Replace list of constants with tuples of constants.
diff --git a/Lib/smtplib.py b/Lib/smtplib.py
index 52a2e1f..5b406af 100755
--- a/Lib/smtplib.py
+++ b/Lib/smtplib.py
@@ -578,7 +578,7 @@
(code, resp) = self.docmd(encode_base64(password, eol=""))
elif authmethod is None:
raise SMTPException("No suitable authentication method found.")
- if code not in [235, 503]:
+ if code not in (235, 503):
# 235 == 'Authentication successful'
# 503 == 'Error: already authenticated'
raise SMTPAuthenticationError(code, resp)