C2901 & C2902: New checks for unnecessary lambda expression usage

ChangeLog

@@ -195,6 +195,13 @@ Release date: TBA
 
   Closes #5670
 
+* ``unnecessary-lambda-assignment``: Lambda expression assigned to a variable.
+  Define a function using the "def" keyword instead.
+  ``unnecessary-direct-lambda-call``: Lambda expression called directly.
+  Execute the expression inline instead.
+
+  Closes #5976
+
 * ``potential-index-error``: Emitted when the index of a list or tuple exceeds its length.
   This checker is currently quite conservative to avoid false positives. We welcome
   suggestions for improvements.

doc/data/messages/u/unnecessary-direct-lambda-call/bad.py

@@ -0,0 +1 @@
+y = (lambda x: x**2 + 2*x + 1)(a)  # [unnecessary-direct-lambda-call]

doc/data/messages/u/unnecessary-direct-lambda-call/good.py

@@ -0,0 +1 @@
+y = a**2 + 2*a + 1

doc/data/messages/u/unnecessary-lambda-assignment/bad.py

@@ -0,0 +1 @@
+foo = lambda x: x**2 + 2*x + 1  # [unnecessary-lambda-assignment]

doc/data/messages/u/unnecessary-lambda-assignment/good.py

@@ -0,0 +1,2 @@
+def foo(x):
+    return x**2 + 2*x + 1

doc/whatsnew/2.14.rst

@@ -35,6 +35,13 @@ New checkers
 
   Closes #5936
 
+* ``unnecessary-lambda-assignment``: Lambda expression assigned to a variable.
+  Define a function using the "def" keyword instead.
+  ``unnecessary-direct-lambda-call``: Lambda expression called directly.
+  Execute the expression inline instead.
+
+  Closes #5976
+
 * ``potential-index-error``: Emitted when the index of a list or tuple exceeds its length.
   This checker is currently quite conservative to avoid false positives. We welcome
   suggestions for improvements.

pylint/checkers/lambda_expressions.py

@@ -0,0 +1,93 @@
+# 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
+
+from itertools import zip_longest
+from typing import TYPE_CHECKING
+
+from astroid import nodes
+
+from pylint.checkers import BaseChecker
+from pylint.interfaces import HIGH
+
+if TYPE_CHECKING:
+    from pylint.lint import PyLinter
+
+
+class LambdaExpressionChecker(BaseChecker):
+    """Check for unnecessary usage of lambda expressions."""
+
+    name = "lambda-expressions"
+    msgs = {
+        "C3001": (
+            "Lambda expression assigned to a variable. "
+            'Define a function using the "def" keyword instead.',
+            "unnecessary-lambda-assignment",
+            "Used when a lambda expression is assigned to variable "
+            'rather than defining a standard function with the "def" keyword.',
+        ),
+        "C3002": (
+            "Lambda expression called directly. Execute the expression inline instead.",
+            "unnecessary-direct-lambda-call",
+            "Used when a lambda expression is directly called "
+            "rather than executing its contents inline.",
+        ),
+    }
+    options = ()
+
+    def visit_assign(self, node: nodes.Assign) -> None:
+        """Check if lambda expression is assigned to a variable."""
+        if isinstance(node.targets[0], nodes.AssignName) and isinstance(
+            node.value, nodes.Lambda
+        ):
+            self.add_message(
+                "unnecessary-lambda-assignment",
+                node=node.value,
+                confidence=HIGH,
+            )
+        elif isinstance(node.targets[0], nodes.Tuple) and isinstance(
+            node.value, (nodes.Tuple, nodes.List)
+        ):
+            # Iterate over tuple unpacking assignment elements and
+            # see if any lambdas are assigned to a variable.
+            # N.B. We may encounter W0632 (unbalanced-tuple-unpacking)
+            # and still need to flag the lambdas that are being assigned.
+            for lhs_elem, rhs_elem in zip_longest(
+                node.targets[0].elts, node.value.elts
+            ):
+                if lhs_elem is None or rhs_elem is None:
+                    # unbalanced tuple unpacking. stop checking.
+                    break
+                if isinstance(lhs_elem, nodes.AssignName) and isinstance(
+                    rhs_elem, nodes.Lambda
+                ):
+                    self.add_message(
+                        "unnecessary-lambda-assignment",
+                        node=rhs_elem,
+                        confidence=HIGH,
+                    )
+
+    def visit_namedexpr(self, node: nodes.NamedExpr) -> None:
+        if isinstance(node.target, nodes.AssignName) and isinstance(
+            node.value, nodes.Lambda
+        ):
+            self.add_message(
+                "unnecessary-lambda-assignment",
+                node=node.value,
+                confidence=HIGH,
+            )
+
+    def visit_call(self, node: nodes.Call) -> None:
+        """Check if lambda expression is called directly."""
+        if isinstance(node.func, nodes.Lambda):
+            self.add_message(
+                "unnecessary-direct-lambda-call",
+                node=node,
+                confidence=HIGH,
+            )
+
+
+def register(linter: PyLinter) -> None:
+    linter.register_checker(LambdaExpressionChecker(linter))

