Feature Request: Snapshot Schema Validation

[BadAsstronaut]

Hi, thank you for creating this library!

I am using snapshottest on a project that has a level of randomization to test interdependent relationships. It would be helpful if snapshottest could validate the schema of the test result instead of the full text.

This could be achieved by adding a snapshot.assert_schema_match(format='json', test_result) or something similar.

[paulmelnikow]

This is similar to the request in #96.

I wonder if we could solve both issues by adding a parameter to assert_match that lets you pass your own comparator.

[medmunds]

I like the idea of custom comparators. It could also address the various "ignore some fields" needs (#21, #32, #102, et al).

JavaScript's jest-snapshot (the inspiration for this package) handles this problem through "asymmetric property matchers". You effectively declare a schema inline in your snapshot test assertion, by placing their any matchers within objects, arrays, and constants that define the (partial) expected shape. (Anything not specified by an any must match the snapshot exactly.)

callee is a Python package that implements something similar, intended for use with mock. I haven't tried it myself yet (just discovered it today), but it looks useful. I wonder how much work it would take to get callee's matchers working with snapshottest? Something like...

    snapshot.assert_match(my_api_response, match={
        "id": callee.Integer(),
        "history": {  # nesting allowed...
            "created_at": callee.InstanceOf(datetime),
            "edit_timestamps": callee.List(callee.Integer()),
        },
        # everything else in my_api_response
        # must exactly match the snapshot
    })