Clarify why we do join(filter(None,...

libcst/metadata/scope_provider.py

@@ -219,7 +219,7 @@ def _index(self) -> int:
     def get_qualified_names_for(self, full_name: str) -> Set[QualifiedName]:
         return {
             QualifiedName(
-                ".".join(filter(None, [self.scope._name_prefix, full_name])),
+                self.scope._maybe_dotted_name(full_name),
                 QualifiedNameSource.LOCAL,
             )
         }
@@ -581,6 +581,11 @@ def accesses(self) -> Accesses:
         """Return an :class:`~libcst.metadata.Accesses` contains all accesses in current scope."""
         return Accesses(self._accesses_by_name)
 
+    # makes a dot separated name but filters out empty strings
+    def _maybe_dotted_name(self, *args: str) -> str:
+        # filter(None, ...) removes all falsey values (ie empty string)
+        return ".".join(filter(None, [self._name_prefix, *args]))
+
 
 class BuiltinScope(Scope):
     """
@@ -688,7 +693,7 @@ def __getitem__(self, name: str) -> Set[BaseAssignment]:
             return self.parent._getitem_from_self_or_parent(name)
 
     def _make_name_prefix(self) -> str:
-        return ".".join(filter(None, [self.parent._name_prefix, self.name, "<locals>"]))
+        return self.parent._maybe_dotted_name(self.name, "<locals>")
 
 
 # even though we don't override the constructor.
@@ -737,7 +742,7 @@ def _contains_in_self_or_parent(self, name: str) -> bool:
         return self.parent._contains_in_self_or_parent(name)
 
     def _make_name_prefix(self) -> str:
-        return ".".join(filter(None, [self.parent._name_prefix, self.name]))
+        return self.parent._maybe_dotted_name(self.name)
 
 
 # even though we don't override the constructor.
@@ -755,7 +760,7 @@ class ComprehensionScope(LocalScope):
     # https://www.python.org/dev/peps/pep-0572/#scope-of-the-target
 
     def _make_name_prefix(self) -> str:
-        return ".".join(filter(None, [self.parent._name_prefix, "<comprehension>"]))
+        return self.parent._maybe_dotted_name("<comprehension>")
 
 
 # Generates dotted names from an Attribute or Name node: