fix: __get_headers mutates caller's config dict on multipart requests

[SiddarthAA]

Problem

So every service class (Audio, Translate, Vision, etc.) creates a single RequestConfig / AsyncRequestConfig in its __init__ and reuses it for every method call on that instance.

The bug was in how Request.__init__ (and the async version) was setting headers:

self.headers = config.get("headers", None) or {"Content-Type": "application/json"}

This isn't copying anything — it's just storing a reference to the same dict object that the service class holds onto. So when __get_headers() does:

self.headers.pop("Content-Type")

...during a multipart upload, it's actually wiping Content-Type out of the shared config for good. Every call after that — JSON POSTs, GETs, whatever — goes out without Content-Type: application/json and you start getting silent 400/422s from the server.

The really annoying part is it only breaks on the second call, so it's easy to miss in basic tests and a pain to track down.

Same issue in both request.py and async_request.py.

Fix

I went with a two-level defence in both files so there's no way either the config or self.headers gets touched:

1. __init__ — own the dict

   raw_headers = config.get("headers", None) or {"Content-Type": "application/json"}
   self.headers: Dict[str, str] = dict(raw_headers)

   Now self.headers is its own copy — whatever happens to it later, the caller's config is safe.

2. __get_headers() — throwaway per-call copy

   caller_headers = dict(self.headers)
   if self.files:
       caller_headers.pop("Content-Type", None)
   h.update(caller_headers)

   .pop() only touches a local copy made fresh each call, so self.headers stays intact.

Also cleaned up the leftover headers.pop("Content-Type", None) in make_request() in both files — __get_headers() already handles that, so it was just dead code sitting there.

Tests

Added test_headers_mutation.py — 12 tests, no API key or network needed:

Test	Covers
test_multipart_does_not_mutate_config_headers	Config dict is untouched after a multipart call
test_repeated_multipart_calls_do_not_degrade	Calling __get_headers() twice returns identical results
test_multipart_output_omits_content_type	Outgoing multipart headers correctly omit Content-Type so the HTTP library can set the boundary
test_json_request_includes_content_type	Plain JSON requests still carry Content-Type: application/json
test_custom_headers_propagated	Caller-supplied extra headers reach the outgoing dict
test_second_request_on_same_config_unaffected	A second Request built from the same config after a multipart call has correct headers — the original regression scenario

All 6 run for both Request (sync) and AsyncRequest (async) — 12/12 pass.

Files changed

• request.py
• async_request.py
• test_headers_mutation.py (new)

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Contact

Thanks for reviewing — happy to fix something / reiterate if needed :D
siddartha_ay@protonmail.com | (everything about me at siddartha-ay.xyz )