Notes on Learning Python

Material in Courses: Info2 Info3 Tags: Python

Some Notes and Tips on Getting Started with the Python Programming Language

There are many, and many good Python resources available, many of them for free - after all, it’s the most frequently used programming language for introductionary courses.

See Python Resources

Suggested Path for Info3 & Info3

As we are already using IntelliJ IDEA, using the Python Flavour of IntelliJ IDEA called “PyCharm” doesn’t have that steep of a learning curve.

https://www.jetbrains.com/pycharm-edu/

PyCharm Community Edition comes with a nice interactive Python Course. You can ~~either install/use PyCharm Edu or~~ install the EduTools plugin for PyCharm Professional.

Go to https://www.jetbrains.com/pycharm-edu/ for a pointer to the interactive course
Video Intro: Start learning with PyCharm Edu - YouTube
Install Pycharm Edu - via the JetBrains Toolbox if you already have that, or download it here: PyCharm Edu
you will need a working python - see https://www.python.org/downloads/ - installation and select it in Preferences -> Python Interpreter -> Preferences (gears upper right) Add… or Show All…
PyCharm will install a virtualenv for the project under .venv

While Python might be a beginner-friendly language, installing Python properly certainly is not. While the basic installation steps are straightforward, you most probably will need to sort out your installation(s) at some point. See my “Notes on Installing Python” below.

Further Tips

You might want to put the project under version control to be able to see a summary of your changes using git!

While you work through the course, the course files (*.yaml) are also modified. To put the python files in a separate commit, you could do:

# git add Introduction/**/*.py
# git commit -m "Introduction - py files"

and then commit the rest (check with git statusthat there are just yaml-files left.

# git add .
# git commit -m "Variables - course files"

Installing packages

If you install packages with pip/pip3, you need to make sure that you use the right python context / installation! As PyCharm usually installs an own virtualenv per project, packages need to be installed in this virtualenv. This can be done by installing packages using the PyCharm interface. To do so, open View -> Tool Windows -> Python Packages Documentation: Install, uninstall, and upgrade packages | PyCharm

Installing Python

Here are some notes on what I learned installing it on MacOs in case you want to dig into it. (If it runs for now, you might want to do that at a later point).

Mac OS comes with a Python 2 installation which should not be removed. You will want to add a current Python 3 installation. This can either be done with homebrew or by using the MacOs Package available here: Python Releases for macOS | Python.org I decided on the latter and installed Python 3.10. Some of my homebrew packages depend on Python 3.9, so I need to keep that version as well as the MacOS python installation.

This results in three different python installations and the need to always be sure which one you’re using and to make sure that packages are indeed installed where you want them to be installed!

Checking your Python Installation and the Path

To find out which python version will be used you can use the command which:

ok htw/courses/info3 # which python3
/Library/Frameworks/Python.framework/Versions/3.10/bin/python3
ok htw/courses/info3 # which pip3
/Library/Frameworks/Python.framework/Versions/3.10/bin/pip3

This looks about right - to check where pip installs its packages, you can list the packages and then get the path for one, e.g.

ok htw/courses/info3 # pip3 list
Package           Version
----------------- --------
attrs             21.2.0
...
Scrapy            2.5.1

and then use pip show to check the installation location of the package:


ok htw/courses/info3 # pip3 show Scrapy
Name: Scrapy
Version: 2.5.1
Summary: A high-level Web Crawling and Web Scraping framework
...
Location: /Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages
...

