Delayed initialization of matchers

To support multiprocessing on Windows/macOS
Issue #1181

libcst/matchers/_visitors.py

@@ -265,17 +265,23 @@ def _check_types(
             )
 
 
-def _gather_matchers(obj: object) -> Set[BaseMatcherNode]:
-    visit_matchers: Set[BaseMatcherNode] = set()
+def _gather_matchers(obj: object) -> Dict[BaseMatcherNode, Optional[cst.CSTNode]]:
+    """
+    Set of gating matchers that we need to track and evaluate. We use these
+    in conjunction with the call_if_inside and call_if_not_inside decorators
+    to determine whether to call a visit/leave function.
+    """
+
+    visit_matchers: Dict[BaseMatcherNode, Optional[cst.CSTNode]] = {}
 
     for func in dir(obj):
         try:
             for matcher in getattr(getattr(obj, func), VISIT_POSITIVE_MATCHER_ATTR, []):
-                visit_matchers.add(cast(BaseMatcherNode, matcher))
+                visit_matchers[cast(BaseMatcherNode, matcher)] = None
             for matcher in getattr(getattr(obj, func), VISIT_NEGATIVE_MATCHER_ATTR, []):
-                visit_matchers.add(cast(BaseMatcherNode, matcher))
+                visit_matchers[cast(BaseMatcherNode, matcher)] = None
         except Exception:
-            # This could be a caculated property, and calling getattr() evaluates it.
+            # This could be a calculated property, and calling getattr() evaluates it.
             # We have no control over the implementation detail, so if it raises, we
             # should not crash.
             pass
@@ -366,6 +372,8 @@ def _visit_matchers(
     node: cst.CSTNode,
     metadata_resolver: cst.MetadataDependent,
 ) -> Dict[BaseMatcherNode, Optional[cst.CSTNode]]:
+    # Initialize matchers if they are empty
+    matchers = matchers or _gather_matchers(metadata_resolver)
     new_matchers: Dict[BaseMatcherNode, Optional[cst.CSTNode]] = {}
     for matcher, existing_node in matchers.items():
         # We don't care about visiting matchers that are already true.
@@ -448,12 +456,8 @@ class MatcherDecoratableTransformer(CSTTransformer):
 
     def __init__(self) -> None:
         CSTTransformer.__init__(self)
-        # List of gating matchers that we need to track and evaluate. We use these
-        # in conjuction with the call_if_inside and call_if_not_inside decorators
-        # to determine whether or not to call a visit/leave function.
-        self._matchers: Dict[BaseMatcherNode, Optional[cst.CSTNode]] = {
-            m: None for m in _gather_matchers(self)
-        }
+        # Do not initialize matchers yet to support multiprocessing
+        self._matchers = None
         # Mapping of matchers to functions. If in the course of visiting the tree,
         # a node matches one of these matchers, the corresponding function will be
         # called as if it was a visit_* method.
@@ -660,12 +664,8 @@ class MatcherDecoratableVisitor(CSTVisitor):
 
     def __init__(self) -> None:
         CSTVisitor.__init__(self)
-        # List of gating matchers that we need to track and evaluate. We use these
-        # in conjuction with the call_if_inside and call_if_not_inside decorators
-        # to determine whether or not to call a visit/leave function.
-        self._matchers: Dict[BaseMatcherNode, Optional[cst.CSTNode]] = {
-            m: None for m in _gather_matchers(self)
-        }
+        # Do not initialize matchers yet to support multiprocessing
+        self._matchers = None
         # Mapping of matchers to functions. If in the course of visiting the tree,
         # a node matches one of these matchers, the corresponding function will be
         # called as if it was a visit_* method.