tests/functional/a/alternative/alternative_union_syntax.py

@@ -1,6 +1,6 @@
 """Test PEP 604 - Alternative Union syntax"""
 # pylint: disable=missing-function-docstring,unused-argument,invalid-name,missing-class-docstring
-# pylint: disable=inherit-non-class,too-few-public-methods
+# pylint: disable=inherit-non-class,too-few-public-methods,unnecessary-direct-lambda-call
 import dataclasses
 import typing
 from dataclasses import dataclass

tests/functional/a/alternative/alternative_union_syntax_error.py

@@ -4,7 +4,7 @@
 For Python 3.7 - 3.9: Everything should fail.
 Testing only 3.8/3.9 to support TypedDict.
 """
-# pylint: disable=missing-function-docstring,unused-argument,invalid-name,missing-class-docstring,inherit-non-class,too-few-public-methods,line-too-long
+# pylint: disable=missing-function-docstring,unused-argument,invalid-name,missing-class-docstring,inherit-non-class,too-few-public-methods,line-too-long,unnecessary-direct-lambda-call
 import dataclasses
 import typing
 from dataclasses import dataclass

tests/functional/a/alternative/alternative_union_syntax_py37.py

@@ -4,7 +4,7 @@
 For Python 3.7 - 3.9: Most things should work.
 Testing only 3.8/3.9 to support TypedDict.
 """
-# pylint: disable=missing-function-docstring,unused-argument,invalid-name,missing-class-docstring,inherit-non-class,too-few-public-methods,line-too-long
+# pylint: disable=missing-function-docstring,unused-argument,invalid-name,missing-class-docstring,inherit-non-class,too-few-public-methods,line-too-long,unnecessary-direct-lambda-call
 from __future__ import annotations
 import dataclasses
 import typing

tests/functional/a/arguments.py

@@ -1,5 +1,6 @@
 # pylint: disable=too-few-public-methods, missing-docstring,import-error,wrong-import-position
 # pylint: disable=wrong-import-order, useless-object-inheritance,unnecessary-lambda, consider-using-f-string
+# pylint: disable=unnecessary-lambda-assignment
 
 def decorator(fun):
     """Decorator"""

