Why doesn't a %en0 suffix work to connect a link-local IPv6 TCP socket in Python?

A week or so ago someone on StackOverflow asked why their Python code for connecting to an IPv6 link-local address wasn't working, and I replied that since it was a link-local address they needed to add a %en0 (or whatever the desired local-interface-name is) suffix to their target IP address. I thought I knew what I was talking about, so I didn't actually test my suggestion before answering (shame on me!).

Today I went to use that same technique for myself, only to find that it doesn't seem to work. :^( That is, this code does not work:

>>> from socket import *
>>> s = socket(AF_INET6, SOCK_STREAM)
>>> s.connect(('fe80::21f:5bff:fe3f:1b36%en0', 2001))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1, in connect
socket.error: [Errno 65] No route to host

The following code, on the other hand, DOES work (with or without the %en0 suffix):

>>> from socket import *
>>> s = socket(AF_INET6, SOCK_STREAM)
>>> s.connect(('fe80::21f:5bff:fe3f:1b36%en0', 2001, 0, 6))
>>> 

... but I don't like doing it that way, because in order to figure out which scope ID integer to supply for the last argument, I have to execute a bunch of not-very-portable code to iterate over the local interfaces list, find the开发者_JS百科 interface named 'en0', and extract its scope ID, which is more complexity overhead than I'd like to have.

Given that connect() is accepting the %en0 suffix to the IP address, why isn't it actually using it as expected to determine the scope ID?

FWIW, I am testing with Python 2.6.1 under MacOS/X 10.6.4.

This is the correct way to do an ipv6 connection:

>>> addrinfo = getaddrinfo('fe80::225:ff:fecd:5aa0%en0', 2001, AF_INET6, SOCK_STREAM)
>>> addrinfo
[(30, 1, 6, '', ('fe80::225:ff:fecd:5aa0%en0', 2001, 0, 4))]
>>> (family, socktype, proto, canonname, sockaddr) = addrinfo[0]
>>> s = socket(family, socktype, proto)
>>> s.connect(sockaddr)

getaddrinfo() will return the correct numerical scope and flow information for you.