[Python-Dev] Assignment expression and coding style: the while True case

[Victor Stinner]

On the 3360 for loops of the stdlib (*), I only found 2 loops which
would benefit of assignment expressions.

It's not easy to find loops which:
- build a list,
- are simple enough to be expressed as list comprehension,
- use a condition (if),
- use an expression different than just a variable name as the list
value (value appended to the list).

diff --git a/Lib/mailbox.py b/Lib/mailbox.py
index 056251dce0..dc61a3a8b6 100644
--- a/Lib/mailbox.py
+++ b/Lib/mailbox.py
@@ -1341,9 +1341,9 @@ class Babyl(_singlefileMailbox):
                 if len(stops) < len(starts):
                     stops.append(line_pos - len(linesep))
                 starts.append(next_pos)
-                labels = [label.strip() for label
-                                        in
self._file.readline()[1:].split(b',')
-                                        if label.strip()]
+                labels = [slabel for label
+                          in self._file.readline()[1:].split(b',')
+                          if (slabel := label.strip())]
                 label_lists.append(labels)
             elif line == b'\037' or line == b'\037' + linesep:
                 if len(stops) < len(starts):

diff --git a/Lib/nntplib.py b/Lib/nntplib.py
index 5961a28ab7..a5d13e35be 100644
--- a/Lib/nntplib.py
+++ b/Lib/nntplib.py
@@ -843,11 +843,9 @@ class _NNTPBase:
                       DeprecationWarning, 2)
         line_pat = re.compile('^([^ \t]+)[ \t]+(.*)$')
         resp, raw_lines = self._longcmdstring('XGTITLE ' + group, file)
-        lines = []
-        for raw_line in raw_lines:
-            match = line_pat.search(raw_line.strip())
-            if match:
-                lines.append(match.group(1, 2))
+        lines = [match.group(1, 2)
+                 for raw_line in raw_lines
+                 if (match := line_pat.search(raw_line.strip()))]
         return resp, lines

     def xpath(self, id):

(*) Command used to count the number of for loops in the stdlib:

$ grep '\bfor\b' Lib/*py
Lib/{asyncio,logging,multiprocessing}/*.py|grep -v '"""'|grep -v
"'''"|grep -v '\.py: *#'|wc -l
3360

Victor