Fix from the Madusa mailing list:

tim-one · tim-one · commit 6fd71206169c · 2001-04-08T07:23:44.000Z
http://groups.yahoo.com/group/medusa/message/333 It's clear that Medusa should not be checking for an empty buffer via "buf is ''". The patch merely changes "is" to "==". However, there's a mystery here all the same: Python attempts to store null strings uniquely, so it's unclear why "buf is ''" ever returned false when buf actually was empty. *Some* string operations produce non-unique null strings, e.g. >>> "abc"*0 is "abc"*0 0 >>> but they're rare, and I don't see any such operations in asynchat.
diff --git a/Lib/asynchat.py b/Lib/asynchat.py
@@ -165,7 +165,7 @@ def writable (self):
         # return len(self.ac_out_buffer) or len(self.producer_fifo) or (not self.connected)
         # this is about twice as fast, though not as clear.
         return not (
-                (self.ac_out_buffer is '') and
+                (self.ac_out_buffer == '') and
                 self.producer_fifo.is_empty() and
                 self.connected
                 )

Original file line number	Diff line number	Diff line change
`@@ -165,7 +165,7 @@ def writable (self):`
`165`	`165`	`# return len(self.ac_out_buffer) or len(self.producer_fifo) or (not self.connected)`
`166`	`166`	`# this is about twice as fast, though not as clear.`
`167`	`167`	`return not (`
`168`		`- (self.ac_out_buffer is '') and`
	`168`	`+ (self.ac_out_buffer == '') and`
`169`	`169`	`self.producer_fifo.is_empty() and`
`170`	`170`	`self.connected`
`171`	`171`	`)`