Fix Python – How can I replace (or strip) an extension from a filename in Python?

Is there a built-in function in Python that would replace (or remove, whatever) the extension of a filename (if it has one)?
print replace_extension(‘/home/user/somefile.txt’, ‘.jpg’)

In my example: /home/user/somefile.txt would become /home/user/somefile.jpg
I don’t know if it matters, but I need this for a SCons module I’m writing. (So….