Why does Jupyter Notebook "forget" Cython from one cell to the next?

Question

When I compile a cell with cython, it seems Jupyter forgets the compiled function in the next cell. This seems to me to be not right. What is going wrong?

I am using version 5.0.0 of the notebook, and

Python 3.6.1 |Anaconda custom (x86_64)| (default, May 11 2017, 13:04:09) 
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.57)]

Here is a MWE that produces the problem:

Cell 1:

%load_ext Cython

Cell 2:

%%cython
cdef int foo():
    return 3

print(foo())

This produces:

3

In the next cell, I have

print(foo())

This produces:

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-3-9701608cebc0> in <module>()
----> 1 print(foo())

NameError: name 'foo' is not defined

Answer 1

I guess it's because you didn't defined your foo function as available in python (with cpdef) but only only give it a C signature (with cdef) so it can only be called from cython code.
In the cell 2 you can call it because you are still using cython code but in your cell 3 you are back in pure python and the function isn't available. There is various way to get the result from the foo function in python :

%%cython
# Not reachable in pure python:
cdef int foo():
    return 3

# Python visible function signature:
cpdef int foo2():
    return 3

# Or a wrapper around the cython function:
def foo3():
    return foo()

You can now try to call foo2() or foo3() in your python code.

See one of the relevant part of the documentation if you haven't see it.