What is usually meant by "a callback" is a function object. In Python, functions are first class objects. You just use the function name without the parentheses.
print "Executing my_function"
b = my_function # b is now a function object
Likewise, instead of storing it in a global, you might pass it to a method which stores it as an object attribute, or whatever.
Also of interest is that you can easily create partial functions, where some of the parameters are already decided. See the docs for functools.partial
And if you're trying to use a method as a callback, you can store the bound-method, which is effectively a partial including the self parameter.
Finally, don't forget lambda functions, which can be useful if you're trying to create a simple function and don't need a name for it.