Fix unnecessary-lambda false positive for lambdas using its parameters in their body (#8498) (#8506)

Fixes #8496

(cherry picked from commit b621436)

Co-authored-by: cherryblossom <[email protected]>

Author: github-actions[bot]

doc/whatsnew/fragments/8496.false_positive

@@ -0,0 +1,5 @@
+``unnecessary-lambda`` no longer warns on lambdas which use its parameters in
+their body (other than the final arguments), e.g.
+``lambda foo: (bar if foo else baz)(foo)``.
+
+Closes #8496

pylint/checkers/base/basic_checker.py

@@ -519,6 +519,7 @@ def _has_variadic_argument(
         )
 
     @utils.only_required_for_messages("unnecessary-lambda")
+    # pylint: disable-next=too-many-return-statements
     def visit_lambda(self, node: nodes.Lambda) -> None:
         """Check whether the lambda is suspicious."""
         # if the body of the lambda is a call expression with the same
@@ -576,6 +577,13 @@ def visit_lambda(self, node: nodes.Lambda) -> None:
             if arg.name != passed_arg.name:
                 return
 
+        # The lambda is necessary if it uses its parameter in the function it is
+        # calling in the lambda's body
+        # e.g. lambda foo: (func1 if foo else func2)(foo)
+        for name in call.func.nodes_of_class(nodes.Name):
+            if name.lookup(name.name)[0] is node:
+                return
+
         self.add_message("unnecessary-lambda", line=node.fromlineno, node=node)
 
     @utils.only_required_for_messages("dangerous-default-value")

tests/functional/u/unnecessary/unnecessary_lambda.py

@@ -24,6 +24,12 @@
 # +1: [unnecessary-lambda]
 _ = lambda x, y, z, *args, **kwargs: _ANYARGS(x, y, z, *args, **kwargs)
 
+# These don't use their parameters in their body
+# +1: [unnecessary-lambda]
+_ = lambda x: z(lambda x: x)(x)
+# +1: [unnecessary-lambda]
+_ = lambda x, y: z(lambda x, y: x + y)(x, y)
+
 # Lambdas that are *not* unnecessary and should *not* trigger warnings.
 _ = lambda x: x
 _ = lambda x: x()
@@ -50,3 +56,8 @@
 _ = lambda: code().analysis()
 
 _ = lambda **kwargs: dict(bar=42, **kwargs)
+
+# These use the lambda parameters in their body
+_ = lambda x: x(x)
+_ = lambda x, y: x(x, y)
+_ = lambda x: z(lambda y: x + y)(x)

tests/functional/u/unnecessary/unnecessary_lambda.txt

@@ -5,3 +5,5 @@ unnecessary-lambda:19:4:19:33:<lambda>:Lambda may not be necessary:UNDEFINED
 unnecessary-lambda:21:4:21:39:<lambda>:Lambda may not be necessary:UNDEFINED
 unnecessary-lambda:23:4:23:53:<lambda>:Lambda may not be necessary:UNDEFINED
 unnecessary-lambda:25:4:25:71:<lambda>:Lambda may not be necessary:UNDEFINED
+unnecessary-lambda:29:4:29:31:<lambda>:Lambda may not be necessary:UNDEFINED
+unnecessary-lambda:31:4:31:44:<lambda>:Lambda may not be necessary:UNDEFINED