Fix #5112: Prevent used-before-assignment if named expression found first in container

ChangeLog

@@ -205,6 +205,11 @@ Release date: TBA
 
   Closes #367
 
+* Fixed a false positive for ``used-before-assignment`` when a named expression
+  appears as the first value in a container.
+
+  Closes #5112
+
 * ``used-before-assignment`` now assumes that assignments in except blocks
   may not have occurred and warns accordingly.
 

doc/whatsnew/2.13.rst

@@ -241,6 +241,11 @@ Other Changes
 
   Closes #3793
 
+* Fixed a false positive for ``used-before-assignment`` when a named expression
+  appears as the first value in a container.
+
+  Closes #5112
+
 * Fixed false positive for ``used-before-assignment`` with self-referential type
   annotation in conditional statements within class methods.
 

pylint/checkers/variables.py

@@ -1971,7 +1971,7 @@ def _is_variable_violation(
                     # same line as the function definition
                     maybe_before_assign = False
                 elif (
-                    isinstance(  # pylint: disable=too-many-boolean-expressions
+                    isinstance(
                         defstmt,
                         (
                             nodes.Assign,
@@ -1981,26 +1981,7 @@ def _is_variable_violation(
                             nodes.Return,
                         ),
                     )
-                    and (
-                        isinstance(defstmt.value, nodes.IfExp)
-                        or (
-                            isinstance(defstmt.value, nodes.Lambda)
-                            and isinstance(defstmt.value.body, nodes.IfExp)
-                        )
-                        or (
-                            isinstance(defstmt.value, nodes.Call)
-                            and (
-                                any(
-                                    isinstance(kwarg.value, nodes.IfExp)
-                                    for kwarg in defstmt.value.keywords
-                                )
-                                or any(
-                                    isinstance(arg, nodes.IfExp)
-                                    for arg in defstmt.value.args
-                                )
-                            )
-                        )
-                    )
+                    and VariablesChecker._maybe_used_and_assigned_at_once(defstmt)
                     and frame is defframe
                     and defframe.parent_of(node)
                     and stmt is defstmt
@@ -2074,6 +2055,27 @@ def _is_variable_violation(
 
         return maybe_before_assign, annotation_return, use_outer_definition
 
+    @staticmethod
+    def _maybe_used_and_assigned_at_once(defstmt: nodes.Statement) -> bool:
+        """Check if `defstmt` has the potential to use and assign a name in the
+        same statement.
+        """
+        if isinstance(defstmt.value, nodes.BaseContainer) and defstmt.value.elts:
+            # The assignment must happen as part of the first element
+            # e.g. "assert x:= True, x"
+            # NOT "assert x, x:= True"
+            value = defstmt.value.elts[0]
+        else:
+            value = defstmt.value
+        if isinstance(value, nodes.IfExp):
+            return True
+        if isinstance(value, nodes.Lambda) and isinstance(value.body, nodes.IfExp):
+            return True
+        return isinstance(value, nodes.Call) and (
+            any(isinstance(kwarg.value, nodes.IfExp) for kwarg in value.keywords)
+            or any(isinstance(arg, nodes.IfExp) for arg in value.args)
+        )
+
     @staticmethod
     def _is_only_type_assignment(node: nodes.Name, defstmt: nodes.Statement) -> bool:
         """Check if variable only gets assigned a type and never a value."""

tests/functional/u/undefined/undefined_variable_py38.py

@@ -155,3 +155,17 @@ def __init__(self, value):
 
 
 dummy = Dummy(value=val if (val := 'something') else 'anything')
+
+def expression_in_ternary_operator_inside_container():
+    """Named expression in ternary operator: inside container"""
+    return [val2 if (val2 := 'something') else 'anything']
+
+
+def expression_in_ternary_operator_inside_container_tuple():
+    """Same case, using a tuple inside a 1-element list"""
+    return [(val3, val3) if (val3 := 'something') else 'anything']
+
+
+def expression_in_ternary_operator_inside_container_wrong_position():
+    """2-element list where named expression comes too late"""
+    return [val3, val3 if (val3 := 'something') else 'anything']  # [used-before-assignment]

tests/functional/u/undefined/undefined_variable_py38.txt

@@ -5,3 +5,4 @@ undefined-variable:99:6:99:19::Undefined variable 'else_assign_2':INFERENCE
 unused-variable:126:4:126:10:type_annotation_unused_after_comprehension:Unused variable 'my_int':UNDEFINED
 used-before-assignment:134:10:134:16:type_annotation_used_improperly_after_comprehension:Using variable 'my_int' before assignment:HIGH
 used-before-assignment:141:10:141:16:type_annotation_used_improperly_after_comprehension_2:Using variable 'my_int' before assignment:HIGH
+used-before-assignment:171:12:171:16:expression_in_ternary_operator_inside_container_wrong_position:Using variable 'val3' before assignment:HIGH