There are many ways to import a Python file, all with their pros and cons.

Don't just hastily pick the first import strategy that works for you, or else you'll have to rewrite the codebase later on when you find it doesn't meet your needs.

I'll start out explaining the easiest example #1, then I'll move toward the most professional and robust example #7

Example 1, Import a Python module with Python interpreter:

1. Put this in /home/el/foo/fox.py:

   def what_does_the_fox_say():
     print("vixens cry")
   

2. Get into the Python interpreter:

   el@apollo:/home/el/foo$ python
   Python 2.7.3 (default, Sep 26 2013, 20:03:06)
   >>> import fox
   >>> fox.what_does_the_fox_say()
   vixens cry
   

   You imported fox through the Python interpreter, invoked the Python function what_does_the_fox_say() from within fox.py.

Example 2, Use execfile or (exec in Python 3) in a script to execute the other Python file in place:

1. Put this in /home/el/foo2/mylib.py:

   def moobar():
     print("hi")
   

2. Put this in /home/el/foo2/main.py:

   execfile("/home/el/foo2/mylib.py")
   moobar()
   

3. run the file:

    el@apollo:/home/el/foo$ python main.py
    hi
   

   The function moobar was imported from mylib.py and made available in main.py

Example 3, Use from ... import ... functionality:

1. Put this in /home/el/foo3/chekov.py:

   def question():
     print("where are the nuclear wessels?")
   

2. Put this in /home/el/foo3/main.py:

    from chekov import question
    question()
   

3. Run it like this:

    el@apollo:/home/el/foo3$ python main.py
    where are the nuclear wessels?
   

   If you defined other functions in chekov.py, they would not be available unless you import *

Example 4, Import riaa.py if it's in a different file location from where it is imported

1. Put this in /home/el/foo4/stuff/riaa.py:

    def watchout():
      print("computers are transforming into a noose and a yoke for humans")
   

2. Put this in /home/el/foo4/main.py:

    import sys
    import os
    sys.path.append(os.path.abspath("/home/el/foo4/stuff"))
    from riaa import *
    watchout()
   

3. Run it:

   el@apollo:/home/el/foo4$ python main.py
   computers are transforming into a noose and a yoke for humans
   

   That imports everything in the foreign file from a different directory.

Example 5, use os.system("python yourfile.py")

import os
os.system("python yourfile.py")

Example 6, import your file via piggybacking the Python startuphook:

This example used to work for both Python 2 and Python 3, but now only works for Python 2. Python 3 got rid of this user startuphook feature set, because it was abused by low-skill Python library writers, using it to impolitely inject their code into the global namespace, before all user-defined programs. If you want this to work for Python 3, you'll have to get more creative. If I tell you how to do it, Python developers will disable that feature set as well, so you're on your own.

See: https://docs.python.org/2/library/user.html

Put this code into your home directory in ~/.pythonrc.py

class secretclass:
    def secretmessage(cls, myarg):
        return myarg + " is if.. up in the sky, the sky"
    secretmessage = classmethod( secretmessage )

    def skycake(cls):
        return "cookie and sky pie people can't go up and "
    skycake = classmethod( skycake )

Put this code into your main.py (can be anywhere):

import user
msg = "The only way skycake tates good"
msg = user.secretclass.secretmessage(msg)
msg += user.secretclass.skycake()
print(msg + " have the sky pie! SKYCAKE!")

Run it, you should get this:

$ python main.py
The only way skycake tates good is if.. up in the sky,
the skycookie and sky pie people can't go up and  have the sky pie!
SKYCAKE!

If you get an error here: ModuleNotFoundError: No module named 'user' then it means you're using Python 3, startuphooks are disabled there by default.

Credit for this jist goes to: https://github.com/docwhat/homedir-examples/blob/master/python-commandline/.pythonrc.py Send along your up-boats.

Example 7, Most Robust: Import files in Python with the bare import command:

1. Make a new directory /home/el/foo5/

2. Make a new directory /home/el/foo5/herp

3. Make an empty file named __init__.py under herp:

   el@apollo:/home/el/foo5/herp$ touch __init__.py
   el@apollo:/home/el/foo5/herp$ ls
   __init__.py
   

4. Make a new directory /home/el/foo5/herp/derp

5. Under derp, make another __init__.py file:

   el@apollo:/home/el/foo5/herp/derp$ touch __init__.py
   el@apollo:/home/el/foo5/herp/derp$ ls
   __init__.py
   

6. Under /home/el/foo5/herp/derp make a new file called yolo.py Put this in there:

   def skycake():
     print("SkyCake evolves to stay just beyond the cognitive reach of " +
     "the bulk of men. SKYCAKE!!")
   

7. The moment of truth, Make the new file /home/el/foo5/main.py, put this in there;

   from herp.derp.yolo import skycake
   skycake()
   

8. Run it:

   el@apollo:/home/el/foo5$ python main.py
   SkyCake evolves to stay just beyond the cognitive reach of the bulk
   of men. SKYCAKE!!
   

   The empty __init__.py file communicates to the Python interpreter that the developer intends this directory to be an importable package.

If you want to see my post on how to include ALL .py files under a directory see here: https://stackoverflow.com/a/20753073/445131