Q:

import a function from another .ipynb file

I defined a hello world function in a file called 'functions.ipynb'.  Now, I would like to import functions in another file by using "import functions".  I am sure that they are in the same folder.  However, it still shows that "ImportError: No module named functions".  By the way, I am using jupyter notebook.  Thanks a lot! 

A:

You'll want to use the ipynb package/module importer.  You'll need to install it: pip install ipynb. 
Create a Notebook named my_functions.ipynb.  Add a simple function to it. 
def factorial(n):
    if n == 0:
        return 1
    else:
        return n * factorial(n-1)

Then, create a second IPython Notebook and import this function with:
from ipynb.fs.full.my_functions import factorial

Then you can use it as if it was in the same IPython Notebook:
testing = factorial(5)

See the documentation for more details.