perf: cache session snapshot extension discovery

src/syrupy/session.py

@@ -1,7 +1,9 @@
+from collections import defaultdict
 from pathlib import Path
 from typing import (
     TYPE_CHECKING,
     Any,
+    DefaultDict,
     Dict,
     Iterable,
     List,
@@ -34,6 +36,10 @@ class SnapshotSession:
     _assertions: List["SnapshotAssertion"] = attr.ib(factory=list)
     _extensions: Dict[str, "AbstractSyrupyExtension"] = attr.ib(factory=dict)
 
+    _locations_discovered: DefaultDict[str, Set[Any]] = attr.ib(
+        factory=lambda: defaultdict(set)
+    )
+
     @property
     def update_snapshots(self) -> bool:
         return bool(self._pytest_session.config.option.update_snapshots)
@@ -55,6 +61,7 @@ def start(self) -> None:
         self._selected_items = {}
         self._assertions = []
         self._extensions = {}
+        self._locations_discovered = defaultdict(set)
 
     def ran_item(self, nodeid: str) -> None:
         self._selected_items[nodeid] = True
@@ -80,12 +87,17 @@ def finish(self) -> int:
 
     def register_request(self, assertion: "SnapshotAssertion") -> None:
         self._assertions.append(assertion)
-        discovered_extensions = {
-            discovered.location: assertion.extension
-            for discovered in assertion.extension.discover_snapshots()
-            if discovered.has_snapshots
-        }
-        self._extensions.update(discovered_extensions)
+
+        test_location = assertion.extension.test_location.filepath
+        extension_class = assertion.extension.__class__
+        if extension_class not in self._locations_discovered[test_location]:
+            self._locations_discovered[test_location].add(extension_class)
+            discovered_extensions = {
+                discovered.location: assertion.extension
+                for discovered in assertion.extension.discover_snapshots()
+                if discovered.has_snapshots
+            }
+            self._extensions.update(discovered_extensions)
 
     def remove_unused_snapshots(
         self,