The problem
Linux is tricky, and many times the $PATH gets corrupted because of some new installation. So, I would like to keep a reminder here on the website on the process of rectifying this issue.
The Solution
First we need to understand the working of python and pip. pip is the package manager for Python packages. This is like an app store for python modules. Just like we go on the app store to find any app of our liking and install in from there, for python packages we go to pip-store and install the packages with the command
pip install package_name==version_number
In Linux, the package gets installed at the following location
/usr/local/lib/python3.6/site-packages/or/usr/local/lib/python3.6/dist-packages/
You have to update python3.6 in the above locations to the version number of python on your system. Suppose you have python3.9 on your system then the installation path will be
/usr/local/lib/python3.9/site-packages/or/usr/local/lib/python3.9/dist-packages/
Now the computer needs a way to find this package on the system. For this in LINUX we have the $PATH variable which is a system variable. You can think of it as the app drawer or the start menu in windows where you can find all the installed apps.
$PATH variable contains the location of all the directories where system can find an installed package. You can see the content of $PATH variable by typing in terminal the following command
$PATH
On my system I get the following output
bash: /home/fenics/local/lib:/usr/local/lib/:/usr/lib/ccache:/usr/local/gmsh-4.3.0-Linux64/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
As you can see from above, I donot have the pip install directories on my $PATH variable. Thus, my system is unable to find the installed package. To rectify it, I just need to run the following command
export PATH=$PATH:/usr/local/lib/python3.6/dist-packages/
Now I can see my $PATH variable which gives the output
bash: /home/fenics/local/lib:/usr/local/lib/:/usr/lib/ccache:/usr/local/gmsh-4.3.0-Linux64/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/local/lib/python3.6/dist-packages/
And now I can use the packages installed via pip.