In [1]: %reload_ext Cython
In [2]: %%cython --name foobar
        def f(int a, int b):
            return a + b
In [3]: f(5, 3)
Out[3]: 8

In [1]: %reload_ext Cython
In [4]: %%cython --name foobar
        def f(int a, int b):
            return a - b
In [5]: f(5, 3)
Out[5]: 8

%load_ext autoreload
%autoreload 2

%%cython --name foobar_1
def f(int a, int b):
    return a - b

from foobar import f
f(a, b)