tests/functional/a/arguments.txt

@@ -1,39 +1,39 @@
-no-value-for-parameter:46:0:46:16::No value for argument 'first_argument' in function call:UNDEFINED
-too-many-function-args:47:0:47:25::Too many positional arguments for function call:UNDEFINED
-no-value-for-parameter:49:0:49:25::No value for argument 'third_argument' in function call:UNDEFINED
-no-value-for-parameter:51:0:51:17::No value for argument 'first_argument' in function call:UNDEFINED
-no-value-for-parameter:51:0:51:17::No value for argument 'second_argument' in function call:UNDEFINED
-no-value-for-parameter:51:0:51:17::No value for argument 'third_argument' in function call:UNDEFINED
-too-many-function-args:53:0:53:41::Too many positional arguments for function call:UNDEFINED
-no-value-for-parameter:58:0:58:21::No value for argument 'first_argument' in function call:UNDEFINED
-unexpected-keyword-arg:58:0:58:21::Unexpected keyword argument 'bob' in function call:UNDEFINED
-unexpected-keyword-arg:59:0:59:40::Unexpected keyword argument 'coin' in function call:UNDEFINED
-redundant-keyword-arg:61:0:61:30::Argument 'one' passed by position and keyword in function call:UNDEFINED
-no-value-for-parameter:66:0:66:8::No value for argument 'arg' in lambda call:UNDEFINED
-no-value-for-parameter:71:4:71:24:method_tests:No value for argument 'arg' in staticmethod call:UNDEFINED
-no-value-for-parameter:72:4:72:29:method_tests:No value for argument 'arg' in staticmethod call:UNDEFINED
-no-value-for-parameter:74:4:74:23:method_tests:No value for argument 'arg' in classmethod call:UNDEFINED
-no-value-for-parameter:75:4:75:28:method_tests:No value for argument 'arg' in classmethod call:UNDEFINED
-no-value-for-parameter:77:4:77:17:method_tests:No value for argument 'arg' in method call:UNDEFINED
-no-value-for-parameter:78:4:78:26:method_tests:No value for argument 'arg' in unbound method call:UNDEFINED
-no-value-for-parameter:80:4:80:27:method_tests:No value for argument 'arg' in method call:UNDEFINED
-no-value-for-parameter:81:4:81:36:method_tests:No value for argument 'arg' in unbound method call:UNDEFINED
-no-value-for-parameter:110:8:110:19:TestStaticMethod.func:No value for argument 'first' in staticmethod call:UNDEFINED
-too-many-function-args:111:8:111:29:TestStaticMethod.func:Too many positional arguments for staticmethod call:UNDEFINED
-too-many-function-args:119:8:119:27:TypeCheckConstructor.test:Too many positional arguments for constructor call:UNDEFINED
-no-value-for-parameter:121:8:121:20:TypeCheckConstructor.test:No value for argument 'first' in constructor call:UNDEFINED
-no-value-for-parameter:121:8:121:20:TypeCheckConstructor.test:No value for argument 'second' in constructor call:UNDEFINED
-no-value-for-parameter:122:8:122:29:TypeCheckConstructor.test:No value for argument 'second' in constructor call:UNDEFINED
-unexpected-keyword-arg:122:8:122:29:TypeCheckConstructor.test:Unexpected keyword argument 'lala' in constructor call:UNDEFINED
-no-value-for-parameter:133:8:133:18:Test.test:No value for argument 'icon' in method call:UNDEFINED
-too-many-function-args:134:8:134:25:Test.test:Too many positional arguments for method call:UNDEFINED
-no-value-for-parameter:136:0:136:12::No value for argument 'icon' in method call:UNDEFINED
-no-value-for-parameter:163:4:163:29:no_context_but_redefined:No value for argument 'three' in function call:UNDEFINED
-no-value-for-parameter:163:4:163:29:no_context_but_redefined:No value for argument 'two' in function call:UNDEFINED
-no-value-for-parameter:166:4:166:22:no_context_one_elem:No value for argument 'three' in function call:UNDEFINED
-no-value-for-parameter:166:4:166:22:no_context_one_elem:No value for argument 'two' in function call:UNDEFINED
-unexpected-keyword-arg:202:23:202:56:namedtuple_replace_issue_1036:Unexpected keyword argument 'd' in method call:UNDEFINED
-unexpected-keyword-arg:202:23:202:56:namedtuple_replace_issue_1036:Unexpected keyword argument 'e' in method call:UNDEFINED
-no-value-for-parameter:215:0:215:24::No value for argument 'third' in function call:UNDEFINED
-no-value-for-parameter:216:0:216:30::No value for argument 'second' in function call:UNDEFINED
-unexpected-keyword-arg:217:0:217:43::Unexpected keyword argument 'fourth' in function call:UNDEFINED
