Emit possibly-used-before-assignment after if/else switches

doc/whatsnew/fragments/1727.false_negative

@@ -0,0 +1,4 @@
+Emit ``used-before-assignment`` when relying on names after an ``if/else``
+switch when one branch failed to define the name, raise, or return.
+
+Closes #1727

doc/whatsnew/fragments/1727.false_negative.1

@@ -0,0 +1,19 @@
+``used-before-assignment`` is now emitted when relying on variable assignments
+that were not exhaustively made in every if/else branch.
+
+If you rely on a pattern like this:
+```
+if guarded():
+    var = 1
+
+if guarded():
+    print(var)  # now emits used-before-assignment
+```
+
+...you may be concerned that ``used-before-assignment`` is not totally useful
+in this instance. However, consider that pylint, as a static analysis tool, does
+not know if ``guarded()`` is deterministic, has side effects, or talks to
+a database. (Likewise, for ``guarded`` instead of ``guarded()``, any other
+part of your program may have changed the value in the meantime.)
+
+Closes #1727

pylint/checkers/variables.py

@@ -536,6 +536,7 @@ def __init__(self, node: nodes.NodeNG, scope_type: str) -> None:
             copy.copy(node.locals), {}, collections.defaultdict(list), scope_type
         )
         self.node = node
+        self.names_under_always_false_test = set()
 
     def __repr__(self) -> str:
         _to_consumes = [f"{k}->{v}" for k, v in self._atomic.to_consume.items()]
@@ -637,7 +638,7 @@ def get_next_to_consume(self, node: nodes.Name) -> list[nodes.NodeNG] | None:
 
         # Filter out assignments guarded by always false conditions
         if found_nodes:
-            uncertain_nodes = self._uncertain_nodes_in_false_tests(found_nodes, node)
+            uncertain_nodes = self._uncertain_nodes_if_tests(found_nodes, node)
             self.consumed_uncertain[node.name] += uncertain_nodes
             uncertain_nodes_set = set(uncertain_nodes)
             found_nodes = [n for n in found_nodes if n not in uncertain_nodes_set]
@@ -687,8 +688,9 @@ def get_next_to_consume(self, node: nodes.Name) -> list[nodes.NodeNG] | None:
 
         return found_nodes
 
-    @staticmethod
-    def _inferred_to_define_name_raise_or_return(name: str, node: nodes.NodeNG) -> bool:
+    def _inferred_to_define_name_raise_or_return(
+        self, name: str, node: nodes.NodeNG
+    ) -> bool:
         """Return True if there is a path under this `if_node`
         that is inferred to define `name`, raise, or return.
         """
@@ -738,17 +740,14 @@ def _inferred_to_define_name_raise_or_return(name: str, node: nodes.NodeNG) -> b
 
         # Only search else branch when test condition is inferred to be false
         if all_inferred and only_search_else:
-            return NamesConsumer._branch_handles_name(name, node.orelse)
-        # Only search if branch when test condition is inferred to be true
-        if all_inferred and only_search_if:
-            return NamesConsumer._branch_handles_name(name, node.body)
+            self.names_under_always_false_test.add(name)
+            return self._branch_handles_name(name, node.orelse)
         # Search both if and else branches
-        return NamesConsumer._branch_handles_name(
-            name, node.body
-        ) or NamesConsumer._branch_handles_name(name, node.orelse)
+        return self._branch_handles_name(name, node.body) and self._branch_handles_name(
+            name, node.orelse
+        )
 
-    @staticmethod
-    def _branch_handles_name(name: str, body: Iterable[nodes.NodeNG]) -> bool:
+    def _branch_handles_name(self, name: str, body: Iterable[nodes.NodeNG]) -> bool:
         return any(
             NamesConsumer._defines_name_raises_or_returns(name, if_body_stmt)
             or isinstance(
@@ -761,17 +760,15 @@ def _branch_handles_name(name: str, body: Iterable[nodes.NodeNG]) -> bool:
                     nodes.While,
                 ),
             )
-            and NamesConsumer._inferred_to_define_name_raise_or_return(
-                name, if_body_stmt
-            )
+            and self._inferred_to_define_name_raise_or_return(name, if_body_stmt)
             for if_body_stmt in body
         )
 
-    def _uncertain_nodes_in_false_tests(
+    def _uncertain_nodes_if_tests(
         self, found_nodes: list[nodes.NodeNG], node: nodes.NodeNG
     ) -> list[nodes.NodeNG]:
-        """Identify nodes of uncertain execution because they are defined under
-        tests that evaluate false.
+        """Identify nodes of uncertain execution because they are defined under if
+        tests.
 
