Fix false-positive 'used-before-assignment' for assignments in except blocks following try blocks that return (#5506)

Author: jacobtylerwalls

ChangeLog

@@ -11,15 +11,21 @@ Release date: TBA
 ..
   Put new features here and also in 'doc/whatsnew/2.13.rst'
 
+* ``used-before-assignment`` now considers that assignments in a try block
+  may not have occurred when the except or finally blocks are executed.
+
+  Closes #85, #2615
+
 * ``used-before-assignment`` now assumes that assignments in except blocks
   may not have occurred and warns accordingly.
 
   Closes #4761
 
-* ``used-before-assignment`` now considers that assignments in a try block
-  may not have occurred when the except or finally blocks are executed.
+* When evaluating statements after an except block, ``used-before-assignment``
+  assumes that assignments in the except blocks took place if the
+  corresponding try block contained a return statement.
 
-  Closes #85, #2615
+  Closes #5500
 
 * ``used-before-assignment`` now checks names in try blocks.
 

doc/whatsnew/2.13.rst

@@ -43,15 +43,21 @@ Other Changes
 
   Closes #5323
 
+* ``used-before-assignment`` now considers that assignments in a try block
+  may not have occurred when the except or finally blocks are executed.
+
+  Closes #85, #2615
+
 * ``used-before-assignment`` now assumes that assignments in except blocks
   may not have occurred and warns accordingly.
 
   Closes #4761
 
-* ``used-before-assignment`` now considers that assignments in a try block
-  may not have occurred when the except or finally blocks are executed.
+* When evaluating statements after an except block, ``used-before-assignment``
+  assumes that assignments in the except blocks took place if the
+  corresponding try block contained a return statement.
 
-  Closes #85, #2615
+  Closes #5500
 
 * ``used-before-assignment`` now checks names in try blocks.
 

pylint/checkers/variables.py

@@ -408,7 +408,8 @@ def _has_locals_call_after_node(stmt, scope):
         "Emitted when a local variable is accessed before its assignment took place. "
         "Assignments in try blocks are assumed not to have occurred when evaluating "
         "associated except/finally blocks. Assignments in except blocks are assumed "
-        "not to have occurred when evaluating statements outside the block.",
+        "not to have occurred when evaluating statements outside the block, except "
+        "when the associated try block contains a return statement.",
     ),
     "E0602": (
         "Undefined variable %r",
@@ -656,6 +657,25 @@ def get_next_to_consume(self, node):
                     and isinstance(
                         n.statement(future=True).parent.parent, nodes.TryExcept
                     )
+                    # If the try block returns we assume that assignments in the except
+                    # handlers could have happened.
+                    and (
+                        not any(
+                            isinstance(try_statement, nodes.Return)
+                            for try_statement in n.statement(
+                                future=True
+                            ).parent.parent.body
+                        )
+                        # But not if this node is in the final block, which will
+                        # execute before the return.
+                        or (
+                            isinstance(node_statement.parent, nodes.TryFinally)
+                            and node_statement in node_statement.parent.finalbody
+                            and n.statement(future=True).parent.parent.parent.parent_of(
+                                node_statement
+                            )
+                        )
+                    )
                 )
                 or n.statement(future=True).parent.parent_of(node)
             ]

tests/functional/u/use/used_before_assignment_except_handler_for_try_with_return.py

@@ -0,0 +1,15 @@
+"""Tests for used-before-assignment with assignments in except handlers after
+try blocks with return statements.
+See: https://github.com/PyCQA/pylint/issues/5500.
+"""
+def function():
+    """Assume except blocks execute if the try block returns."""
+    try:
+        success_message = "success message"
+        return success_message
+    except ValueError:
+        failure_message = "failure message"
+    finally:
+        print(failure_message)  # [used-before-assignment]
+
+    return failure_message

tests/functional/u/use/used_before_assignment_except_handler_for_try_with_return.txt

@@ -0,0 +1 @@
+used-before-assignment:13:14:13:29:function:Using variable 'failure_message' before assignment:UNDEFINED