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# 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
"""Comparison checker from the basic checker."""
import astroid
from astroid import nodes
from pylint.checkers import utils
from pylint.checkers.base.basic_checker import _BasicChecker
from pylint.interfaces import HIGH
LITERAL_NODE_TYPES = (nodes.Const, nodes.Dict, nodes.List, nodes.Set)
COMPARISON_OPERATORS = frozenset(("==", "!=", "<", ">", "<=", ">="))
TYPECHECK_COMPARISON_OPERATORS = frozenset(("is", "is not", "==", "!="))
TYPE_QNAME = "builtins.type"
def _is_one_arg_pos_call(call: nodes.NodeNG) -> bool:
"""Is this a call with exactly 1 positional argument ?"""
return isinstance(call, nodes.Call) and len(call.args) == 1 and not call.keywords
class ComparisonChecker(_BasicChecker):
"""Checks for comparisons.
- singleton comparison: 'expr == True', 'expr == False' and 'expr == None'
- yoda condition: 'const "comp" right' where comp can be '==', '!=', '<',
'<=', '>' or '>=', and right can be a variable, an attribute, a method or
a function
"""
msgs = {
"C0121": (
"Comparison %s should be %s",
"singleton-comparison",
"Used when an expression is compared to singleton "
"values like True, False or None.",
),
"C0123": (
"Use isinstance() rather than type() for a typecheck.",
"unidiomatic-typecheck",
"The idiomatic way to perform an explicit typecheck in "
"Python is to use isinstance(x, Y) rather than "
"type(x) == Y, type(x) is Y. Though there are unusual "
"situations where these give different results.",
{"old_names": [("W0154", "old-unidiomatic-typecheck")]},
),
"R0123": (
"In '%s', use '%s' when comparing constant literals not '%s' ('%s')",
"literal-comparison",
"Used when comparing an object to a literal, which is usually "
"what you do not want to do, since you can compare to a different "
"literal than what was expected altogether.",
),
"R0124": (
"Redundant comparison - %s",
"comparison-with-itself",
"Used when something is compared against itself.",
),
"R0133": (
"Comparison between constants: '%s %s %s' has a constant value",
"comparison-of-constants",
"When two literals are compared with each other the result is a constant. "
"Using the constant directly is both easier to read and more performant. "
"Initializing 'True' and 'False' this way is not required since Python 2.3.",
),
"W0143": (
"Comparing against a callable, did you omit the parenthesis?",
"comparison-with-callable",
"This message is emitted when pylint detects that a comparison with a "
"callable was made, which might suggest that some parenthesis were omitted, "
"resulting in potential unwanted behaviour.",
),
"W0177": (
"Comparison %s should be %s",
"nan-comparison",
"Used when an expression is compared to NaN "
"values like numpy.NaN and float('nan').",
),
}
def _check_singleton_comparison(
self,
left_value: nodes.NodeNG,
right_value: nodes.NodeNG,
root_node: nodes.Compare,
checking_for_absence: bool = False,
) -> None:
"""Check if == or != is being used to compare a singleton value."""
if utils.is_singleton_const(left_value):
singleton, other_value = left_value.value, right_value
elif utils.is_singleton_const(right_value):
singleton, other_value = right_value.value, left_value
else:
return
singleton_comparison_example = {False: "'{} is {}'", True: "'{} is not {}'"}
# True/False singletons have a special-cased message in case the user is
# mistakenly using == or != to check for truthiness
if singleton in {True, False}:
suggestion_template = (
"{} if checking for the singleton value {}, or {} if testing for {}"
)
truthiness_example = {False: "not {}", True: "{}"}
truthiness_phrase = {True: "truthiness", False: "falsiness"}
# Looks for comparisons like x == True or x != False
checking_truthiness = singleton is not checking_for_absence
suggestion = suggestion_template.format(
singleton_comparison_example[checking_for_absence].format(
left_value.as_string(), right_value.as_string()
),
singleton,
(
"'bool({})'"
if not utils.is_test_condition(root_node) and checking_truthiness
else "'{}'"
).format(
truthiness_example[checking_truthiness].format(
other_value.as_string()
)
),
truthiness_phrase[checking_truthiness],
)
else:
suggestion = singleton_comparison_example[checking_for_absence].format(
left_value.as_string(), right_value.as_string()
)
self.add_message(
"singleton-comparison",
node=root_node,
args=(f"'{root_node.as_string()}'", suggestion),
)
def _check_nan_comparison(
self,
left_value: nodes.NodeNG,
right_value: nodes.NodeNG,
root_node: nodes.Compare,
checking_for_absence: bool = False,
) -> None:
def _is_float_nan(node: nodes.NodeNG) -> bool:
try:
if isinstance(node, nodes.Call) and len(node.args) == 1:
if (
node.args[0].value.lower() == "nan"
and node.inferred()[0].pytype() == "builtins.float"
):
return True
return False
except AttributeError:
return False
def _is_numpy_nan(node: nodes.NodeNG) -> bool:
if isinstance(node, nodes.Attribute) and node.attrname == "NaN":
if isinstance(node.expr, nodes.Name):
return node.expr.name in {"numpy", "nmp", "np"}
return False
def _is_nan(node: nodes.NodeNG) -> bool:
return _is_float_nan(node) or _is_numpy_nan(node)
nan_left = _is_nan(left_value)
if not nan_left and not _is_nan(right_value):
return
absence_text = ""
if checking_for_absence:
absence_text = "not "
if nan_left:
suggestion = f"'{absence_text}math.isnan({right_value.as_string()})'"
else:
suggestion = f"'{absence_text}math.isnan({left_value.as_string()})'"
self.add_message(
"nan-comparison",
node=root_node,
args=(f"'{root_node.as_string()}'", suggestion),
)
def _check_literal_comparison(
self, literal: nodes.NodeNG, node: nodes.Compare
) -> None:
"""Check if we compare to a literal, which is usually what we do not want to do."""
