When using the import statement, the python interpreter looks for the package (or module) in a list of directories
known as the python path.
Let’s find out about what directories constitute the python path:
$ python
>>> import sys
>>> sys.path
['', '/usr/lib/python38.zip', '/usr/lib/python3.8', '/usr/lib/python3.8/lin-dynload', '/home/thibault/python-workshop-venv/lib/python3.8/site-packages/']
The order of this list matters: it is the order in which python looks into the directories that constitute the python path. To begin with, Python first looks in the current directory. If the package/module isn’t found there, the python intepreter looks in the following directories (in this order):
/usr/lib/python38.zip/usr/lib/python3.8/usr/lib/python3.8/lib-dynloadThe above contain the modules and packages in the standard library, i.e the packages and modules that
come “pre-installed” with Python.
Finally, the python interpreter looks inside the directory /home/thibault/python-workshop-venv/lib/python3.8/site-packages/.
The output of sys.path is probaby different on your machine. It depends on many factors,
like your operating system, your version of Python, the location of your current active Python
environment.
For Python to find out package tstools it must be located in one of the directories listed in
the sys.path list. If it is the case, the package is said to be installed.
Looking back at the example in the previous section, let’s list some potential workarounds
for the tstools package to be importable in analysis2/.:
Copy (analysis1/tstools/) in analysis2/.
You end up with two independant packages. If you make changes to one, you have to remember to make the same
changes to the other. It’s the usual copy and paste problems: inefficient and error-prone.
Add analysis1/ to sys.path.
At the beginning of analysis2.py, you could just add
import sys
sys.path.append("../analysis1/")
This approach can be sufficient in some situations, but generally not recommended. What if the package directory is relocated?
Copy analysis1/tstools directory to the site-packages/ directory.
You have to know where the site-packages is. This depends on your current system and python environment (see below).
The location on your macine may very well be differnt from the location on your colleague’s machine.
More generally, the three above approaches overlook a very important point: dependencies. Our package has two: numpy and matplotlib. If you were to give your package to a colleague, nothing guarantees that they have both packages installed. This is a pedagogical example, as it is likely that they would have both installed, given the popularity of these packages. However, if your package relies on less widespread packages, specific versions of them or maybe a long list of packages, it is important to make sure that they are available.
Note that all three above approaches work.
However, unless you have a good reason to use one of them, they are not recommended for the
reasons above. In the next section, we look at the recommended way to install a package, using
setuptools and pip.