Fix false positive for 'used-before-assignment' with terminating func

doc/whatsnew/fragments/7563.false_positive

@@ -0,0 +1,3 @@
+Fix ``used-before-assignment`` false positive when else branch calls ``sys.exit`` or similar terminating functions.
+
+Closes #7563

pylint/checkers/base/basic_checker.py

@@ -668,27 +668,6 @@ def _check_misplaced_format_function(self, call_node: nodes.Call) -> None:
             ):
                 self.add_message("misplaced-format-function", node=call_node)
 
-    @staticmethod
-    def _is_terminating_func(node: nodes.Call) -> bool:
-        """Detect call to exit(), quit(), os._exit(), or sys.exit()."""
-        if (
-            not isinstance(node.func, nodes.Attribute)
-            and not (isinstance(node.func, nodes.Name))
-            or isinstance(node.parent, nodes.Lambda)
-        ):
-            return False
-
-        qnames = {"_sitebuiltins.Quitter", "sys.exit", "posix._exit", "nt._exit"}
-
-        try:
-            for inferred in node.func.infer():
-                if hasattr(inferred, "qname") and inferred.qname() in qnames:
-                    return True
-        except (StopIteration, astroid.InferenceError):
-            pass
-
-        return False
-
     @utils.only_required_for_messages(
         "eval-used",
         "exec-used",
@@ -698,7 +677,7 @@ def _is_terminating_func(node: nodes.Call) -> bool:
     )
     def visit_call(self, node: nodes.Call) -> None:
         """Visit a Call node."""
-        if self._is_terminating_func(node):
+        if utils.is_terminating_func(node):
             self._check_unreachable(node, confidence=INFERENCE)
         self._check_misplaced_format_function(node)
         if isinstance(node.func, nodes.Name):

pylint/checkers/utils.py

@@ -2138,3 +2138,24 @@ def is_singleton_const(node: nodes.NodeNG) -> bool:
     return isinstance(node, nodes.Const) and any(
         node.value is value for value in SINGLETON_VALUES
     )
+
+
+def is_terminating_func(node: nodes.Call) -> bool:
+    """Detect call to exit(), quit(), os._exit(), or sys.exit()."""
+    if (
+        not isinstance(node.func, nodes.Attribute)
+        and not (isinstance(node.func, nodes.Name))
+        or isinstance(node.parent, nodes.Lambda)
+    ):
+        return False
+
+    qnames = {"_sitebuiltins.Quitter", "sys.exit", "posix._exit", "nt._exit"}
+
+    try:
+        for inferred in node.func.infer():
+            if hasattr(inferred, "qname") and inferred.qname() in qnames:
+                return True
+    except (StopIteration, astroid.InferenceError):
+        pass
+
+    return False

pylint/checkers/variables.py

@@ -713,7 +713,14 @@ def _uncertain_nodes_in_except_blocks(
                 isinstance(else_statement, nodes.Return)
                 for else_statement in closest_try_except.orelse
             )
-            if try_block_returns or else_block_returns:
+            else_block_exits = any(
+                isinstance(else_statement, nodes.Expr)
+                and isinstance(else_statement.value, nodes.Call)
+                and utils.is_terminating_func(else_statement.value)
+                for else_statement in closest_try_except.orelse
+            )
+
+            if try_block_returns or else_block_returns or else_block_exits:
                 # Exception: if this node is in the final block of the other_node_statement,
                 # it will execute before returning. Assume the except statements are uncertain.
                 if (

tests/functional/u/used/used_before_assignment_else_return.py

@@ -1,5 +1,6 @@
 """If the else block returns, it is generally safe to rely on assignments in the except."""
-
+# pylint: disable=missing-function-docstring, invalid-name
+import sys
 
 def valid():
     """https://github.com/PyCQA/pylint/issues/6790"""
@@ -59,3 +60,15 @@ def invalid_4():
     else:
         print(error)  # [used-before-assignment]
         return
+
+def valid_exit():
+    try:
+        pass
+    except SystemExit as e:
+        lint_result = e.code
+    else:
+        sys.exit("Bad")
+    if lint_result != 0:
+        sys.exit("Error is 0.")
+
+    print(lint_result)

tests/functional/u/used/used_before_assignment_else_return.txt

@@ -1,4 +1,4 @@
-used-before-assignment:25:14:25:19:invalid:Using variable 'error' before assignment:CONTROL_FLOW
-used-before-assignment:38:14:38:19:invalid_2:Using variable 'error' before assignment:CONTROL_FLOW
-used-before-assignment:50:14:50:19:invalid_3:Using variable 'error' before assignment:CONTROL_FLOW
-used-before-assignment:60:14:60:19:invalid_4:Using variable 'error' before assignment:CONTROL_FLOW
+used-before-assignment:26:14:26:19:invalid:Using variable 'error' before assignment:CONTROL_FLOW
+used-before-assignment:39:14:39:19:invalid_2:Using variable 'error' before assignment:CONTROL_FLOW
+used-before-assignment:51:14:51:19:invalid_3:Using variable 'error' before assignment:CONTROL_FLOW
+used-before-assignment:61:14:61:19:invalid_4:Using variable 'error' before assignment:CONTROL_FLOW