Installing Python

By Ryan Wiles

Why do I care about the Python version and being able to switch between them

Typically, mature programming languages, don’t tend to change too drastically from one version to the next. Allowing programs written for one version to continue working on newer versions with few if any changes. When changes are made to a programming language, it’s usually adding new features or making minor refinements to existing ones. Sometimes a more drastic decision is made to remove what are considered to be problematic aspects of the language. While this is done to improve the language overall, it comes at the cost of breaking backwards compatibility. The jump from Python 2 to Python 3 was one of these releases that broke backwards compatibility.

Python 3 is represents the future of the Python language. If you’re new to the language, that’s where you should focus your efforts. These tutorials are going to focus principally on Python 3, however there is still a lot of Python 2 code that hasn’t yet been ported to 3 yet. So, it is prudent to know what some of the changes between versions are and how to run Python 2 code without breaking your Python 3 environment. We’ll cover that later when we discuss sandboxes and virtual environments.

Some of the breaking changes between Python 2 and 3

Python 2 Python 3
Two types of strings str and unicode All strings are unicode
Two types of integers int and long All integers are called int, but have the range of a long
Division returns int if operands are integers Division always returns float
round() returns float round() returns int
Unorderable types can be compared Unorderable types raise TypeError if compared
print was a statement print() is now a function
range(), map(), filter(), zip(), dict.items(), dict.keys(), dict.values() return a list Now return an object for lazy evaluation
xrange() returns an object for lazy evaluation Removed (replaced by range())
input() ran eval() on the input input() returns input as a string
raw_input() returns input as a string Removed (replaced by input())
raise and except Changed argument structure
iter.next() Replaced with by next(iterator)
Comprehension loop variables leak to the global namespace Comprehension loop variables don’t leak
Source files default to ASCII encoding Source files default to UTF-8 encoding
lambdas supported tuple unpacking tuples are no longer unpacked

There’s a fairly comprehensive list of all the differences at Porting Code to Python 3 with 2to3

Prerequisites for MacOS

Note: MacOS currently comes pre-installed with Python 2.7.10 located at /usr/bin/python. This is convenient if you want to quickly play around with Python. However, you’ll quickly find that you want to start installing additional packages or switch between Python 2 & 3 and it’s best not mess up the OSes bundled Python environment.

Instead, we’re going to install Python using Homebrew. Homebrew will install Python 2/3 in /usr/local/bin. But since that directory is usually added to the PATH variable after /usr/bin. This means that typing in python is still going to run the MacOS’s version of Python. To run Homebrew’s Python executables, we’ll type python2 or python3 instead.

If you really want to have the shell run the Homebrew version when you type python, you would have to change your user’s PATH environment variable or set up an alias. I’m not going to go into that process, since we’re going to introduce a more flexible approach later on.

Installing Homebrew

Similar to the apt and yum package managers on Linux. Homebrew is a command line package manager for MacOS allowing you to install and manage many popular UNIX/Linux software packages.

/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"

Full installation instructions for Homebrew are here: https://docs.brew.sh/Installation

Installing Python

On Windows: https://docs.python.org/3.3/using/windows.html

On Mac: https://docs.python.org/3.3/using/mac.html

brew install python # installs Python 3
brew install python@2 # installs Python 2

On Linux: https://docs.python.org/3.3/using/unix.html

Verifying Your Environment

$ which python2
/usr/local/bin/python2 # This is where HomeBrew places it

$ python2 --version
Python 2.7.15 # The 2 is the important part, the exact minor versions may vary

$ which python3
/usr/local/bin/python3 # This is where HomeBrew places it

$ python3 --version
Python 3.7.0 # The 3 is the important part, the exact minor versions may vary

$ python2 # to enter the Python 2 REPL shell
$ python3 # to enter the Python 3 REPL shell

References

© 2018 Ryan Wiles