feat: allow extensions to override snapshot equality check

README.md

@@ -288,6 +288,7 @@ See examples of how syrupy can be used and extended in the [test examples](https
 - [Custom snapshot directory](https://github.com/tophat/syrupy/tree/master/tests/examples/test_custom_snapshot_directory.py)
 - [Custom snapshot name](https://github.com/tophat/syrupy/tree/master/tests/examples/test_custom_snapshot_name.py)
 - [Custom object snapshots](https://github.com/tophat/syrupy/tree/master/tests/examples/test_custom_object_repr.py)
+- [Custom comparator](https://github.com/tophat/syrupy/tree/master/tests/integration/test_custom_comparator.py)
 - [JPEG image extension](https://github.com/tophat/syrupy/tree/master/tests/examples/test_custom_image_extension.py)
 - [Built-in image extensions](https://github.com/tophat/syrupy/blob/master/tests/syrupy/extensions/image/test_image_svg.py)
 

src/syrupy/assertion.py

@@ -189,7 +189,9 @@ def _assert(self, data: "SerializableData") -> bool:
         try:
             snapshot_data = self._recall_data()
             serialized_data = self._serialize(data)
-            matches = snapshot_data is not None and serialized_data == snapshot_data
+            matches = snapshot_data is not None and self.extension.matches(
+                serialized_data=serialized_data, snapshot_data=snapshot_data
+            )
             assertion_success = matches
             if not matches and self._update_snapshots:
                 self.extension.write_snapshot(

src/syrupy/extensions/base.py

@@ -378,7 +378,23 @@ def __strip_ends(self, line: str) -> str:
         return line.rstrip("".join(self._ends.keys()))
 
 
-class AbstractSyrupyExtension(SnapshotSerializer, SnapshotFossilizer, SnapshotReporter):
+class SnapshotComparator(ABC):
+    def matches(
+        self,
+        *,
+        serialized_data: "SerializableData",
+        snapshot_data: "SerializableData",
+    ) -> bool:
+        """
+        Compares serialized data and snapshot data and returns
+        whether they match.
+        """
+        return bool(serialized_data == snapshot_data)
+
+
+class AbstractSyrupyExtension(
+    SnapshotSerializer, SnapshotFossilizer, SnapshotReporter, SnapshotComparator
+):
     def __init__(self, test_location: "PyTestLocation"):
         self._test_location = test_location
 

tests/integration/test_custom_comparator.py

@@ -0,0 +1,84 @@
+import pytest
+
+
+@pytest.fixture
+def testcases_initial(testdir):
+    testdir.makeconftest(
+        """
+        import pytest
+        import math
+
+        from syrupy.extensions.amber import AmberSnapshotExtension
+
+        class CustomSnapshotExtension(AmberSnapshotExtension):
+            def matches(self, *, serialized_data, snapshot_data):
+                try:
+                    a = float(serialized_data)
+                    b = float(snapshot_data)
+                    return math.isclose(a, b, rel_tol=1e-5)
+                except:
+                    return False
+
+        @pytest.fixture
+        def snapshot_custom(snapshot):
+            return snapshot.use_extension(CustomSnapshotExtension)
+        """
+    )
+    return {
+        "passed": (
+            """
+            def test_passed_custom(snapshot_custom):
+                assert snapshot_custom == 3.0
+            """
+        ),
+        "failed": (
+            """
+            def test_passed_custom(snapshot_custom):
+                # this comment is required or the test breaks
+                assert snapshot_custom == 4.0
+            """
+        ),
+    }
+
+
+@pytest.fixture
+def generate_snapshots(testdir, testcases_initial):
+    testdir.makepyfile(test_file=testcases_initial["passed"])
+    result = testdir.runpytest("-v", "--snapshot-update")
+    return result, testdir, testcases_initial
+
+
+def test_generated_snapshots(generate_snapshots):
+    result = generate_snapshots[0]
+    result.stdout.re_match_lines((r"1 snapshot generated\."))
+    assert "snapshots unused" not in result.stdout.str()
+    assert result.ret == 0
+
+
+def test_approximate_match(generate_snapshots):
+    testdir = generate_snapshots[1]
+    testdir.makepyfile(
+        test_file="""
+            def test_passed_custom(snapshot_custom):
+                assert snapshot_custom == 3.2
+            """
+    )
+    result = testdir.runpytest("-v")
+    result.stdout.re_match_lines((r"test_file.py::test_passed_custom PASSED"))
+    assert result.ret == 0
+
+
+def test_failed_snapshots(generate_snapshots):
+    testdir = generate_snapshots[1]
+    testdir.makepyfile(test_file=generate_snapshots[2]["failed"])
+    result = testdir.runpytest("-v")
+    result.stdout.re_match_lines((r"1 snapshot failed\."))
+    assert result.ret == 1
+
+
+def test_updated_snapshots(generate_snapshots):
+    _, testdir, initial = generate_snapshots
+    testdir.makepyfile(test_file=initial["failed"])
+    result = testdir.runpytest("-v", "--snapshot-update")
+    result.stdout.re_match_lines((r"1 snapshot updated\."))
+    assert result.ret == 0