Fix false negative for used-before-assignment when some except handlers don't define a name

ChangeLog

@@ -210,6 +210,12 @@ Release date: TBA
 
   Closes #5500
 
+* Fixed a false negative for ``used-before-assignment`` when some but not all
+  except handlers defined a name relied upon after an except block when the
+  corresponding try block contained a return statement.
+
+  Partially closes #5524
+
 * When evaluating statements in the ``else`` clause of a loop, ``used-before-assignment``
   assumes that assignments in the except blocks took place if the
   except handlers constituted the only ways for the loop to finish without

doc/whatsnew/2.13.rst

@@ -251,6 +251,12 @@ Other Changes
 
   Closes #5500
 
+* Fixed a false negative for ``used-before-assignment`` when some but not all
+  except handlers defined a name relied upon after an except block when the
+  corresponding try block contained a return statement.
+
+  Partially closes #5524
+
 * When evaluating statements in the ``else`` clause of a loop, ``used-before-assignment``
   assumes that assignments in the except blocks took place if the
   except handlers constituted the only ways for the loop to finish without

pylint/checkers/variables.py

@@ -703,17 +703,18 @@ def _uncertain_nodes_in_except_blocks(
         for other_node in found_nodes:
             other_node_statement = other_node.statement(future=True)
             # Only testing for statements in the except block of TryExcept
-            if not (
-                isinstance(other_node_statement.parent, nodes.ExceptHandler)
-                and isinstance(other_node_statement.parent.parent, nodes.TryExcept)
-            ):
+            closest_except_handler = utils.get_node_first_ancestor_of_type(
+                other_node_statement, nodes.ExceptHandler
+            )
+            if not closest_except_handler:
                 continue
             # If the other node is in the same scope as this node, assume it executes
-            if other_node_statement.parent.parent_of(node):
+            if closest_except_handler.parent_of(node):
                 continue
+            closest_try_except: nodes.TryExcept = closest_except_handler.parent
             try_block_returns = any(
                 isinstance(try_statement, nodes.Return)
-                for try_statement in other_node_statement.parent.parent.body
+                for try_statement in closest_try_except.body
             )
             # If the try block returns, assume the except blocks execute.
             if try_block_returns:
@@ -722,28 +723,56 @@ def _uncertain_nodes_in_except_blocks(
                 if (
                     isinstance(node_statement.parent, nodes.TryFinally)
                     and node_statement in node_statement.parent.finalbody
-                    # We have already tested that other_node_statement has two parents
-                    # and it was TryExcept, so getting one more parent is safe.
-                    and other_node_statement.parent.parent.parent.parent_of(
-                        node_statement
-                    )
+                    and closest_try_except.parent.parent_of(node_statement)
                 ):
                     uncertain_nodes.append(other_node)
-                else:
-                    # Assume the except blocks execute. Possibility for a false negative
-                    # if one of the except blocks does not define the name in question,
-                    # raise, or return. See: https://github.com/PyCQA/pylint/issues/5524.
+                # Assume the except blocks execute, so long as each handler
+                # defines the name, raises, or returns.
+                elif all(
+                    NamesConsumer._defines_name_raises_or_returns(node.name, handler)
+                    for handler in closest_try_except.handlers
+                ):
                     continue
 
-            if NamesConsumer._check_loop_finishes_via_except(
-                node, other_node_statement.parent.parent
-            ):
+            if NamesConsumer._check_loop_finishes_via_except(node, closest_try_except):
                 continue
 
             # Passed all tests for uncertain execution
             uncertain_nodes.append(other_node)
         return uncertain_nodes
 
+    @staticmethod
+    def _defines_name_raises_or_returns(
+        name: str, handler: nodes.ExceptHandler
+    ) -> bool:
+        """Return True if some child of `handler` defines the name `name`,
+        raises, or returns.
+        """
+        for stmt in handler.get_children():
+            if isinstance(stmt, (nodes.Raise, nodes.Return)):
+                return True
+            if isinstance(stmt, nodes.Assign):
+                for target in stmt.targets:
+                    for elt in utils.get_all_elements(target):
+                        if isinstance(elt, nodes.AssignName) and elt.name == name:
+                            return True
+            if isinstance(stmt, nodes.If):
+                if (
+                    isinstance(stmt.test, nodes.NamedExpr)
+                    and stmt.test.target.name == name
+                ):
+                    return True
+                if isinstance(stmt.test, nodes.Call):
+                    for arg_or_kwarg in stmt.test.args + [
+                        kw.value for kw in stmt.test.keywords
+                    ]:
+                        if (
+                            isinstance(arg_or_kwarg, nodes.NamedExpr)
+                            and arg_or_kwarg.target.name == name
+                        ):
+                            return True
+        return False
+
     @staticmethod
     def _check_loop_finishes_via_except(
         node: nodes.NodeNG, other_node_try_except: nodes.TryExcept

tests/functional/u/use/used_before_assignment_except_handler_for_try_with_return.py