         Don't identify a node if there is a path that is inferred to
         define the name, raise, or return (e.g. any executed if/elif/else branch).
@@ -807,7 +804,7 @@ def _uncertain_nodes_in_false_tests(
                 continue
 
             # Name defined in the if/else control flow
-            if NamesConsumer._inferred_to_define_name_raise_or_return(name, outer_if):
+            if self._inferred_to_define_name_raise_or_return(name, outer_if):
                 continue
 
             uncertain_nodes.append(other_node)
@@ -1974,9 +1971,12 @@ def _report_unfound_name_definition(
         ):
             return
 
-        confidence = (
-            CONTROL_FLOW if node.name in current_consumer.consumed_uncertain else HIGH
-        )
+        confidence = HIGH
+        if node.name in current_consumer.consumed_uncertain:
+            confidence = CONTROL_FLOW
+        if node.name in current_consumer.names_under_always_false_test:
+            confidence = INFERENCE
+
         self.add_message(
             "used-before-assignment",
             args=node.name,

tests/functional/u/undefined/undefined_variable.txt

@@ -27,7 +27,7 @@ undefined-variable:166:4:166:13::Undefined variable 'unicode_2':UNDEFINED
 undefined-variable:171:4:171:13::Undefined variable 'unicode_3':UNDEFINED
 undefined-variable:226:25:226:37:LambdaClass4.<lambda>:Undefined variable 'LambdaClass4':UNDEFINED
 undefined-variable:234:25:234:37:LambdaClass5.<lambda>:Undefined variable 'LambdaClass5':UNDEFINED
-used-before-assignment:255:26:255:34:func_should_fail:Using variable 'datetime' before assignment:CONTROL_FLOW
+used-before-assignment:255:26:255:34:func_should_fail:Using variable 'datetime' before assignment:INFERENCE
 undefined-variable:291:18:291:24:not_using_loop_variable_accordingly:Undefined variable 'iteree':UNDEFINED
 undefined-variable:308:27:308:28:undefined_annotation:Undefined variable 'x':UNDEFINED
 used-before-assignment:309:7:309:8:undefined_annotation:Using variable 'x' before assignment:HIGH

tests/functional/u/undefined/undefined_variable_py38.txt

@@ -7,4 +7,4 @@ undefined-variable:106:6:106:19::Undefined variable 'else_assign_2':INFERENCE
 used-before-assignment:141:10:141:16:type_annotation_used_improperly_after_comprehension:Using variable 'my_int' before assignment:HIGH
 used-before-assignment:148:10:148:16:type_annotation_used_improperly_after_comprehension_2:Using variable 'my_int' before assignment:HIGH
 used-before-assignment:186:9:186:10::Using variable 'z' before assignment:HIGH
-used-before-assignment:193:6:193:19::Using variable 'NEVER_DEFINED' before assignment:CONTROL_FLOW
+used-before-assignment:193:6:193:19::Using variable 'NEVER_DEFINED' before assignment:INFERENCE

tests/functional/u/used/used_before_assignment.py

@@ -70,7 +70,7 @@ def redefine_time_import_with_global():
         VAR5 = True
     else:
         VAR5 = True
-if VAR5:
+if VAR5:  # [used-before-assignment]
     pass
 
 if FALSE:
@@ -116,7 +116,7 @@ def redefine_time_import_with_global():
     VAR11 = num
     if VAR11:
         VAR12 = False
-print(VAR12)
+print(VAR12)  # [used-before-assignment]
 
 def turn_on2(**kwargs):
     """https://github.com/pylint-dev/pylint/issues/7873"""

tests/functional/u/used/used_before_assignment.txt

@@ -4,9 +4,11 @@ used-before-assignment:10:4:10:9:outer:Using variable 'inner' before assignment:
 used-before-assignment:19:20:19:40:ClassWithProperty:Using variable 'redefine_time_import' before assignment:HIGH
 used-before-assignment:23:0:23:9::Using variable 'calculate' before assignment:HIGH
 used-before-assignment:31:10:31:14:redefine_time_import:Using variable 'time' before assignment:HIGH
-used-before-assignment:45:3:45:7::Using variable 'VAR2' before assignment:CONTROL_FLOW
-used-before-assignment:63:3:63:7::Using variable 'VAR4' before assignment:CONTROL_FLOW
-used-before-assignment:78:3:78:7::Using variable 'VAR6' before assignment:CONTROL_FLOW
-used-before-assignment:113:6:113:11::Using variable 'VAR10' before assignment:CONTROL_FLOW
-used-before-assignment:144:10:144:14::Using variable 'SALE' before assignment:CONTROL_FLOW
-used-before-assignment:176:10:176:18::Using variable 'ALL_DONE' before assignment:CONTROL_FLOW
+used-before-assignment:45:3:45:7::Using variable 'VAR2' before assignment:INFERENCE
+used-before-assignment:63:3:63:7::Using variable 'VAR4' before assignment:INFERENCE
+used-before-assignment:73:3:73:7::Using variable 'VAR5' before assignment:INFERENCE
+used-before-assignment:78:3:78:7::Using variable 'VAR6' before assignment:INFERENCE
+used-before-assignment:113:6:113:11::Using variable 'VAR10' before assignment:INFERENCE
+used-before-assignment:119:6:119:11::Using variable 'VAR12' before assignment:CONTROL_FLOW
+used-before-assignment:144:10:144:14::Using variable 'SALE' before assignment:INFERENCE
+used-before-assignment:176:10:176:18::Using variable 'ALL_DONE' before assignment:INFERENCE

