bytes(obj) rejected even when obj implements __bytes__

[bmerry]

Sample code:

#!/usr/bin/env python3

class A(object):
    def __bytes__(self) -> bytes:
        return b'An A'

print(bytes(A()))

Output from mypy 0.521, Python 3.5.2:

test.py:7: error: No overload variant of "bytes" matches argument types [test.A]

Looking in typeshed, it has this overload:

class bytes(ByteString):
    @overload
    def __init__(self, o: SupportsBytes) -> None: ...

which doesn't seem to be matching.

[ilevkivskyi]

The problem is that SupportsBytes (and several other classes) is not yet defined as a protocol. This will be fixed by python/typeshed#1220, a temporary workaround is to explicitly inherit A from typing.SupportsBytes.

[bmerry]

Unfortunately the workaround breaks using isinstance with the class:

#!/usr/bin/env python3

import typing

class A(typing.SupportsBytes):
    def __bytes__(self) -> bytes:
        return b'An A'

print(bytes(A()))
assert isinstance(None, A)

at runtime gives

b'An A'
Traceback (most recent call last):
  File "./test.py", line 10, in <module>
    assert isinstance(None, A)
  File "/usr/lib/python3.5/typing.py", line 1269, in __instancecheck__
    raise TypeError("Protocols cannot be used with isinstance().")
TypeError: Protocols cannot be used with isinstance().

I managed to work around it by replacing bytes(a) with bytes(cast(SupportsBytes, a)).

[ilevkivskyi]

Concerning the isinstance() error, it looks like you are using an older version of typing, this is fixed in latest version. You could try updating Python (or upgrading to 3.6), but your workaround (using cast) is perfectly OK.

[bmerry]

It works with the current version of mypy (0.570), so closing.