Fix bug with cell-var-from-loop and kw_defaults (#5045)

* Fix bug with cell-var-from-loop and kw_defaults
* Use more efficient generator for defaults and add issue link
* Fix test output for cellvar_escaping_loop

Co-authored-by: Pierre Sassoulas <pierre.sassoulas@gmail.com>

ChangeLog

@@ -29,6 +29,11 @@ Release date: TBA
 
   Closes #5029
 
+* Fixed false positive for ``cell-var-from-loop`` when variable is used as the default
+  value for a keyword-only parameter.
+
+  Closes #5012
+
 
 What's New in Pylint 2.11.1?
 ============================

pylint/checkers/utils.py

@@ -402,10 +402,15 @@ def is_default_argument(
     if not scope:
         scope = node.scope()
     if isinstance(scope, (nodes.FunctionDef, nodes.Lambda)):
-        for default_node in scope.args.defaults:
-            for default_name_node in default_node.nodes_of_class(nodes.Name):
-                if default_name_node is node:
-                    return True
+        all_defaults = itertools.chain(
+            scope.args.defaults, (d for d in scope.args.kw_defaults if d is not None)
+        )
+        return any(
+            default_name_node is node
+            for default_node in all_defaults
+            for default_name_node in default_node.nodes_of_class(nodes.Name)
+        )
+
     return False
 
 

tests/functional/c/cellvar_escaping_loop.py

@@ -93,6 +93,22 @@ def good_case_issue3107():
         next(filter(lambda j, ix=i[0]: j == ix, [1, 3]))
 
 
+def good_case_issue_5012():
+    """Eager binding of cell variable when used as the default value of a keyword-only argument.
+    https://github.com/PyCQA/pylint/issues/5012
+    """
+    funs = []
+    for i in range(5):
+        def func(*, _i=i):
+            print(_i)
+        funs.append(func)
+
+        def func2(_i=i):
+            print(_i)
+        funs.append(func2)
+
+    return funs
+
 def bad_case():
     """Closing over a loop variable."""
     lst = []

tests/functional/c/cellvar_escaping_loop.txt

@@ -1,13 +1,13 @@
-cell-var-from-loop:101:27:bad_case.<lambda>:Cell variable i defined in loop:HIGH
-cell-var-from-loop:106:20:bad_case2.<lambda>:Cell variable i defined in loop:HIGH
-cell-var-from-loop:114:27:bad_case3.<lambda>:Cell variable j defined in loop:HIGH
-cell-var-from-loop:124:19:bad_case4.nested:Cell variable i defined in loop:HIGH
-cell-var-from-loop:145:20:bad_case5.<lambda>:Cell variable i defined in loop:HIGH
-cell-var-from-loop:153:27:bad_case6.<lambda>:Cell variable i defined in loop:HIGH
-cell-var-from-loop:161:12:bad_case7.<lambda>:Cell variable x defined in loop:HIGH
-cell-var-from-loop:162:14:bad_case7.<lambda>:Cell variable y defined in loop:HIGH
-cell-var-from-loop:171:27:bad_case8.<lambda>:Cell variable j defined in loop:HIGH
-cell-var-from-loop:181:27:bad_case9.<lambda>:Cell variable i defined in loop:HIGH
-cell-var-from-loop:190:26:bad_case10.func.func2:Cell variable i defined in loop:HIGH
-cell-var-from-loop:202:17:bad_case_issue2846.<lambda>:Cell variable n defined in loop:HIGH
-cell-var-from-loop:207:18:bad_case_issue2846.<lambda>:Cell variable n defined in loop:HIGH
+cell-var-from-loop:117:27:bad_case.<lambda>:Cell variable i defined in loop:HIGH
+cell-var-from-loop:122:20:bad_case2.<lambda>:Cell variable i defined in loop:HIGH
+cell-var-from-loop:130:27:bad_case3.<lambda>:Cell variable j defined in loop:HIGH
+cell-var-from-loop:140:19:bad_case4.nested:Cell variable i defined in loop:HIGH
+cell-var-from-loop:161:20:bad_case5.<lambda>:Cell variable i defined in loop:HIGH
+cell-var-from-loop:169:27:bad_case6.<lambda>:Cell variable i defined in loop:HIGH
+cell-var-from-loop:177:12:bad_case7.<lambda>:Cell variable x defined in loop:HIGH
+cell-var-from-loop:178:14:bad_case7.<lambda>:Cell variable y defined in loop:HIGH
+cell-var-from-loop:187:27:bad_case8.<lambda>:Cell variable j defined in loop:HIGH
+cell-var-from-loop:197:27:bad_case9.<lambda>:Cell variable i defined in loop:HIGH
+cell-var-from-loop:206:26:bad_case10.func.func2:Cell variable i defined in loop:HIGH
+cell-var-from-loop:218:17:bad_case_issue2846.<lambda>:Cell variable n defined in loop:HIGH
+cell-var-from-loop:223:18:bad_case_issue2846.<lambda>:Cell variable n defined in loop:HIGH