Better example for W0640: cell-var-from-loop (#8960)

Co-authored-by: Pierre Sassoulas <[email protected]>

Author: SubaruArai

doc/data/messages/c/cell-var-from-loop/bad.py

@@ -1,10 +1,23 @@
 def teacher_greeting(names):
+    greetings = []
     for name in names:
 
         def greet():
+            # do something
             print(f"Hello, {name}!")  # [cell-var-from-loop]
 
+        if name.isalpha():
+            greetings.append(greet)
+
+    for greet in greetings:
+        # the "name" variable is evaluated when the function is called here,
+        # which is the last value it had in the loop - "Not-A-Name"
         greet()
 
 
 teacher_greeting(["Graham", "John", "Terry", "Eric", "Terry", "Michael"])
+# "Hello, Michael!"
+# "Hello, Michael!"
+# "Hello, Michael!"
+# "Hello, Michael!"
+# "Hello, Michael!"

doc/data/messages/c/cell-var-from-loop/good.py

@@ -0,0 +1,23 @@
+import functools
+
+
+def teacher_greeting(names):
+    greetings = []
+    for name in names:
+        if name.isalpha():
+            # "name" is evaluated when the partial is created here, so this
+            # does not do lazy evaluation
+            greetings.append(functools.partial(print, f"Hello, {name}!"))
+
+    for greet in greetings:
+        # `partial`s are called like functions, but you've already passed the
+        # arguments to them
+        greet()
+
+
+teacher_greeting(["Graham", "John", "Terry", "Eric", "Terry", "Michael"])
+# "Hello, Graham!"
+# "Hello, John!"
+# "Hello, Eric!"
+# "Hello, Terry!"
+# "Hello, Michael!"

doc/data/messages/c/cell-var-from-loop/good/functools.partial.py

@@ -0,0 +1,17 @@
+def teacher_greeting(names):
+    def greet(name):
+        # do something
+        print(f"Hello, {name}!")
+
+    for name in names:
+        if name.isalpha():
+            # we're passing the value of "name" to the function here
+            greet(name)
+
+
+teacher_greeting(["Graham", "John", "Terry", "Eric", "Terry", "Michael"])
+# "Hello, Graham!"
+# "Hello, John!"
+# "Hello, Eric!"
+# "Hello, Terry!"
+# "Hello, Michael!"