[F823] Warn when variable name is referenced before assignment

ghstack-source-id: 0f31d5c2224059c7f0a8d1e1015aee9370803b42
Pull Request resolved: #277

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)]
+            """
+        ),
+    ]