snapshot stored value will be changed after assert_match

[lucemia]

e.g.

test case

def test_f(snapshot):
    x = {}
    snapshot.assert_match(x)  # should be {}

    x["a"] = 1
    snapshot.assert_match(x)

snapshot

snapshots['test_f 1'] = {
    'a': 1  # <-- wrong value
}

snapshots['test_f 2'] = {
    'a': 1
}

[paulmelnikow]

Would welcome a PR to fix this! Not sure whether that will be easy or not.

[medmunds]

This looks like it will be non-trivial to fix.

The problem is that during updating, SnapshotTest.assert_match just stores a simple reference to the value it's passed. The value isn't actually serialized into a snapshot until later (after all tests in a particular module). So if anything modifies that value in between, the snapshot will be written with the wrong value.

Couple of options:

• Store a deep copy of the value in assert_match. (This seems simple, but there are a lot of corner cases, and I'm not even sure it's possible for all the value types supported.)
• Serialize the snapshot immediately in SnapshotTest.store (or BaseFormatter.store). I think this is the right fix, but it involves a substantial reworking of the "formatters" code, and how they are later combined into a single snapshot file (merging imports, etc.)

[paulmelnikow]

Thanks for looking into this.

Serialize the snapshot immediately in SnapshotTest.store (or BaseFormatter.store). I think this is the right fix, but it involves a substantial reworking of the "formatters" code, and how they are later combined into a single snapshot file (merging imports, etc.)

Agree that this would be the way to go.