# Licensed under the GPL: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html # For details: https://github.com/PyCQA/pylint/blob/main/LICENSE # Copyright (c) https://github.com/PyCQA/pylint/blob/main/CONTRIBUTORS.txt from __future__ import annotations import astroid from astroid import bases, nodes, util from pylint import checkers from pylint.checkers import utils from pylint.interfaces import HIGH, INFERENCE class ImplicitBooleanessChecker(checkers.BaseChecker): """Checks for incorrect usage of comparisons or len() inside conditions. Incorrect usage of len() Pep8 states: For sequences, (strings, lists, tuples), use the fact that empty sequences are false. Yes: if not seq: if seq: No: if len(seq): if not len(seq): Problems detected: * if len(sequence): * if not len(sequence): * elif len(sequence): * elif not len(sequence): * while len(sequence): * while not len(sequence): * assert len(sequence): * assert not len(sequence): * bool(len(sequence)) Incorrect usage of empty literal sequences; (), [], {}, For empty sequences, (dicts, lists, tuples), use the fact that empty sequences are false. Yes: if variable: if not variable No: if variable == empty_literal: if variable != empty_literal: Problems detected: * comparison such as variable == empty_literal: * comparison such as variable != empty_literal: """ name = "refactoring" msgs = { "C1802": ( "Do not use `len(SEQUENCE)` without comparison to determine if a sequence is empty", "use-implicit-booleaness-not-len", "Used when Pylint detects that len(sequence) is being used " "without explicit comparison inside a condition to determine if a sequence is empty. " "Instead of coercing the length to a boolean, either " "rely on the fact that empty sequences are false or " "compare the length against a scalar.", {"old_names": [("C1801", "len-as-condition")]}, ), "C1803": ( "'%s' can be simplified to '%s' as an empty %s is falsey", "use-implicit-booleaness-not-comparison", "Used when Pylint detects that collection literal comparison is being " "used to check for emptiness; Use implicit booleaness instead " "of a collection classes; empty collections are considered as false", ), } options = () @utils.only_required_for_messages("use-implicit-booleaness-not-len") def visit_call(self, node: nodes.Call) -> None: # a len(S) call is used inside a test condition # could be if, while, assert or if expression statement # e.g. `if len(S):` if not utils.is_call_of_name(node, "len"): return # the len() call could also be nested together with other # boolean operations, e.g. `if z or len(x):` parent = node.parent while isinstance(parent, nodes.BoolOp): parent = parent.parent # we're finally out of any nested boolean operations so check if # this len() call is part of a test condition if not utils.is_test_condition(node, parent): return len_arg = node.args[0] generator_or_comprehension = ( nodes.ListComp, nodes.SetComp, nodes.DictComp, nodes.GeneratorExp, ) if isinstance(len_arg, generator_or_comprehension): # The node is a generator or comprehension as in len([x for x in ...]) self.add_message( "use-implicit-booleaness-not-len", node=node, confidence=HIGH, ) return try: instance = next(len_arg.infer()) except astroid.InferenceError: # Probably undefined-variable, abort check return mother_classes = self.base_names_of_instance(instance) affected_by_pep8 = any( t in mother_classes for t in ("str", "tuple", "list", "set") ) if "range" in mother_classes or ( affected_by_pep8 and not self.instance_has_bool(instance) ): self.add_message( "use-implicit-booleaness-not-len", node=node, confidence=INFERENCE, ) @staticmethod def instance_has_bool(class_def: nodes.ClassDef) -> bool: try: class_def.getattr("__bool__") return True except astroid.AttributeInferenceError: ... return False @utils.only_required_for_messages("use-implicit-booleaness-not-len") def visit_unaryop(self, node: nodes.UnaryOp) -> None: """`not len(S)` must become `not S` regardless if the parent block is a test condition or something else (boolean expression) e.g. `if not len(S):`. """ if ( isinstance(node, nodes.UnaryOp) and node.op == "not" and utils.is_call_of_name(node.operand, "len") ): self.add_message( "use-implicit-booleaness-not-len", node=node, confidence=HIGH ) @utils.only_required_for_messages("use-implicit-booleaness-not-comparison") def visit_compare(self, node: nodes.Compare) -> None: self._check_use_implicit_booleaness_not_comparison(node) def _check_use_implicit_booleaness_not_comparison( self, node: nodes.Compare ) -> None: """Check for left side and right side of the node for empty literals.""" is_left_empty_literal = utils.is_base_container( node.left ) or utils.is_empty_dict_literal(node.left) # Check both left-hand side and right-hand side for literals for operator, comparator in node.ops: is_right_empty_literal = utils.is_base_container( comparator ) or utils.is_empty_dict_literal(comparator) # Using Exclusive OR (XOR) to compare between two side. # If two sides are both literal, it should be different error. if is_right_empty_literal ^ is_left_empty_literal: # set target_node to opposite side of literal target_node = node.left if is_right_empty_literal else comparator literal_node = comparator if is_right_empty_literal else node.left # Infer node to check target_instance = utils.safe_infer(target_node) if target_instance is None: continue mother_classes = self.base_names_of_instance(target_instance) is_base_comprehension_type = any( t in mother_classes for t in ("tuple", "list", "dict", "set") ) # Only time we bypass check is when target_node is not inherited by # collection literals and have its own __bool__ implementation. if not is_base_comprehension_type and self.instance_has_bool( target_instance ): continue # No need to check for operator when visiting compare node if operator in {"==", "!=", ">=", ">", "<=", "<"}: self.add_message( "use-implicit-booleaness-not-comparison", args=self._implicit_booleaness_message_args( literal_node, operator, target_node ), node=node, confidence=HIGH, ) def _get_node_description(self, node: nodes.NodeNG) -> str: return { nodes.List: "list", nodes.Tuple: "tuple", nodes.Dict: "dict", nodes.Const: "str", }.get(type(node), "iterable") def _implicit_booleaness_message_args( self, literal_node: nodes.NodeNG, operator: str, target_node: nodes.NodeNG ) -> tuple[str, str, str]: """Helper to get the right message for "use-implicit-booleaness-not-comparison".""" description = self._get_node_description(literal_node) collection_literal = { "list": "[]", "tuple": "()", "dict": "{}", }.get(description, "iterable") instance_name = "x" if isinstance(target_node, nodes.Call) and target_node.func: instance_name = f"{target_node.func.as_string()}(...)" elif isinstance(target_node, (nodes.Attribute, nodes.Name)): instance_name = target_node.as_string() original_comparison = f"{instance_name} {operator} {collection_literal}" suggestion = f"{instance_name}" if operator == "!=" else f"not {instance_name}" return original_comparison, suggestion, description @staticmethod def base_names_of_instance( node: util.UninferableBase | bases.Instance, ) -> list[str]: """Return all names inherited by a class instance or those returned by a function. The inherited names include 'object'. """ if isinstance(node, bases.Instance): return [node.name] + [x.name for x in node.ancestors()] return []