fix: prevent TogetherException repr crash on non-JSON-serializable headers

[giulio-leone]

Problem

TogetherException.__repr__ uses json.dumps() to serialize the exception's attributes (including headers) into a JSON string. When headers contains a non-JSON-serializable object like aiohttp's CIMultiDictProxy, this raises a TypeError:

TypeError: Object of type CIMultiDictProxy is not JSON serializable

This makes it impossible to print(), repr(), or log the exception — the debugging tool itself crashes.

Root Cause

json.dumps() in __repr__ has no fallback for non-serializable types:

def __repr__(self) -> str:
    repr_message = json.dumps(
        {
            "response": self._message,
            "status": self.http_status,
            "request_id": self.request_id,
            "headers": self.headers,  # ← CIMultiDictProxy crashes here
        }
    )

Fix

Add default=str to json.dumps() so non-serializable objects fall back to their string representation:

repr_message = json.dumps(
    {...},
    default=str,  # ← fallback for non-serializable objects
)

This is the standard Python pattern for graceful JSON serialization of arbitrary objects.

Tests

Added 7 unit tests covering:

• Normal dict headers (regression)
• CIMultiDictProxy-like headers (the reported bug)
• None headers (default case)
• String headers
• Nested non-serializable values
• Valid output format verification
• All exception subclasses inherit the fix

Verified Locally

OLD CODE CRASHES: Object of type FakeMultiDictProxy is not JSON serializable
NEW CODE: OK -> {"headers": "<FakeMultiDictProxy({'Content-Type': 'application/json'})>"}

All 7 new tests pass. All 181 existing unit tests continue to pass.

Fixes #108

---

Note

Low Risk
Low risk: change is confined to exception stringification and adds tests; behavior only affects how errors are represented/logged when headers are non-JSON-serializable.

Overview
Fixes TogetherException.__repr__ to never crash while logging/printing errors when headers contains non-JSON-serializable types (e.g., CIMultiDictProxy).

Adds _json_safe_headers plus json.dumps(..., default=str) to safely stringify/normalize headers, and introduces a focused unit test suite covering dict/None/string headers, CIMultiDictProxy, nested non-serializable values, and subclass behavior.

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[giulio-leone]

Friendly ping — CI is green, tests pass, ready for review whenever convenient. Happy to address any feedback. Thanks! 🙏

[giulio-leone]

Friendly ping — this branch has been refreshed on the latest upstream base and all current review feedback has been addressed. It should be ready for review whenever you have a chance. Happy to make any further changes quickly.

[giulio-leone]

Friendly ping — rebased on latest and ready for review. Happy to address any feedback!

[giulio-leone]

Refreshed this branch on the latest main and strengthened the fix a bit further.

What changed in this follow-up:

• kept mapping-like headers structured in TogetherException.__repr__ instead of falling back to an opaque string when the object is something like CIMultiDictProxy
• replaced the fake proxy-only regression with a real multidict.CIMultiDictProxy repro so the test now matches the actual async header shape from the reported issue

Validation (double pass):

• PYTHONPATH="$wt/src" python -m ruff check src/together/error.py tests/unit/test_error_repr.py
• PYTHONPATH="$wt/src" python -m pytest tests/unit/test_error_repr.py tests/unit/test_client.py tests/unit/test_async_client.py

Runtime proof (double pass):

• reproduced TogetherException(message="boom", headers=CIMultiDictProxy(...), http_status=500, request_id="req_123")
• confirmed repr(exc) now renders successfully as structured JSON with headers preserved as an object, e.g. {\"content-type\": \"application/json\"}