bytes(obj) rejected even when obj implements __bytes__ #3989

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1. ilevkivskyi commented on Sep 23, 2017

   

   ilevkivskyi

   on Sep 23, 2017

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   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.
2. ilevkivskyi

   added

   topic-protocols

   on Sep 23, 2017
3. bmerry commented on Sep 23, 2017

   

   bmerry

   on Sep 23, 2017

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   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)).
4. ilevkivskyi commented on Sep 23, 2017

   

   ilevkivskyi

   on Sep 23, 2017

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   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.
5. bmerry commented on Mar 11, 2018

   

   bmerry

   on Mar 11, 2018

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

   closed this as completedon Mar 11, 2018