[F823] Warn when variable name is referenced before assignment

src/fixit/rules/pyflakes/no_reference_before_assignment.py

@@ -0,0 +1,97 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+import libcst as cst
+
+from fixit import CstLintRule, InvalidTestCase as Invalid, ValidTestCase as Valid
+
+
+class NoReferenceBeforeAssignmentRule(CstLintRule):
+ """
+ Local variables should be defined before the are referenced.
+
+ Autofix: N/A
+ """
+
+ CUSTOM_MESSAGE = ("Local variable {name} was referenced without "
+ "being defined in the current scope. Either define"
+ " it locally or use the `global` keyword to access"
+ " a global variable.")
+
+ METADATA_DEPENDENCIES = (cst.metadata.ScopeProvider,)
+
+ def __init__(self) -> None:
+ super().__init__()
+
+ def visit_Name(self, node: cst.Name) -> None:
+ try:
+ metadata = self.get_metadata(cst.metadata.ScopeProvider, node)
+ assert metadata is not None
+ accesses = metadata.accesses
+ except KeyError:
+ # Not all Names have ExpressionContext
+ return
+ for access in accesses:
+ if access.node is node and len(access.referents) == 0:
+ assert isinstance(access.node, cst.Name)
+ name = access.node.value
+ self.report(node, self.CUSTOM_MESSAGE.format(name=name))
+
+ VALID = [
+ Valid(
+ """
+ y = 2 + 3
+ """
+ ),
+ Valid(
+ """
+ a = 2
+ y = a + 3
+ """
+ ),
+ Valid(
+ """
+ a = 2
+
+ def fn():
+ return a + 3
+ """
+ ),
+ Valid(
+ """
+ a = abs(1.333)
+ """
+ ),
+ Valid(
+ """
+ a = [2**i for i in (1, 2, 3)]
+ """
+ ),
+ ]
+
+ INVALID = [
+ Invalid(
+ """
+ y = a + 3
+ a = 2
+ """
+ ),
+ Invalid(
+ """
+ def fn():
+ return a + 3
+ """
+ ),
+ Invalid(
+ """
+ a = foo(1.333)
+ """
+ ),
+ Invalid(
+ """
+ a = [2**j for i in (1, 2, 3)]
+ """
+ ),
+ ]