tests/functional/u/used/used_before_assignment_except_handler_for_try_with_return.txt

@@ -1,7 +1,7 @@
 used-before-assignment:16:14:16:29:function:Using variable 'failure_message' before assignment:CONTROL_FLOW
 used-before-assignment:120:10:120:13:func_invalid1:Using variable 'msg' before assignment:CONTROL_FLOW
 used-before-assignment:131:10:131:13:func_invalid2:Using variable 'msg' before assignment:CONTROL_FLOW
-used-before-assignment:150:10:150:13:func_invalid3:Using variable 'msg' before assignment:CONTROL_FLOW
+used-before-assignment:150:10:150:13:func_invalid3:Using variable 'msg' before assignment:INFERENCE
 used-before-assignment:163:10:163:13:func_invalid4:Using variable 'msg' before assignment:CONTROL_FLOW
 used-before-assignment:175:10:175:13:func_invalid5:Using variable 'msg' before assignment:CONTROL_FLOW
 used-before-assignment:187:10:187:13:func_invalid6:Using variable 'msg' before assignment:CONTROL_FLOW

tests/functional/u/used/used_before_assignment_issue1081.txt

@@ -2,6 +2,6 @@ used-before-assignment:7:7:7:8:used_before_assignment_1:Using variable 'x' befor
 redefined-outer-name:8:12:8:13:used_before_assignment_1:Redefining name 'x' from outer scope (line 3):UNDEFINED
 used-before-assignment:13:7:13:8:used_before_assignment_2:Using variable 'x' before assignment:HIGH
 redefined-outer-name:15:4:15:5:used_before_assignment_2:Redefining name 'x' from outer scope (line 3):UNDEFINED
-used-before-assignment:19:7:19:8:used_before_assignment_3:Using variable 'x' before assignment:HIGH
+used-before-assignment:19:7:19:8:used_before_assignment_3:Using variable 'x' before assignment:CONTROL_FLOW
 redefined-outer-name:21:12:21:13:used_before_assignment_3:Redefining name 'x' from outer scope (line 3):UNDEFINED
 redefined-outer-name:30:4:30:5:not_used_before_assignment_2:Redefining name 'x' from outer scope (line 3):UNDEFINED

tests/functional/u/used/used_before_assignment_issue2615.txt

@@ -1,3 +1,3 @@
-used-before-assignment:12:14:12:17:main:Using variable 'res' before assignment:CONTROL_FLOW
+used-before-assignment:12:14:12:17:main:Using variable 'res' before assignment:INFERENCE
 used-before-assignment:30:18:30:35:nested_except_blocks:Using variable 'more_bad_division' before assignment:CONTROL_FLOW
-used-before-assignment:31:18:31:21:nested_except_blocks:Using variable 'res' before assignment:CONTROL_FLOW
+used-before-assignment:31:18:31:21:nested_except_blocks:Using variable 'res' before assignment:INFERENCE

tests/functional/u/used/used_before_assignment_postponed_evaluation.txt

@@ -1 +1 @@
-used-before-assignment:10:6:10:9::Using variable 'var' before assignment:CONTROL_FLOW
+used-before-assignment:10:6:10:9::Using variable 'var' before assignment:INFERENCE

tests/functional/u/used/used_before_assignment_scoping.txt

@@ -1,2 +1,2 @@
-used-before-assignment:10:13:10:21:func_two:Using variable 'datetime' before assignment:CONTROL_FLOW
-used-before-assignment:16:12:16:20:func:Using variable 'datetime' before assignment:CONTROL_FLOW
+used-before-assignment:10:13:10:21:func_two:Using variable 'datetime' before assignment:INFERENCE
+used-before-assignment:16:12:16:20:func:Using variable 'datetime' before assignment:INFERENCE

tests/functional/u/used/used_before_assignment_typing.py

@@ -167,32 +167,32 @@ class ConditionalImportGuardedWhenUsed:  # pylint: disable=too-few-public-method
 
 class TypeCheckingMultiBranch:  # pylint: disable=too-few-public-methods,unused-variable
     """Test for defines in TYPE_CHECKING if/elif/else branching"""
-    def defined_in_elif_branch(self) -> calendar.Calendar:
-        print(bisect)
+    def defined_in_elif_branch(self) -> calendar.Calendar:  # [used-before-assignment]
+        print(bisect)  # [used-before-assignment]
         return calendar.Calendar()
 
     def defined_in_else_branch(self) -> urlopen:
-        print(zoneinfo)
+        print(zoneinfo)  # [used-before-assignment]
         print(pprint())
         print(collections())
         return urlopen
 
