HttpRequest.accepted_types incorrectly splits Accept header on commas inside quoted parameter values

[Naveed Qadir]

The accepted_types property in HttpRequest uses str.split(",") to parse the Accept header, which incorrectly splits on commas that appear inside quoted parameter values.

Example

# Accept header with quoted parameter containing comma
header = 'text/plain; param="a,b", application/json'

# Current behavior (WRONG):
header.split(",")
# Returns: ['text/plain; param="a', 'b"', ' application/json']
# 3 parts - comma inside quotes was incorrectly treated as separator

# Expected behavior (per RFC 7231):
# Should return 2 media types:
# 1. text/plain; param="a,b"
# 2. application/json

RFC Reference

RFC 7231 Section 5.3.2 specifies that media-type parameters can contain quoted-string values, and RFC 7230 Section 3.2.6 allows commas within quoted strings.

Proposed Fix

Add a split_header_words() helper function to django/utils/http.py that splits on commas while respecting quoted strings, similar to how _parseparam() handles semicolons.

A patch with tests is available.

[Jacob Walls]

Do you have an example of real-world HTTP traffic that sends params like that for the accept header?

Looking at the ​provided patch, this is too much complexity for the benefit. I'd also expect to block this on a resolution for #35440, with the hope that we can leverage some existing pattern for param parsing using python's stdlib.

[Naveed Qadir]

Thanks for the feedback and for pointing me to #35440 — I agree that reusing a stdlib-based approach would be preferable if we can do so without performance regressions.
Regarding real-world usage: I’m not aware of common browsers or clients emitting Accept headers with quoted parameters containing commas or escaped quotes. The change was motivated by spec-permitted behavior and to avoid incorrect parsing when such headers do appear, but I agree this is rare in practice.
Given the complexity concerns and the direction of #35440, I’m happy to defer this and continue the discussion there.
Replying to Jacob Walls:

Do you have an example of real-world HTTP traffic that sends params like that for the accept header?

Looking at the ​provided patch, this is too much complexity for the benefit. I'd also expect to block this on a resolution for #35440, with the hope that we can leverage some existing pattern for param parsing using python's stdlib.