The parameter to the listen() method is not optional, but was marked as

freddrake · freddrake · commit c80d4dd69504 · 2001-05-29T15:37:45.000Z
optional in the documentation.

This closes SF bug #427985.
diff --git a/Doc/lib/libasyncore.tex b/Doc/lib/libasyncore.tex
@@ -136,7 +136,7 @@ \section{\module{asyncore} ---
   Read at most \var{buffer_size} bytes from the socket.
 \end{methoddesc}
 
-\begin{methoddesc}{listen}{\optional{backlog}}
+\begin{methoddesc}{listen}{backlog}
   Listen for connections made to the socket.  The \var{backlog}
   argument specifies the maximum number of queued connections
   and should be at least 1; the maximum value is
