Werkzeug `EnvironHeaders` is not considered a `Mapping`

[libre-man]

It seems that the EnvironHeaders class of werkzeug (and probably the base class Headers too) is not compatible with Mapping, however it looks like it should be compatible (it implements all needed methods, and functions like one).

[srittau]

Do you have an example that you think should type check but doesn't?

[libre-man]

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>]".

[srittau]

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.

[gward]

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.

[srittau]

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

[srittau]

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