PEP 572: Add additional motivation and examples

pep-0572.rst

@@ -31,6 +31,14 @@ the worst of the confusions, we change the definition of comprehensions,
 causing some edge cases to be interpreted differently, but maintaining the
 existing behaviour in the majority of situations.
 
+Additionally, naming sub-parts of a large expression can assist an interactive
+debugger, providing useful display hooks and partial results. Without a way to
+capture sub-expressions inline, this would require refactoring of the original
+code; with assignment expressions, this merely requires the insertion of a few
+``name :=`` markers. Removing the need to refactor reduces the likelihood that
+the code be inadvertently changed as part of debugging (a common cause of
+Heisenbugs), and is easier to dictate to a student or junior programmer.
+
 
 Syntax and semantics
 ====================
@@ -234,6 +242,12 @@ an ``if`` or ``while`` statement::
     # of this effect
     if match := re.search(pat, text):
         print("Found:", match.group(0))
+    # The same syntax chains nicely into 'elif' statements, unlike the
+    # equivalent using assignment statements.
+    elif match := re.search(otherpat, text):
+        print("Alternate found:", match.group(0))
+    elif match := re.search(third, text):
+        print("Fallback found:", match.group(0))
 
     # Reading socket data until an empty string is returned
     while data := sock.read():