+no-value-for-parameter:47:0:47:16::No value for argument 'first_argument' in function call:UNDEFINED
+too-many-function-args:48:0:48:25::Too many positional arguments for function call:UNDEFINED
+no-value-for-parameter:50:0:50:25::No value for argument 'third_argument' in function call:UNDEFINED
+no-value-for-parameter:52:0:52:17::No value for argument 'first_argument' in function call:UNDEFINED
+no-value-for-parameter:52:0:52:17::No value for argument 'second_argument' in function call:UNDEFINED
+no-value-for-parameter:52:0:52:17::No value for argument 'third_argument' in function call:UNDEFINED
+too-many-function-args:54:0:54:41::Too many positional arguments for function call:UNDEFINED
+no-value-for-parameter:59:0:59:21::No value for argument 'first_argument' in function call:UNDEFINED
+unexpected-keyword-arg:59:0:59:21::Unexpected keyword argument 'bob' in function call:UNDEFINED
+unexpected-keyword-arg:60:0:60:40::Unexpected keyword argument 'coin' in function call:UNDEFINED
+redundant-keyword-arg:62:0:62:30::Argument 'one' passed by position and keyword in function call:UNDEFINED
+no-value-for-parameter:67:0:67:8::No value for argument 'arg' in lambda call:UNDEFINED
+no-value-for-parameter:72:4:72:24:method_tests:No value for argument 'arg' in staticmethod call:UNDEFINED
+no-value-for-parameter:73:4:73:29:method_tests:No value for argument 'arg' in staticmethod call:UNDEFINED
+no-value-for-parameter:75:4:75:23:method_tests:No value for argument 'arg' in classmethod call:UNDEFINED
+no-value-for-parameter:76:4:76:28:method_tests:No value for argument 'arg' in classmethod call:UNDEFINED
+no-value-for-parameter:78:4:78:17:method_tests:No value for argument 'arg' in method call:UNDEFINED
+no-value-for-parameter:79:4:79:26:method_tests:No value for argument 'arg' in unbound method call:UNDEFINED
+no-value-for-parameter:81:4:81:27:method_tests:No value for argument 'arg' in method call:UNDEFINED
+no-value-for-parameter:82:4:82:36:method_tests:No value for argument 'arg' in unbound method call:UNDEFINED
+no-value-for-parameter:111:8:111:19:TestStaticMethod.func:No value for argument 'first' in staticmethod call:UNDEFINED
+too-many-function-args:112:8:112:29:TestStaticMethod.func:Too many positional arguments for staticmethod call:UNDEFINED
+too-many-function-args:120:8:120:27:TypeCheckConstructor.test:Too many positional arguments for constructor call:UNDEFINED
+no-value-for-parameter:122:8:122:20:TypeCheckConstructor.test:No value for argument 'first' in constructor call:UNDEFINED
+no-value-for-parameter:122:8:122:20:TypeCheckConstructor.test:No value for argument 'second' in constructor call:UNDEFINED
+no-value-for-parameter:123:8:123:29:TypeCheckConstructor.test:No value for argument 'second' in constructor call:UNDEFINED
+unexpected-keyword-arg:123:8:123:29:TypeCheckConstructor.test:Unexpected keyword argument 'lala' in constructor call:UNDEFINED
+no-value-for-parameter:134:8:134:18:Test.test:No value for argument 'icon' in method call:UNDEFINED
+too-many-function-args:135:8:135:25:Test.test:Too many positional arguments for method call:UNDEFINED
+no-value-for-parameter:137:0:137:12::No value for argument 'icon' in method call:UNDEFINED
+no-value-for-parameter:164:4:164:29:no_context_but_redefined:No value for argument 'three' in function call:UNDEFINED
+no-value-for-parameter:164:4:164:29:no_context_but_redefined:No value for argument 'two' in function call:UNDEFINED
+no-value-for-parameter:167:4:167:22:no_context_one_elem:No value for argument 'three' in function call:UNDEFINED
+no-value-for-parameter:167:4:167:22:no_context_one_elem:No value for argument 'two' in function call:UNDEFINED
+unexpected-keyword-arg:203:23:203:56:namedtuple_replace_issue_1036:Unexpected keyword argument 'd' in method call:UNDEFINED
+unexpected-keyword-arg:203:23:203:56:namedtuple_replace_issue_1036:Unexpected keyword argument 'e' in method call:UNDEFINED
+no-value-for-parameter:216:0:216:24::No value for argument 'third' in function call:UNDEFINED
+no-value-for-parameter:217:0:217:30::No value for argument 'second' in function call:UNDEFINED
+unexpected-keyword-arg:218:0:218:43::Unexpected keyword argument 'fourth' in function call:UNDEFINED