-    def defined_in_nested_if_else(self) -> heapq:
+    def defined_in_nested_if_else(self) -> heapq:  # [used-before-assignment]
         print(heapq)
         return heapq
 
-    def defined_in_try_except(self) -> array:
-        print(types)
-        print(copy)
-        print(numbers)
+    def defined_in_try_except(self) -> array:  # [used-before-assignment]
+        print(types)  # [used-before-assignment]
+        print(copy)  # [used-before-assignment]
+        print(numbers)  # [used-before-assignment]
         return array
 
-    def defined_in_loops(self) -> json:
-        print(email)
-        print(mailbox)
-        print(mimetypes)
+    def defined_in_loops(self) -> json:  # [used-before-assignment]
+        print(email)  # [used-before-assignment]
+        print(mailbox)  # [used-before-assignment]
+        print(mimetypes)  # [used-before-assignment]
         return json
 
-    def defined_in_with(self) -> base64:
-        print(binascii)
+    def defined_in_with(self) -> base64:  # [used-before-assignment]
+        print(binascii)  # [used-before-assignment]
         return base64

tests/functional/u/used/used_before_assignment_typing.txt

@@ -1,5 +1,19 @@
 undefined-variable:69:21:69:28:MyClass.incorrect_typing_method:Undefined variable 'MyClass':UNDEFINED
 undefined-variable:74:26:74:33:MyClass.incorrect_nested_typing_method:Undefined variable 'MyClass':UNDEFINED
 undefined-variable:79:20:79:27:MyClass.incorrect_default_method:Undefined variable 'MyClass':UNDEFINED
-used-before-assignment:140:35:140:39:MyFourthClass.is_close:Using variable 'math' before assignment:CONTROL_FLOW
-used-before-assignment:153:20:153:28:VariableAnnotationsGuardedByTypeChecking:Using variable 'datetime' before assignment:CONTROL_FLOW
+used-before-assignment:140:35:140:39:MyFourthClass.is_close:Using variable 'math' before assignment:INFERENCE
+used-before-assignment:153:20:153:28:VariableAnnotationsGuardedByTypeChecking:Using variable 'datetime' before assignment:INFERENCE
+used-before-assignment:170:40:170:48:TypeCheckingMultiBranch.defined_in_elif_branch:Using variable 'calendar' before assignment:INFERENCE
+used-before-assignment:171:14:171:20:TypeCheckingMultiBranch.defined_in_elif_branch:Using variable 'bisect' before assignment:INFERENCE
+used-before-assignment:175:14:175:22:TypeCheckingMultiBranch.defined_in_else_branch:Using variable 'zoneinfo' before assignment:INFERENCE
+used-before-assignment:180:43:180:48:TypeCheckingMultiBranch.defined_in_nested_if_else:Using variable 'heapq' before assignment:INFERENCE
+used-before-assignment:184:39:184:44:TypeCheckingMultiBranch.defined_in_try_except:Using variable 'array' before assignment:INFERENCE
+used-before-assignment:185:14:185:19:TypeCheckingMultiBranch.defined_in_try_except:Using variable 'types' before assignment:INFERENCE
+used-before-assignment:186:14:186:18:TypeCheckingMultiBranch.defined_in_try_except:Using variable 'copy' before assignment:INFERENCE
+used-before-assignment:187:14:187:21:TypeCheckingMultiBranch.defined_in_try_except:Using variable 'numbers' before assignment:INFERENCE
+used-before-assignment:190:34:190:38:TypeCheckingMultiBranch.defined_in_loops:Using variable 'json' before assignment:INFERENCE
+used-before-assignment:191:14:191:19:TypeCheckingMultiBranch.defined_in_loops:Using variable 'email' before assignment:INFERENCE
+used-before-assignment:192:14:192:21:TypeCheckingMultiBranch.defined_in_loops:Using variable 'mailbox' before assignment:INFERENCE
+used-before-assignment:193:14:193:23:TypeCheckingMultiBranch.defined_in_loops:Using variable 'mimetypes' before assignment:INFERENCE
+used-before-assignment:196:33:196:39:TypeCheckingMultiBranch.defined_in_with:Using variable 'base64' before assignment:INFERENCE
+used-before-assignment:197:14:197:22:TypeCheckingMultiBranch.defined_in_with:Using variable 'binascii' before assignment:INFERENCE