import os, inspect

source_org = """
def foo():
    pass
"""

source_new = """
def foo():
    return 1
"""

somefile = os.tmpnam()
file(somefile, 'w').write(source_org)
execfile(somefile)
print inspect.findsource(foo)
file(somefile, 'w').write(source_new)
print inspect.findsource(foo)