[yashdhumane]

Passing a reversed() iterator or any non-list iterable to generate_content caused confusing errors like 'contents are required' or 'file uri and mime_type are required' instead of a clear message.

Fix converts iterables (generators, reversed, map, etc.) to a list before validation, so they work correctly.

Fixes #1272

[chrikrah]

@yashdhumane this breaks a single Part or File passed as contents, and #1272 still fails through generate_content. Pydantic models define __iter__, so list(contents) yields (field, value) tuples. And _GenerateContentParameters validates contents before t_contents runs, which turns reversed([...]) into [File()].

blocking: materialize an iterator in generate_content and generate_content_stream, sync and async, before the parameter model is built, or raise a TypeError there. The t_contents line can then go, which also clears the red mypy (3.11) check.

# probe: client.models.generate_content(model="gemini-2.5-flash", contents=x), _api_client.request mocked
# CPython 3.12.6, pydantic 2.12.0, google.genai imported from trees exported with git archive
                      base c0e9c54                             head f7f459d
reversed([Content])   ValueError: file uri and mime_type ...   ValueError: file uri and mime_type ...
generator of Content  ValueError: file uri and mime_type ...   ValueError: file uri and mime_type ...
Part(text="hi")       ok, 1 content sent                       ValueError: Unsupported content type: <class 'tuple'>

$ python -m pytest google/genai/tests/transformers -q -p no:cacheprovider
181 passed in 6.15s            # base c0e9c54
1 failed, 180 passed in 6.13s  # head f7f459d, test_file

$ mypy google/genai/           # mypy 2.4.0, CPython 3.11
Success: no issues found in 41 source files   # base c0e9c54
Found 1 error in 1 file (checked 41 source files)   # head f7f459d, _transformers.py:477 arg-type on list(contents)

# not run: the replay tests and anything outside tests/transformers

Could the #1272 test call generate_content with a mocked request, where it fails today?

[yashdhumane]

Thank you for the detailed analysis and the probe script — you were completely right.

The __iter__ check on Pydantic models was the core mistake: Part(text="hi") iterates as (field, value) tuples, breaking single-Part inputs. And the fix was in the wrong place — _GenerateContentParameters already materializes reversed([...]) before t_contents sees it.

Updated fix:

• Reverted the _transformers.py change
• Added collections.abc.Iterator / collections.abc.Generator check in all 4 generate_content / generate_content_stream methods (sync + async) before the parameter model is built
• 180 tests pass (the 1 failing test_function_with_optional_and_union_types is pre-existing and unrelated)

[chrikrah]

@yashdhumane 1754f71 sends an iterator whole on all four paths, and a single Part works again. From my perspective this can be merged. It does accept more than ContentListUnion, which still says list, so mypy flags reversed(...); widening the generated annotation is a maintainer call.

# probe: generate_content(contents=x) with _api_client request methods stubbed; Python 3.12, pydantic 2.12.0, trees from git archive
                              base c0e9c54                      head 1754f71
reversed([Content, Content])  ValueError: file uri and mime...  sync, stream, async, astream: 2 contents sent
generator of Content          ValueError: file uri and mime...  sync, stream, async, astream: 2 contents sent
Part(text="hi")               1 content sent                    1 content sent
$ mypy google/genai/   # mypy 2.4.0, CPython 3.11, head 1754f71
Success: no issues found in 41 source files
$ python -m pytest google/genai/tests/transformers google/genai/tests/models/test_gc_continuation_token.py -q -p no:cacheprovider
215 passed   # head 1754f71
215 passed   # base c0e9c54

non-blocking: the _transformers.py hunk is now a lone trailing blank line, and the title still names t_contents. No test tells the two commits apart, as the last two lines show. Can you add a mocked reversed([...]) case to test_gc_continuation_token.py?

[yashdhumane]

Added mocked reversed([...]) test case to test_gc_continuation_token.py and removed the trailing blank line from _transformers.py. 35 tests pass on Python 3.12.