@@ -49,3 +49,66 @@ def func_ok3(var):
     except ZeroDivisionError:
         msg = "Division by 0"
     print(msg)
+
+
+def func_ok4(var):
+    """Define "msg" with a chained assignment."""
+    try:
+        return 1 / var.some_other_func()
+    except AttributeError:
+        msg2 = msg = "Division by 0"
+        print(msg2)
+    except ZeroDivisionError:
+        msg = "Division by 0"
+    print(msg)
+
+
+def func_ok5(var):
+    """Define 'msg' via unpacked iterable."""
+    try:
+        return 1 / var.some_other_func()
+    except AttributeError:
+        msg, msg2 = ["Division by 0", "Division by 0"]
+        print(msg2)
+    except ZeroDivisionError:
+        msg = "Division by 0"
+    print(msg)
+
+
+def func_invalid1(var):
+    """'msg' is not defined in one handler."""
+    try:
+        return 1 / var.some_other_func()
+    except AttributeError:
+        pass
+    except ZeroDivisionError:
+        msg = "Division by 0"
+    print(msg)  # [used-before-assignment]
+
+
+def func_invalid2(var):
+    """'msg' is not defined in one handler."""
+    try:
+        return 1 / var.some_other_func()
+    except AttributeError:
+        msg: str
+    except ZeroDivisionError:
+        msg = "Division by 0"
+    print(msg)  # [used-before-assignment]
+
+
+def func_invalid3(var):
+    """'msg' is not defined in one handler, but is defined in another
+    nested under an if.
+    """
+    err_message = False
+    try:
+        return 1 / var.some_other_func()
+    except AttributeError:
+        msg: str
+    except ZeroDivisionError:
+        if err_message:
+            msg = "Division by 0"
+        else:
+            msg = None
+    print(msg)  # [used-before-assignment]

tests/functional/u/use/used_before_assignment_except_handler_for_try_with_return.txt

@@ -1 +1,4 @@
 used-before-assignment:16:14:16:29:function:Using variable 'failure_message' before assignment:CONTROL_FLOW
+used-before-assignment:86:10:86:13:func_invalid1:Using variable 'msg' before assignment:CONTROL_FLOW
+used-before-assignment:97:10:97:13:func_invalid2:Using variable 'msg' before assignment:CONTROL_FLOW
+used-before-assignment:114:10:114:13:func_invalid3:Using variable 'msg' before assignment:CONTROL_FLOW

tests/functional/u/use/used_before_assignment_except_handler_for_try_with_return_py38.py

@@ -0,0 +1,46 @@
+"""Tests for used-before-assignment with assignments in except handlers after
+try blocks with return statements.
+See: https://github.com/PyCQA/pylint/issues/5500.
+"""
+# pylint: disable=inconsistent-return-statements
+
+
+# Named expressions
+def func_ok_namedexpr_1(var):
+    """'msg' is defined in one handler with a named expression under an if."""
+    try:
+        return 1 / var.some_other_func()
+    except AttributeError:
+        if (msg := var.get_msg()):
+            pass
+    except ZeroDivisionError:
+        msg = "Division by 0"
+    print(msg)
+
+
+def func_ok_namedexpr_2(var):
+    """'msg' is defined in one handler with a named expression occurring
+    in a call used in an if test.
+    """
+    try:
+        return 1 / var.some_other_func()
+    except AttributeError:
+        if print(msg := var.get_msg()):
+            pass
+    except ZeroDivisionError:
+        msg = "Division by 0"
+    print(msg)
+
+
+def func_ok_namedexpr_3(var):
+    """'msg' is defined in one handler with a named expression occurring
+    as a keyword in a call used in an if test.
+    """
+    try:
+        return 1 / var.some_other_func()
+    except AttributeError:
+        if print("zero!", "here", sep=(msg := var.get_sep())):
+            pass
+    except ZeroDivisionError:
+        msg = "Division by 0"
+    print(msg)

tests/functional/u/use/used_before_assignment_except_handler_for_try_with_return_py38.rc

@@ -0,0 +1,2 @@
+[testoptions]
+min_pyver=3.8