isinstance + class indirectly deriving from SupportsInt results in "TypeError: Protocols cannot be used with isinstance()"

[jstasiak]

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?

[gvanrossum]

This is definitely a problem. A quick workaround would be to replace the import with

from typing import TYPE_CHECKING
if TYPE_CHECKING:
    from typing import SupportsInt
else:
    SupportsInt = object

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).

diff --git a/src/typing.py b/src/typing.py
index c4f0fdc..a87500c 100644
--- a/src/typing.py
+++ b/src/typing.py
@@ -1440,6 +1440,8 @@ class _ProtocolMeta(GenericMeta):
     """

     def __instancecheck__(self, obj):
+        if _Protocol not in self.__bases__:
+            return super().__instancecheck__(obj)
         raise TypeError("Protocols cannot be used with isinstance().")

     def __subclasscheck__(self, cls):

@ilevkivskyi What do you think of that fix?

[gvanrossum]

PS. The fix only works on top of #295.

[ilevkivskyi]

@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]

Thanks @gvanrossum, I totally forgot about the TYPE_CHECKING variable, I'll use the workaround you mentioned for the time being.