Unexpected (by me) exec behavior

I have a script that I use to control our build process. It is a
general purpose script that exec's other scripts that contain project
specific python code. In one of those other, project-specific,
scripts I exec a third script. That third script is failing because
it can't seem to 'import' successfully. It seems that if one 'exec's
a string that in turn 'exec's another string, the 'import's don't work
in the second string. I'm assuming that I'm misunderstandin g
something about Python namespaces, but here's an example that
illustrates the problem:

----------------------------
# filename = t.py
s1 = """
s2 = \"\"\"
import socket
def xyz():
print socket.gethostb yname('somehost ')
if __name__ == '__main__':
xyz()
\"\"\"

def abc():
exec s2
if __name__ == '__main__':
abc()
"""

exec s1
-----------------------------

Traceback (most recent call last):
File "t.py", line 24, in ?
exec s1
File "<string>", line 13, in ?
File "<string>", line 11, in abc
File "<string>", line 6, in ?
File "<string>", line 4, in xyz
NameError: global name 'socket' is not defined

Can anyone explain this to me?

Mark