in 1 file (checked 1 source file)

Interestingly enough, this error does not fire if you assign to something other than seq:

def foo(seq: Sequence[_T]) -> None:
    not_seq = tuple(map(str, seq))  # no problem, no mypy error

• Mypy version used: mypy 1.9.0 (compiled: yes)
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: Python 3.12.2

[erictraut]

You're attempting to assign a value of type Sequence[str] to a parameter that has a declared type of Sequence[_T]. This is a type violation, so mypy is correct to generate an error here.

Mypy's error message could arguably be made more clear in this case. For example, here is now pyright reports the error for the same code.

[Jacobfaib]

You're attempting to assign a value of type Sequence[str] to a parameter that has a declared type of Sequence[_T]. This is a type violation, so mypy is correct to generate an error here.

Sure, and mypy is indeed silenced if you run with --allow-redefinition, but the point here is:

1. It is not clear from the diagnostic that the original sin is type redefinition.
2. mypy's diagnostic message is arguably wrong. str (or more specifically, type[T] which accepts a single argument) is a sub-type of Callable[[T], T] no?

[JelleZijlstra]

str is not a subtype of Callable[[T], T], because such a callable should return its input type unchanged.

[AnkitSharma-29]

The issue you're encountering is with the map() function call. The error message indicates that there's an incompatible type being passed as the first argument to map(). In this case, map(str, seq), you're trying to convert each element of seq to a string using the str function.

To fix this issue and make it type-check cleanly with mypy, you can explicitly specify the function type for map() by using a lambda function. Here's how you can do it:
`from collections.abc import Sequence
from typing import TypeVar, Callable

_T = TypeVar("_T")

def foo(seq: Sequence[_T]) -> None:
seq = tuple(map(lambda x: str(x), seq)) # Use lambda to specify the function type
# Now mypy should not raise any errors
`

By using a lambda function, you're explicitly specifying the type of the function being passed to map(), which resolves the incompatible type error.

[JukkaL]

The issue here is that the error message is hard to figure out. We can leave this issue open, but it will be about improving the error message (I'll update the title).