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Example Code

from pprint import pprint
import fastapi

response = fastapi.Response()
response.delete_cookie("cookie1")
response.delete_cookie("cookie2")

pprint(list(response.headers.items()))
# [('content-length', '0'),
#  ('set-cookie',
#   'cookie1=""; expires=Thu, 07 Nov 2024 08:26:12 GMT; Max-Age=0; Path=/; '
#   'SameSite=lax'),
#  ('set-cookie',
#   'cookie2=""; expires=Thu, 07 Nov 2024 08:26:12 GMT; Max-Age=0; Path=/; '
#   'SameSite=lax')]

headers = fastapi.datastructures.Headers(
    raw=[(b"set-cookie", x.encode()) for x in response.headers.getlist("set-cookie")]
)

exc = fastapi.HTTPException(400, headers=headers)
# Argument of type "Headers" cannot be assigned to parameter "headers" of type "Dict[str, str] | None" in function "__init__"
#   Type "Headers" is not assignable to type "Dict[str, str] | None"
#     "Headers" is not assignable to "Dict[str, str]"
#     "Headers" is not assignable to "None"PylancereportArgumentType

Description

HTTP Headers cannot be represented by a dict, as they can have duplicate entries, as shown in the example: two set-cookie headers.

As such type of fastapi.HTTPException's headers field should be typing.Mapping instead of dict.

In the example I tried assigning fastapi.datastructures.Headers to the parameter headers. It worked at runtime, but pyright is screaming about wrong types in the editor and CLI.

fastapi.Request's headers field already has the type MutableHeaders, which is a distant descendant of typing.Mapping already.

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.111.1

Pydantic Version

2.8.2

Python Version

Python 3.12.4

Additional Context

No response