Werkzeug EnvironHeaders is not considered a Mapping #3569

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1. srittau commented on Jan 3, 2020

   

   srittau

   on Jan 3, 2020

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   Do you have an example that you think should type check but doesn't?
2. libre-man commented on Jan 3, 2020

   

   libre-man

   on Jan 3, 2020

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   Here is small example:

   from flask import request
   import typing as t
   
   T = t.TypeVar('T')
   Z = t.TypeVar('Z')
   
   def get(kv: t.Mapping[T, Z], key: T) -> Z:
       return kv[key]
   
   get(request.headers, 'a_header')

   On which mypy errors with: test.py:10: error: Argument 1 to "get" has incompatible type "EnvironHeaders"; expected "Mapping[str, <nothing>]".
3. srittau commented on Jan 5, 2020

   

   srittau

   on Jan 5, 2020

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   I opened #3576 for the underlying issue. My best suggestion for now is to use a custom protocol for your own code and - depending on the outcome of the discussion in #3576 - submit PRs for code in typeshed.
4. gward commented on Jul 7, 2020

   

   gward

   on Jul 7, 2020

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   For posterity, here is a custom protocol that works for me in one particular place:

   from typing import overload
   import typing_extensions
   
   class HeadersType(typing_extensions.Protocol):
       """common subtype of Mapping[str, str] and werkzeug.datastructures.EnvironHeaders"""
       @overload
       def get(self, k: str) -> Optional[str]:
           ...
   
       @overload
       def get(self, k: str, default: str) -> str:
           ...
   

   This is not a general solution to the problem! But I just spent at least an hour banging my head against this until I got it working. Hopefully this can save someone else some time in the future.

   I just had to change my code from

   headers: Mapping[str, str]
   

   to

   headers: HeadersType
   

   and now mypy is happy.
5. srittau

   added a commit that references this issue on Jul 10, 2020

   Introduce Mapping-like protocols

   ...

   68e097f
6. srittau commented on Jul 10, 2020

   

   srittau

   on Jul 10, 2020

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   A brief addendum on why we can't just derive Headers from Mapping: Since Mapping is not a protocol, the following code returns False at runtime:

   from collections.abc import Mapping
   from werkzeug.datastructures import Headers
   
   isinstance(Headers(), Mapping)  # False

   Deriving Headers from Mapping would introduce a false negatives. For example, cgi.FieldStorage takes a headers arguments, but checks for Mapping explicitly:

   from cgi import FieldStorage
   from werkzeug.datastructures import Headers
   
   FieldStorage(headers=Headers())  # throws a TypeError
7. JelleZijlstra

   added a commit that references this issue on Jul 12, 2020

   Introduce Mapping-like protocols (#4325)

   ...

   9ad8ed3
8. srittau commented on May 18, 2021

   

   srittau

   on May 18, 2021

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   Closing as the werkzeug stubs will be removed in a few months.
9. srittau

   closed this as completedon May 18, 2021