tests/functional/a/assignment/assignment_expression.py

@@ -1,6 +1,8 @@
 """Test assignment expressions"""
 # pylint: disable=missing-docstring,unused-argument,unused-import,invalid-name
 # pylint: disable=blacklisted-name,unused-variable,pointless-statement,unused-variable
+# pylint: disable=unnecessary-lambda-assignment
+
 import re
 
 if (a := True):

tests/functional/a/assignment/assignment_expression.txt

@@ -1,3 +1,3 @@
-used-before-assignment:21:7:21:12::Using variable 'err_a' before assignment:HIGH
-used-before-assignment:22:6:22:11::Using variable 'err_b' before assignment:HIGH
-used-before-assignment:24:13:24:18::Using variable 'err_d' before assignment:HIGH
+used-before-assignment:23:7:23:12::Using variable 'err_a' before assignment:HIGH
+used-before-assignment:24:6:24:11::Using variable 'err_b' before assignment:HIGH
+used-before-assignment:26:13:26:18::Using variable 'err_d' before assignment:HIGH

tests/functional/c/cellvar_escaping_loop.py

@@ -1,4 +1,4 @@
-# pylint: disable=unnecessary-comprehension,missing-docstring,too-few-public-methods
+# pylint: disable=unnecessary-comprehension,missing-docstring,too-few-public-methods,unnecessary-direct-lambda-call
 """Tests for loopvar-in-closure."""
 from __future__ import print_function
 

tests/functional/c/class_scope.py

@@ -1,4 +1,4 @@
-# pylint: disable=too-few-public-methods, useless-object-inheritance
+# pylint: disable=too-few-public-methods, useless-object-inheritance, unnecessary-lambda-assignment
 """check for scope problems"""
 
 __revision__ = None

tests/functional/d/decorator_scope.py

@@ -1,4 +1,4 @@
-# -*- pylint: disable=too-few-public-methods, useless-object-inheritance
+# -*- pylint: disable=too-few-public-methods, useless-object-inheritance, unnecessary-lambda-assignment
 """Test that decorators sees the class namespace - just like
 function default values does but function body doesn't.
 

