feat: forbid initializing multiple variables on one line (fixes #1901)

Author: f1sherFM

CHANGELOG.md

@@ -21,6 +21,7 @@ Semantic versioning in our case means:
 ### Features
 
 - Adds `WPS367`: forbid redundant trailing colon in slice, #1071
+- Adds `WPS482`: forbid initializing multiple variables on one line, #1901
 
 ## 1.6.2
 

tests/test_visitors/test_ast/test_builtins/test_assign/test_multiple_variables_initialization.py

@@ -0,0 +1,72 @@
+import pytest
+
+from wemake_python_styleguide.violations.best_practices import (
+    MultipleVariablesInitializationViolation,
+)
+from wemake_python_styleguide.visitors.ast.builtins import (
+    MultipleVariablesInitializationVisitor,
+)
+
+# Correct usages:
+
+function_unpacking = 'first, second = some_tuple()'
+swap_assignment = 'first, second = second, first'
+single_assignment = 'constant = 1'
+spread_assignment = 'first, *_, second = [1, 2, 4, 3]'
+
+# Wrong usages:
+
+tuple_assignment = 'first, second = (1, 2)'
+list_unpacking = 'first, second = [], []'
+mixed_unpacking = 'first, second = (1, [])'
+
+
+@pytest.mark.parametrize(
+    'code',
+    [
+        single_assignment,
+        spread_assignment,
+        function_unpacking,
+        swap_assignment,
+    ],
+)
+def test_correct_assignments(
+    assert_errors,
+    parse_ast_tree,
+    code,
+    default_options,
+):
+    """Testing that correct assignments work."""
+    tree = parse_ast_tree(code)
+
+    visitor = MultipleVariablesInitializationVisitor(
+        default_options, tree=tree,
+    )
+    visitor.run()
+
+    assert_errors(visitor, [])
+
+
+@pytest.mark.parametrize(
+    'code',
+    [
+        tuple_assignment,
+        list_unpacking,
+        mixed_unpacking,
+    ],
+)
+def test_multiple_variables_initialization(
+    assert_errors,
+    parse_ast_tree,
+    code,
+    default_options,
+):
+    """Testing that multiple variables initialization is restricted."""
+    tree = parse_ast_tree(code)
+
+    visitor = MultipleVariablesInitializationVisitor(
+        default_options, tree=tree,
+    )
+    visitor.run()
+
+    assert_errors(visitor, [MultipleVariablesInitializationViolation])

wemake_python_styleguide/presets/types/tree.py

@@ -56,6 +56,7 @@
     builtins.WrongStringVisitor,
     builtins.WrongFormatStringVisitor,
     builtins.WrongAssignmentVisitor,
+    builtins.MultipleVariablesInitializationVisitor,
     builtins.WrongCollectionVisitor,
     operators.UselessOperatorsVisitor,
     operators.WrongMathOperatorVisitor,

wemake_python_styleguide/violations/best_practices.py

@@ -3019,3 +3019,35 @@ class Some:
 
     error_template = 'Found a leaking ``for`` loop in a class or module body'
     code = 481
+
+
+@final
+class MultipleVariablesInitializationViolation(ASTViolation):
+    """
+    Forbid initializing multiple variables on a single line.
+
+    Reasoning:
+        Assigning multiple variables in one line reduces readability.
+        It is harder to see where each variable is initialized.
+
+    Solution:
+        Use separate assignment statements for each variable.
+
+    Example::
+
+        # Correct:
+        ab = []
+        cd = []
+        a, b = some_tuple()
+        x, y = y, x
+
+        # Wrong:
+        ab, cd = [], []
+        x, y = (1, 2)
+
+    .. versionadded:: 1.7.0
+
+    """
+
+    error_template = 'Found multiple variables initialized on one line'
+    code = 482

wemake_python_styleguide/visitors/ast/builtins.py

@@ -374,6 +374,34 @@ def _check_unpacking_target_types(self, node: ast.AST | None) -> None:
             )
 
 
+@final
+class MultipleVariablesInitializationVisitor(base.BaseNodeVisitor):
+    """Ensures that multiple variables are not initialized on one line."""
+
+    def visit_Assign(self, node: ast.Assign) -> None:
+        """Checks assignments for multiple variables initialization."""
+        self._check_multiple_variables_initialization(node)
+        self.generic_visit(node)
+
+    def _check_multiple_variables_initialization(
+        self,
+        node: ast.Assign,
+    ) -> None:
+        if len(node.targets) != 1:
+            return
+        target = node.targets[0]
+        if not isinstance(target, ast.Tuple):
+            return
+        if not isinstance(node.value, ast.Tuple):
+            return
+        # Allow swapping: x, y = y, x (all elements in value are ast.Name)
+        if all(isinstance(elt, ast.Name) for elt in node.value.elts):
+            return
+        self.add_violation(
+            best_practices.MultipleVariablesInitializationViolation(node),
+        )
+
+
 @final
 class WrongCollectionVisitor(base.BaseNodeVisitor):
     """Ensures that collection definitions are correct."""