Python

General-purpose, high-level programming language supporting multiple programming paradigms.

Multiple Python interpreters

If you are working on a piece of Python software, you probably want to test it on multiple Python interpreters. On Fedora, that’s easy: all you have to do is use dnf to install what you need. Currently Fedora has the following Pythons ready for you in the repositories:

  • CPython 3.7 (development version)
  • CPython 3.6
  • CPython 3.5
  • CPython 3.4
  • CPython 3.3
  • CPython 2.7
  • CPython 2.6
  • PyPy
  • PyPy 3
  • Jython
  • MicroPython

Quite a nest, isn’t it? You can install them like this:

$ sudo dnf install python37  # to get CPython 3.7
$ sudo dnf install python34  # to get CPython 3.4
$ sudo dnf install python26  # to get CPython 2.6
$ sudo dnf install pypy pypy3 jython python35  # to get more at once

After that, you can run an interactive console or your script with, let’s say, CPython 3.4:

$ python3.4
Python 3.4.3 (default, Jul 11 2016, 11:30:44) 
[GCC 5.3.1 20160406 (Red Hat 5.3.1-6)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 

Warning: For production purposes you should use python3 or python2 packages only. Other CPython versions might be unstable or even dangerous (either because they are extremely old or quite the contrary alpha/beta quality) and are intended solely for development.

Getting it and running it all with tox

Tox is tool that helps you test your Python code on multiple Pythons. If you install it on Fedora via the dnf package manager, you’ll automatically get all the CPythons (except the development version) and PyPys:

$ sudo dnf install tox

If you are not yet familiar with tox, don’t worry. This short example will show you how to start.

Let’s create a directory and a simple Python file in it that will say something nice:

# say.py
print('Fedora is the best OS for Python developers', end='\n\n')

Now we’ll test if it works with all the Pythons, with tox. We’ll create a simple configuration file for tox, tox.ini, in the same directory:

[tox]
envlist = py27,py35,py36,pypy,pypy3
skipsdist = True
[testenv]
commands=python say.py

The envlist directive defines the list of Pythons to test on. Normally, tox assumes you are testing a project with its own setup.py. For the simlicity of this demo, we are not using it, and we need to tell this to tox via the skipsdist option. Finally the commands in [testenv] section tells tox what commands to run for the test, normally that would be python setup.py test, py.test or similar.

With tox.ini, just run tox in the same directory:

$ tox
[...]
ERROR:   py27: commands failed
  py35: commands succeeded
  py36: commands succeeded
ERROR:   pypy: commands failed
  pypy3: commands succeeded

As you can see, there’s something wrong with the script: it only works on Python 3. The full tox output (omitted here) contains the exact error. If you want to support old Python 2 as well, you’ll have to fix it:

# say.py
from __future__ import print_function
print('Fedora is the best OS for Python developers', end='\n\n')
$ tox
[...]
  py27: commands succeeded
  py35: commands succeeded
  py36: commands succeeded
  pypy: commands succeeded
  pypy3: commands succeeded
  congratulations :)

If you want to use tox for your projects, you can learn more at the documentation.

Creating virtualenvs and installing packages

Fedora only packages Python modules for current versions of python2 and python3. For all other interpreters, you will need to install packages from PyPI, the Python Package Index.

The best way is to use Python virtual environments (virtualenvs, or venvs). The invocation to create them differs for different Python versions. Packages installed in a virtualenv are only available once the virtualenv is activated. Here you can see two demos that create virtualenv in a folder named env and install some package into it.

Python 3.4+ only

Recent versions of Python include the venv module, which can create virtual environments.

$ python3.5 -m venv env  # create the virtualenv
$ . env/bin/activate  # activate it
(env)$ python -m pip install requests  # install a package with pip
...
(env)$ python  # run python from that virtualenv
Python 3.5.2 (default, Aug 16 2016, 21:50:46) 
[GCC 5.3.1 20160406 (Red Hat 5.3.1-6)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import requests
>>> ...
(env)$ deactivate  # go back to "normal"

Python 2.x, 3.x, PyPys, Jython

For other Python versions, a tool called virtualenv can create virtual environments:

$ dnf install python-virtualenv  # install the necessary tool
$ virtualenv --python /usr/bin/python2.7 env  # create the virtualenv
Running virtualenv with interpreter /usr/bin/python2.7
New python executable in env/bin/python2.7
Also creating executable in env/bin/python
Installing setuptools, pip...done.
$ . env/bin/activate  # activate it
(env)$ python -m pip install requests  # install a package with pip
...
(env)$ python  # run python from that virtualenv
Python 2.7.11 (default, Jul  8 2016, 19:45:00) 
[GCC 5.3.1 20160406 (Red Hat 5.3.1-6)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import requests
>>> ...
(env)$ deactivate  # go back to "normal"

If you don’t wish to use Python 2 at all, you might install python3-virtualenv in the first step and than use virtualenv-3 command instead of virtualenv.

To learn more about virtualenvs, visit The Hitchhiker’s Guide to Python.

Others

MicroPython does not support virtual environments. It does have a rudimentary pip replacement called upip, which you can use to install packages that support MicroPython. Run it to find out more:

$ micropython -m upip

Authors: Miro Hrončok