[UNKNOWN H4CK3R (unknownhackerworld)]

Overview

HeadersCollection.add() now replaces the value of a single value header (content-type, content-encoding, content-length) when the value is passed as a string, the same way it already does when the value is passed as a list.

Before this change:

headers = HeadersCollection()
headers.add("Content-Type", "application/json")
headers.add("Content-Type", "text/plain")
headers.get("content-type")  # {'application/json', 'text/plain'}

After: {'text/plain'}.

Related Issue

Fixes #818

Notes

Root cause: in packages/abstractions/kiota_abstractions/headers_collection.py, add() has two branches. The list branch checks self.SINGLE_VALUE_HEADERS and replaces the set. The string branch skips that check and goes straight to values.add(header_values) on the existing set. add_all() calls add(key, value) with one string at a time, so merging collections hits the same branch.

Fix: wrap a string value in a one-item list at the top of add(), so both kinds of input go through one path that applies SINGLE_VALUE_HEADERS. The last value wins for single value headers. This matches kiota-dotnet's RequestHeaders.Add, which replaces single value headers no matter how many values are passed. It also removes the duplicated string branch, so add() is shorter than before.

Tests: test_add_single_value_header_to_existing_header used a subset check ({"value2"} <= headers.get("content-type")), so it passed while both values were kept. I changed it to an equality check, and added test_add_all_replaces_single_value_header for the add_all() path. Both fail on main and pass with this change.

Tradeoffs and alternatives:

• try_add() is left as is. It already refuses to overwrite an existing header, which is its documented behavior.
• I first added the SINGLE_VALUE_HEADERS check to the string branch as a one-line fix. That worked but pushed add() over SonarCloud's cognitive complexity limit (16 vs 15) and kept two copies of the same logic, so I normalized the input instead.
• Behavior change: code that relied on add() keeping two Content-Type values would now see only the last one. The class name and existing test say single value headers should hold one value, so this looks like the intended behavior.

AI disclosure: this change was found and written with help from an AI coding assistant.

Testing Instructions

• cd packages/abstractions && pip install -e . pytest pytest-asyncio
• pytest tests/test_request_header.py: 13 passed (2 fail on main)
• pytest tests: 158 passed
• yapf --diff and mypy on kiota_abstractions/headers_collection.py are clean

[UNKNOWN H4CK3R (unknownhackerworld)]

@microsoft-github-policy-service agree

[Copilot]

🟢 Approval recommended

The focused fix preserves existing list and multi-value behavior, with regression coverage for both affected paths.

No open issues 🎉

_{Review effort: Balanced}

[Vincent Biret (baywet)]

Thanks for the contribution!

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud