How to Change Default Python Version?

[updated for 2021]

(Regardless if you are on Mac, Linux, or Windows:)

If you are confused about how to start the latest version of python, on most platforms it is the case that python3 leaves your python2 installation intact (due to the above compatibility reasons); thus you can start python3 with the python3 command.

Historically...

The naming convention is that generally, most scripts will call python2 or python3 explicitly. This happened due to a need for backwards compatibility.

Even though technically python doesn't even guarantee backwards compatibility between minor versions, Python3 really breaks backwards compatibility. At the time, programs invoking 'python' were expecting python2 (which was the main version at the time). Extremely old systems may have programs and scripts which expect python=python2, and changing this would break those programs and scripts.

At the time this answer was written, OP should not have changed this due to maintaining compatibility for old scripts.

Circa year 2021...

Nowadays, many years after the python2->python3 transition, most software explicitly refers to python2 or python3 (at least on Linux). For example, they might call #!/usr/bin/env python2 or #!/usr/bin/env python3. This has for example (python-is-python3-package) freed up the python command to be settable to a user default, but it really depends on the operating system.

The prescription for how distributions should handle the python command was written up in 2011 as PEP 394 -- The "python" Command on Unix-Like Systems. It was last updated in June 2019.

Basically if you are writing a library, you should use the most specific version(s) of python you can use. Otherwise as an end user, you should feel free to rename this for your own personal use (though your OS or distribution may not make that easy).

Shell alias:

You could, however, make a custom alias in your shell. The way you do so depends on the shell, but perhaps you could do alias py=python3, and put it in your shell startup file. This will only work on your local computer (as it should), and is somewhat unnecessary compared to just typing it out (unless you invoke the command constantly).

Confused users should not try to create aliases or virtual environments or similar that make python execute python3; this is poor form.This is acceptable nowadays, but PEP 394 suggests encouraging users to use a virtualenv instead.