Not sure if there is a way to do this, though if there is I'd welcome a PR to add it to the documentation.

I don't believe --update-snapshot is implemented when using raw unitttest. Unittest doesn't seem to have a way to add new command-line options for unittest.main().

You could implement it yourself by setting snapshot_should_update = True on the snapshottest.unittest.TestCase class when you want to update snapshots. Something like this (warning, untested code):

if __name__ == "__main__":
    import snapshottest.unittest
    import sys
    argv = sys.argv
    if "--update-snapshot" in argv:
        snapshottest.unittest.TestCase.snapshot_should_update = True
        argv = argv.copy()  # (don't modify sys.argv itself)
        argv.remove("--update-snapshot")
    unittest.main(argv=argv)

It might make sense to add a new snapshottest.unitttest.main() providing this functionality, which people could use in place of unittest.main().

An alternative could be to set it in an environment variable. What do you think about that approach?

I'm of mixed opinions on using an environment variable to request snapshot updates:

• Pro: it would work with python -m unittest discover, which is another common way to invoke unittest.
• Con: it's different from all of the other test packages, which support --update-snapshot on the command line.
• Con (or Pro?): people would probably expect that environment variable to work with the other test packages then, too.

I think I'd lean toward providing the unittest.main() override, to parallel the other test packages. (Switching your unittest's __main__ is the equivalent of switching your Django TEST_RUNNER.) And then as a separate enhancement, also adding an environment variable supported for all the test packages. (And working out how that interacts with CI environments—I saw there was some activity around snapshot updates in CI, but haven't looked at the discussion yet.)

Of course, the other way to achieve this is deleting your existing snapshot files and re-running the tests, which will generate new snapshots. Maybe just document that for unittest?

the other way to achieve this is deleting your existing snapshot files and re-running the tests, which will generate new snapshots

Ah, I see, that's going away in #73. So I guess this does need to get addressed some other way, or it'll be really hard for unittest users to get started with snapshots.

I added an environment variable option to #112 so that it doesn't make things worse for unittest users.

I think it would be better if someone who uses and understands unittest added a main override if we decide we do want one.

I'm much more familiar with Django's unittest wrapper (which has its own test runner and command-line handling), but could take a shot at a main override.

Just realized that raw unittest is probably also missing the end-of-test-session handling (delete unvisited snapshots and report summary), which are coded separately for each of the other three test frameworks.