Fix used-before-assignment for variable annotations guarded by TYPE_CHECKING (#7810)

Author: jacobtylerwalls

doc/whatsnew/fragments/7609.false_positive

@@ -0,0 +1,4 @@
+Fix a false positive for ``used-before-assignment`` for imports guarded by
+``typing.TYPE_CHECKING`` later used in variable annotations.
+
+Closes #7609

pylint/checkers/variables.py

@@ -1998,12 +1998,22 @@ def _is_variable_violation(
                     )
 
             # Look for type checking definitions inside a type checking guard.
-            if isinstance(defstmt, (nodes.Import, nodes.ImportFrom)):
+            # Relevant for function annotations only, not variable annotations (AnnAssign)
+            if (
+                isinstance(defstmt, (nodes.Import, nodes.ImportFrom))
+                and isinstance(defstmt.parent, nodes.If)
+                and defstmt.parent.test.as_string() in TYPING_TYPE_CHECKS_GUARDS
+            ):
                 defstmt_parent = defstmt.parent
 
-                if (
-                    isinstance(defstmt_parent, nodes.If)
-                    and defstmt_parent.test.as_string() in TYPING_TYPE_CHECKS_GUARDS
+                maybe_annotation = utils.get_node_first_ancestor_of_type(
+                    node, nodes.AnnAssign
+                )
+                if not (
+                    maybe_annotation
+                    and utils.get_node_first_ancestor_of_type(
+                        maybe_annotation, nodes.FunctionDef
+                    )
                 ):
                     # Exempt those definitions that are used inside the type checking
                     # guard or that are defined in both type checking guard branches.

tests/functional/u/used/used_before_assignment_typing.py

@@ -2,8 +2,10 @@
 # pylint: disable=missing-function-docstring
 
 
-from typing import List, Optional
+from typing import List, Optional, TYPE_CHECKING
 
+if TYPE_CHECKING:
+    import datetime
 
 class MyClass:
     """Type annotation or default values for first level methods can't refer to their own class"""
@@ -74,3 +76,17 @@ def function(self, var: int) -> None:
 
     def other_function(self) -> None:
         _x: MyThirdClass = self
+
+
+class VariableAnnotationsGuardedByTypeChecking:  # pylint: disable=too-few-public-methods
+    """Class to test conditional imports guarded by TYPE_CHECKING then used in
+    local (function) variable annotations, which are not evaluated at runtime.
+
+    See: https://github.com/PyCQA/pylint/issues/7609
+    """
+
+    still_an_error: datetime.date  # [used-before-assignment]
+
+    def print_date(self, date) -> None:
+        date: datetime.date = date
+        print(date)

tests/functional/u/used/used_before_assignment_typing.txt

@@ -1,3 +1,4 @@
-undefined-variable:12:21:12:28:MyClass.incorrect_typing_method:Undefined variable 'MyClass':UNDEFINED
-undefined-variable:17:26:17:33:MyClass.incorrect_nested_typing_method:Undefined variable 'MyClass':UNDEFINED
-undefined-variable:22:20:22:27:MyClass.incorrect_default_method:Undefined variable 'MyClass':UNDEFINED
+undefined-variable:14:21:14:28:MyClass.incorrect_typing_method:Undefined variable 'MyClass':UNDEFINED
+undefined-variable:19:26:19:33:MyClass.incorrect_nested_typing_method:Undefined variable 'MyClass':UNDEFINED
+undefined-variable:24:20:24:27:MyClass.incorrect_default_method:Undefined variable 'MyClass':UNDEFINED
+used-before-assignment:88:20:88:28:VariableAnnotationsGuardedByTypeChecking:Using variable 'datetime' before assignment:HIGH