My lisp is a bit rusty, but it looks like what you're doing there is returning a function which gets redefined every time you reuse the function.

The equivalent Python would be something like this:

    def function():
        x = 3
        def internal(x, foo=[]):
            foo.append([7])
            foo.append(x)
            return foo
        return internal(x)
        
    print function()
    print function()

Which does what you would expect:

    [[7], 3]
    [[7], 3]