how to use an older version of python under Ubuntu 20.04
Ubuntu 20.04 comes with an official python package of version 3.8.5. But we always want to keep our projects work after doing an upgrade and some of them could be tied to older python versions such as 3.7.x or even 3.6.x. Unfortunately Ubuntu doesn’t encourage users to roll back to older versions of any package through apt and we have to try some other approaches.
Gather the Pieces from Ubuntu Package Server
Even though those packages are no longer maintained by the latest release we still can find them one by one from the ubuntu website.
Let’s take Python 3.6.9 as an example, following packages can be downloaded from https://packages.ubuntu.com/bionic-updates/python/ and installed in the dependency order:
- libpython3.6-minimal
- libpython3.6-stdlib
- libpython3.6
- libpython3.6-dev
- python3.6-minimal
- python3.6
- python3.6-dev
- python-pip-whl
- python-pip
Installation command using dpkg is as follows:
$ sudo dpkg -i ${PACKAGE}.deb
We can manage different python packages by installing alternatives for one named package under Ubuntu.
sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.8 3
sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.6 3
Then we can switch between them using ‘sudo update-alternatives –config python3’
The downside is: we can’t completely separate different python packages due to the fact that some of the packages are shared in the Linux distribution, e.g. python3-disutils. The issue caused by the mismatch of python3-disutils and active python sometimes could be very painstaking, which breaks pip and apt.
Build from the Source
One of them is to build and install the specific version of python from source and method actually is much simpler than it sounds if default configuration is accepted. Detailed guide is provided by the official site which can be simplified further as following steps:
$ ./configure
$ make -j8
$ sudo make install
Lower number helps if there is less than 8 cores in the build system.
The downside is:
- There could be some minor difference in the installation path from the official Ubuntu package, if the project is tied to some system path (not typical) tweaks are required.
- Uninstall must be done manually, say, through the infamous ‘rm -rf *’
Use pyenv
Aboves are quick solutions when someone occasionally needs a different version of python from what system provides, however in the case that one must switch among several python versions regularly (to maintain different project that requires different python version), it is recommended to use pyenv - an incredible tool to manage multiple python packages in a more flexible manner. Pyenv downloads and builds the packages from source and install them to user’s home directory, which effectively avoids breaking the existing python dependent applications installed across the system.
In addition to supporting different version of standard python packages, pyenv even allows different implementations of python to co-exist to the system, such as pipy, jython, … Users can switch between different pythons either globally or locally, in the later case the required python package can be designated to a specific project folder.
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