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2020-06-21 Python

Finding Modules And Packages

Finding Modules and Packages#

Python most times is simple but sometimes it appears complex and even complicated when your modules or packages just won’t import.

Usually you will get an error:

ImportError: No module named 'xxx'

Before we get over this hurdle let’s take a step back and understand some of the fundamentals

Fundamentals of Modules and Packages#

  • A module is a single python file
  • A package is a folder containing python files along with a __init__.py that tells python it is a package to import files from

Remember all these files have .py extensions but importing them with the .py is not needed

Where does python look for modules and packages#

Python looks in the environment variable: $PYTHONPATH

More on $PYHTONPATH

Printing out the environment variable does not work for me:

printenv PYTHONPATH

To find out the directories where python is looking for modules and packages use:

import sys
print(sys.path)

How do I add a directory to the PYTHONPATH#

One way is in your file:

import sys
sys.path.insert(0, "/path/to/your/package_or_module")

Another way is to add it to your environmetn variables permanently:

Add the following line to your ~/.profile or ~/.bashrc file.

export PYTHONPATH=$PYTHONPATH:/path/you/want/to/add

But this method is not the best because usually you will be in a virtual environment for a specific project and don’t want to globally set your environment

Finding where a module is coming from#

The module __file__ attribute

Say you have imported a module but you are unsure where it is coming from you can use the:

my_module.__file__

which will output the location of the directory that module is coming from

Note: The file attribute is not present for C modules that are statically linked into the interpreter

Recent changes to PYTHONPATH#

The current working directory is not added if PYTHONPATH is empty

An empty PYTHONPATH was equivalent to setting it to . previously.

How to add the current working directory in the python path without modifying the package#

SAy you are getting import errors and need to add the current working directory to the PYTHONPATH but because this is a package that others are going to use you do not want to use the methods above.

You can set the python path in your virtual environemnt:

vim /env/bin/activate

and add:

export PYTHON_PATH='/my-module/working/dir'

Note this only works with an activated pipenv shell and not when riunning pipenv run

Sources#