Where are my python modules?

When we are developing software, it doesn’t matter which language, it is a best practice to split the code in small pieces, it helps the legibility and code organization. When working with C, for example, we create header (*.h) files and implementation files (*.c). When working with python there are module files which have extension .py. To load a module we use the import keyword.

A question often asked is how to find the location of my python modules. For example this error message: ImportError: No module named XXX. Has it ever happened to you? :D Here we will try to understand a little bit more about this problem.

To start I will create a pub directory and add to it a module named drink.py. As I’m living in England, to drink a pint is part of my culture now. \o/

$ mkdir pub
$ touch pub/drink.py
$ echo "print('give me a pint')" > pub/drink.py

Lets try to import the module drink.py.

$ python
Python 2.7.10 (default, Jul 14 2015, 19:46:27) 
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.39)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import drink
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ImportError: No module named drink

ImportError: No module named drink. The message is clear and tell us that python doesn’t know where our module is. Maybe you are thinking, go to pub directory and run the interpreter from there.

$ cd pub/
$ python
>>> import drink
give me a pint

It works! Why? And if I have modules in different directories, how to import all at the same time? In this case we can use sys.path that is a list of strings that specifies the search path for modules.

Ok, lets go back to the previous directory and add pub in the sys.path.

$ cd ..
$ python
>>> import sys
>>> sys.path
['', '/Library/Python/2.7/site-packages/npm-0.1.1-py2.7.egg',
'/Library/Python/2.7/site-packages/optional_django-0.1.0-py2.7.egg', '/tmp',

Have a look in the first value of the list ‘’. It means that the module will be firstly searched in the current directory, as a result when we ran the interpreter inside the pub directory the module was found.

>>> import sys
>>> sys.path.append('pub')
>>> import drink
give me a pint

Another way to add a directory in sys.path is through the PYTHONPATH environment variable. Come on! Test it!

$ export PYTHONPATH=pub
$ python
>>> import drink
give me a pint

This was a basic hint, we can find much more information at docs.python.org. I hope I’ve helped and if you have comments leave me a comment.

Happy Coding!


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