Checking your path

If the default python version is not the one you want or expected, it is not the first one found in your path. To check your path, use

echo $PATH

if you replace the colons with newlines it’s easier to see:

echo $PATH | sed -e "s/:/\n/g"

Python Versions on my machine

Apart from Homebrew, it boils down to the System 2.7 and my newly installed 3.10:

ok ~ # ls /Library/Frameworks/Python.framework/Versions
3.10	Current
ok ~ # ls /System/Library/Frameworks/Python.framework/Versions
2.3	2.5	2.6	2.7	Current

/usr/bin/python points to the os installation.

ok ~ # ls -l /usr/bin/python*
lrwxr-xr-x  1 root  wheel      75 Jan  1  2020 /usr/bin/python -> ../../System/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7
lrwxr-xr-x  1 root  wheel      82 Jan  1  2020 /usr/bin/python-config -> ../../System/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7-config
lrwxr-xr-x  1 root  wheel      75 Jan  1  2020 /usr/bin/python2 -> ../../System/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7
lrwxr-xr-x  1 root  wheel      75 Jan  1  2020 /usr/bin/python2.7 -> ../../System/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7
lrwxr-xr-x  1 root  wheel      82 Jan  1  2020 /usr/bin/python2.7-config -> ../../System/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7-config
-rwxr-xr-x  1 root  wheel  137616 Jan  1  2020 /usr/bin/python3

/usr/local/bin/python points to the 3.10 installation:

ok ~ # ls -lart /usr/local/bin/python3*
lrwxr-xr-x  1 root  wheel  78 Oct 21 14:26 /usr/local/bin/python3-intel64 -> ../../../Library/Frameworks/Python.framework/Versions/3.10/bin/python3-intel64
lrwxr-xr-x  1 root  wheel  70 Oct 21 14:26 /usr/local/bin/python3 -> ../../../Library/Frameworks/Python.framework/Versions/3.10/bin/python3

Plus the Homebrew installation…

… which can be accessed via symlinks in /usr/local/opt/python* which point to HomeBrews´ cellar:

brew deps --tree --installed
brew info python3

ok ~ # ls -lart   /usr/local/opt/python@3*
lrwxr-xr-x  1 kleinen  admin  28 Oct 21 14:31 /usr/local/opt/python@3.9 -> ../Cellar/python@3.9/3.9.7_1
lrwxr-xr-x  1 kleinen  admin  28 Oct 21 14:31 /usr/local/opt/python@3 -> ../Cellar/python@3.9/3.9.7_1
lrwxr-xr-x  1 kleinen  admin  30 Nov  7 11:59 /usr/local/opt/python@3.10 -> ../Cellar/python@3.10/3.10.0_2

Together with all the symlinks (I’ve only shown a part of them here) this results in a rather long and confusing list that PyCharm generates to select the correct Python interpreter!

Also; I expect that the next update will struggle to get ahead in the path and thus change my default python.

IntelliJ/Virtualenv

in PyCharm, you’ll either let IntelliJ create or use a given vitualenv environment. This basically fixates the python environment for this project by creating a folder with a link to the appropriate binaries - independent from the path - and a folder for the installed packages:


ok  (main✗)python/virtualenv/pycharm-virtualenv # ls
bin		lib		pyvenv.cfg

ok  (main✗)python/virtualenv/pycharm-virtualenv # ls -lart bin
total 112
lrwxr-xr-x   1 kleinen  staff    62 Nov 14 20:30 python -> /Library/Frameworks/Python.framework/Versions/3.10/bin/python3
...

ok  (main✗)python/virtualenv/pycharm-virtualenv # ls -lart lib/python3.10/site-packages/
__pycache__/                  _virtualenv.py                pip-21.1.2.dist-info/         setuptools/                   wheel/
_distutils_hack/              distutils-precedence.pth      pip-21.1.2.virtualenv         setuptools-57.0.0.dist-info/  wheel-0.36.2.dist-info/
_virtualenv.pth               pip/                          pkg_resources/                setuptools-57.0.0.virtualenv  wheel-0.36.2.virtualenv

Which means that, if you want to run your PyCharm scripts from the command line in the same context (e.g. to give them run-time parameters without fiddling with runtime configurations), you’ll need to use this one. This can be done easiest by using the Terminal integrated in IntelliJ/PyCharm.