Fix pylint-dev#5500: Fix false-positive used-before-assignment for assignments in except blocks following try blocks that return

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

@@ -656,6 +656,12 @@ 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
+                    )
                 )
                 or n.statement(future=True).parent.parent_of(node)
             ]

tests/functional/u/use/used_before_assignment_issue5500.py

@@ -0,0 +1,9 @@
+"""used-before-assignment (E0601)"""
+def function():
+    """Assume except blocks execute if the try block returns."""
+    try:
+        return "success message"
+    except ValueError:
+        failure_message = "failure message"
+
+    return failure_message