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isinstance + class indirectly deriving from SupportsInt results in "TypeError: Protocols cannot be used with isinstance()" #297
Description
The following assumes mypy 0.4.5 and CPython 3.5.2.
Let's have the code:
# code3.py
class C1:
def __int__(self) -> int:
return 42
class C2(C1):
pass
c = C2()
print(int(c))
print(isinstance(c, C1))The code works:
% python code3.py
42
True
mypy, however, will complain (as expected):
% mypy code3.py
code3.py:12: error: Argument 1 to "int" has incompatible type "C2"; expected "Union[SupportsInt, str, bytes]"
Let's try to make mypy happy:
# code3.py
from typing import SupportsInt
class C1(SupportsInt):
def __int__(self) -> int:
return 42
class C2(C1):
pass
c = C2()
print(int(c))
print(isinstance(c, C1))Now mypy is happy but the code fails at runtime:
% python code3.py
42
Traceback (most recent call last):
File "code3.py", line 16, in <module>
print(isinstance(c, C1))
File "/usr/local/Cellar/python3/3.5.2_2/Frameworks/Python.framework/Versions/3.5/lib/python3.5/typing.py", line 1269, in __instancecheck__
raise TypeError("Protocols cannot be used with isinstance().")
TypeError: Protocols cannot be used with isinstance().
It seems that I'm put in a situation where I have to choose between static type safety and having the code actually work, am I missing something here? If not - what's the recommended solution to a problem like this?
Activity
gvanrossum commentedon Oct 12, 2016
This is definitely a problem. A quick workaround would be to replace the import with
but that's pretty gross.
I have a pretty straightforward fix, which still disallows isinstance() checks on classes that directly derive from _Protocol (such as SupportsAbs) but allows it for subclasses thereof (like your C1 and C2).
@ilevkivskyi What do you think of that fix?
gvanrossum commentedon Oct 12, 2016
PS. The fix only works on top of #295.
ilevkivskyi commentedon Oct 12, 2016
@gvanrossum As a quick fix this is perfectly OK. I would say that for a permanent fix we could think about making the rules the same as for ordinary generics. But before deciding on permanent solution, I would ask @JukkaL for his opinion (the permanent fix would probably depend on resolution of #11).
jstasiak commentedon Oct 13, 2016
Thanks @gvanrossum, I totally forgot about the
TYPE_CHECKINGvariable, I'll use the workaround you mentioned for the time being.Fix isinstance() for Protocols (#330)