tests/functional/d/defined_and_used_on_same_line.py

@@ -1,5 +1,5 @@
 """Check for definitions and usage happening on the same line."""
-#pylint: disable=missing-docstring,multiple-statements,wrong-import-position,unnecessary-comprehension,unspecified-encoding
+#pylint: disable=missing-docstring,multiple-statements,wrong-import-position,unnecessary-comprehension,unspecified-encoding,unnecessary-lambda-assignment
 from __future__ import print_function
 print([index
        for index in range(10)])

tests/functional/ext/typing/typing_consider_using_alias.py

@@ -3,7 +3,7 @@
 'py-version' needs to be set to '3.7' or '3.8' and 'runtime-typing=no'.
 With 'from __future__ import annotations' present.
 """
-# pylint: disable=missing-docstring,invalid-name,unused-argument,line-too-long
+# pylint: disable=missing-docstring,invalid-name,unused-argument,line-too-long,unnecessary-direct-lambda-call
 from __future__ import annotations
 
 import collections

tests/functional/ext/typing/typing_consider_using_alias_without_future.py

@@ -2,7 +2,7 @@
 
 'py-version' needs to be set to '3.7' or '3.8' and 'runtime-typing=no'.
 """
-# pylint: disable=missing-docstring,invalid-name,unused-argument,line-too-long,unsubscriptable-object
+# pylint: disable=missing-docstring,invalid-name,unused-argument,line-too-long,unsubscriptable-object,unnecessary-direct-lambda-call
 import collections
 import collections.abc
 import typing

tests/functional/ext/typing/typing_consider_using_union.py

@@ -4,7 +4,7 @@
 With 'from __future__ import annotations' present.
 """
 # pylint: disable=missing-docstring,invalid-name,unused-argument,line-too-long
-# pylint: disable=consider-using-alias
+# pylint: disable=consider-using-alias,unnecessary-direct-lambda-call
 from __future__ import annotations
 from dataclasses import dataclass
 import typing

tests/functional/ext/typing/typing_consider_using_union_py310.py

@@ -3,7 +3,7 @@
 'py-version' needs to be set to >= '3.10'.
 """
 # pylint: disable=missing-docstring,invalid-name,unused-argument,line-too-long
-# pylint: disable=deprecated-typing-alias
+# pylint: disable=deprecated-typing-alias,unnecessary-direct-lambda-call
 from dataclasses import dataclass
 import typing
 from typing import Dict, List, Optional, Union, TypedDict

tests/functional/ext/typing/typing_consider_using_union_without_future.py

@@ -2,7 +2,7 @@
 
 'py-version' needs to be set to >= '3.7' and 'runtime-typing=no'.
 """
-# pylint: disable=missing-docstring,invalid-name,unused-argument,line-too-long
+# pylint: disable=missing-docstring,invalid-name,unused-argument,line-too-long,unnecessary-direct-lambda-call
 # pylint: disable=consider-using-alias
 from dataclasses import dataclass
 import typing

tests/functional/ext/typing/typing_deprecated_alias.py

@@ -2,7 +2,7 @@
 
 'py-version' needs to be set to >= '3.9'.
 """
-# pylint: disable=missing-docstring,invalid-name,unused-argument,line-too-long,unsubscriptable-object
+# pylint: disable=missing-docstring,invalid-name,unused-argument,line-too-long,unsubscriptable-object,unnecessary-direct-lambda-call
 import collections
 import collections.abc
 import typing

tests/functional/g/generic_alias/generic_alias_related.py

@@ -7,7 +7,7 @@
 """
 # flake8: noqa
 # pylint: disable=missing-docstring,pointless-statement,expression-not-assigned
-# pylint: disable=too-few-public-methods,multiple-statements,line-too-long
+# pylint: disable=too-few-public-methods,multiple-statements,line-too-long, unnecessary-lambda-assignment
 from abc import ABCMeta, abstractmethod
 import typing
 

tests/functional/i/invalid/invalid_bool_returned.py

@@ -1,6 +1,6 @@
 """Check invalid value returned by __bool__ """
 
-# pylint: disable=too-few-public-methods,missing-docstring,no-self-use,import-error, useless-object-inheritance
+# pylint: disable=too-few-public-methods,missing-docstring,no-self-use,import-error, useless-object-inheritance, unnecessary-lambda-assignment
 import six
 
 from missing import Missing