fix: SamSegmenter crashed on every run — checkpoint=None

[leiverkus]

The SAM backend has never worked. Found by installing the sam extra locally for the first time.

The bug

SamSegmenter.segment passed checkpoint=self.checkpoint unconditionally. samgeo's constructor:

if "checkpoint" in kwargs:
    checkpoint = kwargs["checkpoint"]
    if not os.path.exists(checkpoint):   # ← None
        ...
else:
    checkpoint = download_checkpoint(model_type, checkpoint_dir, hq)

With the default checkpoint=None this takes the first branch and raises before a single mask is generated:

TypeError: stat: path should be string, bytes, os.PathLike or integer, not NoneType

Omitting the key entirely takes the else branch, which downloads the default weights for the model type — the intended behaviour.

Why it survived

tests/test_sam.py opens with pytest.importorskip("samgeo"), and CI installs neither the sam nor the cellpose extra. The only test covering this path has always skipped. v0.3.0 ships the backend as done, and nothing has exercised it since.

Where the regression test lives

In tests/test_make_segmenter.py, not beside the test it fixes. That module deliberately runs without the heavy dependencies — its existing test asserts that constructing the backends does not import samgeo/cellpose/torch. Putting the new test in test_sam.py would have made it skip in CI too, guarding nothing.

It asserts both directions: the key is absent when unset, and forwarded as a string when set.

A related gap, not fixed here

Installing the extras un-skips two tests that CI never runs. That is a standing blind spot rather than a one-off: any breakage in the SAM or Cellpose path is invisible until someone installs several GB of torch by hand. Worth deciding separately whether CI should carry that cost, or whether these backends need smoke tests that stub the heavy call.

Verification

ruff clean, mypy clean on 26 files, test_make_segmenter.py 4 passed in 0.01 s — with the extras absent, which is the condition CI runs under.

🤖 Generated with Claude Code