You can connect to a IPv4 addr "a.b.c.d" from a IPv6
socket simply connecting to "::FFFF:a.b.c.d" IPv6 address.

When you issue a "connect" call on a IPv6 socket using
a pure IPv4 addr "a.b.c.d", this format SHOULD be
converted "automagically" to "::FFFF:a.b.c.d",
intenally. Currently, this operation gives an error.

The real problem is "connect" call using a hostname
(not resolved to a IPv4 or IPv6 addr). Since I don't
know to which IP address type the host will resolve, I
think the correct course should be to promote IPv4 to
IPv6 address space (if the socket is IPv6) and to
connect normally.

Obviously, trying to connect to a IPv6 addr using a
IPv4 socket should fail, like just now.

When a hostname used in "connect" call resolves to both
IPv4 and IPv6 addresses, it would use IPv4 if the
socket is IPv4, and IPv6 if the socket is IPv6.

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I fail to see a Python bug here. All calls are delegated to
the C library, which delegates most of it to the kernel. So
complain to your operating system vendor.

I don't think your complaint is valid, though: the sockaddr
is computed by looking at its format. If it does not
contains a colon, it is not an AF_INET6.

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The problem, as I said, is when you use a hostname in the
"connect" call. Since the program has no control over the
address type the DNS lookup will resolve, I, in my soft,
can't search for colons in order to create IF_INET or
IF_INET6 sockets, since that info is processed inside the
"connect" call.

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Can you please post an example connect call?

Did you consider the IPv6 example in

http://www.python.org/doc/current/lib/socket-example.html ?

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>>> import socket
>>> a=socket.socket(socket.AF_INET6,socket.SOCK_STREAM)
>>> a.connect(("www.elmundo.es",80))

If "www.elmundo.es" is a IPv4 ONLY server, the connect
"fails". Nevertheless, you can reach IPv4 hosts from IPv6
sockets, using the "::FFFF:a.b.c.d" format...

You suggest using "getaddinfo". That could be an option but,
then, this problem should be documented in docs, I think :-p

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Please understand that we expose the functionality from the
C library. If you do

socket.getaddrinfo("www.elmundo.es",None,0,socket.SOCK_STREAM)

you get

[(2, 1, 6, '', ('193.110.128.200', 0))]

i.e. there is *no* IPv4 mapped address reported from the C
library. See

http://www.wcug.wwu.edu/lists/netdev/199702/msg00179.html

why there is no need to report one, and

http://lwn.net/Articles/8646/

why IPv4 mapped addresses are considered harmful.

See the examples of how to use getaddrinfo to solve this
problem: getaddrinfo tells you to open a IPv4 socket if
there is no IPv6 connectivity to the requested service.

I see no need to change any documentation. If you think
something should change, please submit a patch.