is_other_literal = isinstance(literal, (nodes.List, nodes.Dict, nodes.Set))
is_const = False
if isinstance(literal, nodes.Const):
if isinstance(literal.value, bool) or literal.value is None:
# Not interested in these values.
return
is_const = isinstance(literal.value, (bytes, str, int, float))
if is_const or is_other_literal:
incorrect_node_str = node.as_string()
if "is not" in incorrect_node_str:
equal_or_not_equal = "!="
is_or_is_not = "is not"
else:
equal_or_not_equal = "=="
is_or_is_not = "is"
fixed_node_str = incorrect_node_str.replace(
is_or_is_not, equal_or_not_equal
)
self.add_message(
"literal-comparison",
args=(
incorrect_node_str,
equal_or_not_equal,
is_or_is_not,
fixed_node_str,
),
node=node,
confidence=HIGH,
)
def _check_logical_tautology(self, node: nodes.Compare) -> None:
"""Check if identifier is compared against itself.
:param node: Compare node
:Example:
val = 786
if val == val: # [comparison-with-itself]
pass
"""
left_operand = node.left
right_operand = node.ops[0][1]
operator = node.ops[0][0]
if isinstance(left_operand, nodes.Const) and isinstance(
right_operand, nodes.Const
):
left_operand = left_operand.value
right_operand = right_operand.value
elif isinstance(left_operand, nodes.Name) and isinstance(
right_operand, nodes.Name
):
left_operand = left_operand.name
right_operand = right_operand.name
if left_operand == right_operand:
suggestion = f"{left_operand} {operator} {right_operand}"
self.add_message("comparison-with-itself", node=node, args=(suggestion,))
def _check_constants_comparison(self, node: nodes.Compare) -> None:
"""When two constants are being compared it is always a logical tautology."""
left_operand = node.left
if not isinstance(left_operand, nodes.Const):
return
right_operand = node.ops[0][1]
if not isinstance(right_operand, nodes.Const):
return
operator = node.ops[0][0]
self.add_message(
"comparison-of-constants",
node=node,
args=(left_operand.value, operator, right_operand.value),
confidence=HIGH,
)
def _check_callable_comparison(self, node: nodes.Compare) -> None:
operator = node.ops[0][0]
if operator not in COMPARISON_OPERATORS:
return
bare_callables = (nodes.FunctionDef, astroid.BoundMethod)
left_operand, right_operand = node.left, node.ops[0][1]
# this message should be emitted only when there is comparison of bare callable
# with non bare callable.
number_of_bare_callables = 0
for operand in left_operand, right_operand:
inferred = utils.safe_infer(operand)
# Ignore callables that raise, as well as typing constants
# implemented as functions (that raise via their decorator)
if (
isinstance(inferred, bare_callables)
and "typing._SpecialForm" not in inferred.decoratornames()
and not any(isinstance(x, nodes.Raise) for x in inferred.body)
):
number_of_bare_callables += 1
if number_of_bare_callables == 1:
self.add_message("comparison-with-callable", node=node)
@utils.only_required_for_messages(
"singleton-comparison",
"unidiomatic-typecheck",
"literal-comparison",
"comparison-with-itself",
"comparison-of-constants",
"comparison-with-callable",
"nan-comparison",
)
def visit_compare(self, node: nodes.Compare) -> None:
self._check_callable_comparison(node)
self._check_logical_tautology(node)
self._check_unidiomatic_typecheck(node)
self._check_constants_comparison(node)
# NOTE: this checker only works with binary comparisons like 'x == 42'
# but not 'x == y == 42'
if len(node.ops) != 1:
return
left = node.left
operator, right = node.ops[0]
if operator in {"==", "!="}:
self._check_singleton_comparison(
left, right, node, checking_for_absence=operator == "!="
)
if operator in {"==", "!=", "is", "is not"}:
self._check_nan_comparison(
left, right, node, checking_for_absence=operator in {"!=", "is not"}
)
if operator in {"is", "is not"}:
self._check_literal_comparison(right, node)
def _check_unidiomatic_typecheck(self, node: nodes.Compare) -> None:
operator, right = node.ops[0]
if operator in TYPECHECK_COMPARISON_OPERATORS:
left = node.left
if _is_one_arg_pos_call(left):
self._check_type_x_is_y(node, left, operator, right)
def _check_type_x_is_y(
self,
node: nodes.Compare,
left: nodes.NodeNG,
operator: str,
right: nodes.NodeNG,
) -> None:
"""Check for expressions like type(x) == Y."""
left_func = utils.safe_infer(left.func)
if not (
isinstance(left_func, nodes.ClassDef) and left_func.qname() == TYPE_QNAME
):
return
if operator in {"is", "is not"} and _is_one_arg_pos_call(right):
right_func = utils.safe_infer(right.func)
if (
isinstance(right_func, nodes.ClassDef)
and right_func.qname() == TYPE_QNAME
):
# type(x) == type(a)
right_arg = utils.safe_infer(right.args[0])
if not isinstance(right_arg, LITERAL_NODE_TYPES):
# not e.g. type(x) == type([])
return
self.add_message("unidiomatic-typecheck", node=node)
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