Dict[str, Callable[...]] Not Handled Properly

[ameily]

I'm trying to annotate a dict of supported hashing algorithms.

from typing import Callable, Optional, Dict
import hashlib
from _hashlib import HASH

#: Hash algorithm, as returned by hashlib.new()
HashAlgorithm = Callable[[Optional[bytes]], HASH]

ALGORITHMS = {
    'md5': hashlib.md5,
    'sha1': hashlib.sha1,
    'sha224': hashlib.sha224,
    'sha256': hashlib.sha256,
    'sha384': hashlib.sha384,
    'sha512': hashlib.sha512
}  # type: Dict[str, HashAlgorithm]

This is resulting in a weird / confusing error message (ignore line numbers).

file.py823: error: Dict entry 0 has incompatible type "str": "Callable[[Union[bytes, bytearray, memoryview]], _Hash]"; expected "str": "Callable[[Optional[bytes]], Any]"
file.py:823: error: Dict entry 1 has incompatible type "str": "Callable[[Union[bytes, bytearray, memoryview]], _Hash]"; expected "str": "Callable[[Optional[bytes]], Any]"
# same error for each entry in the dict

Switching the value type to just Callbable fixes the issue, but it seems to me that there's an underlying problem here.

Version info:

python -V
python 3.7.3

mypy -V
mypy 0.720

[JelleZijlstra]

Isn't the issue just that your HashAlgorithm alias allows None as an argument, but the hashlib functions do not?

[ilevkivskyi]

Yes, this is a legitimate error, I tried some methods and they are actually failing with None:

>>> hashlib.md5(None)
TypeError: object supporting the buffer API required

If you wanted an optional argument, not optional type, then define a callback protocol:

class HashAlgorithm(Protocol):
    def __call__(self, arg: bytes = ...) -> HASH: ...

Finally, I don't see why do you need this annotation here, mypy is able to infer a decent type itself.