How to Check All Versions of Python Installed on Osx and Centos

How to check all versions of python installed on osx and centos

Use,

yum list installed

command to find the packages you installed.

How can I check all the installed Python versions on Windows?

I just got the answer. By typing "py -h" or "py --help" I got the help message:

C:\Users\admin>py -h
Python Launcher for Windows Version 3.7.1150.1013

usage:
py [launcher-args] [python-args] script [script-args]

Launcher arguments:

-2     : Launch the latest Python 2.x version
-3     : Launch the latest Python 3.x version
-X.Y   : Launch the specified Python version
     The above all default to 64 bit if a matching 64 bit python is present.
-X.Y-32: Launch the specified 32bit Python version
-X-32  : Launch the latest 32bit Python X version
-X.Y-64: Launch the specified 64bit Python version
-X-64  : Launch the latest 64bit Python X version
-0  --list       : List the available pythons
-0p --list-paths : List with paths

Which tells me that "-0" (zero, not letter "O") lists the available pythons:

C:\Users\admin>py -0
Installed Pythons found by py Launcher for Windows
 -3.7-64 *
 -3.7-32
 -2.7-64
 -2.7-32

While "-0p" lists not only the versions, but also the paths:

C:\Users\admin>py -0p
Installed Pythons found by py Launcher for Windows
 -3.7-64        C:\Users\admin\AppData\Local\Programs\Python\Python37\python.exe *
 -3.7-32        C:\Users\admin\AppData\Local\Programs\Python\Python37-32\python.exe
 -2.7-64        C:\Python27_64\python.exe
 -2.7-32        C:\Python27_32\python.exe

Multiple Python versions on the same machine?

I think it is totally independent. Just install them, then you have the commands e.g. /usr/bin/python2.5 and /usr/bin/python2.6. Link /usr/bin/python to the one you want to use as default.

All the libraries are in separate folders (named after the version) anyway.

If you want to compile the versions manually, this is from the readme file of the Python source code:

Installing multiple versions

On Unix and Mac systems if you intend to install multiple versions of Python
using the same installation prefix (--prefix argument to the configure
script) you must take care that your primary python executable is not
overwritten by the installation of a different version. All files and
directories installed using "make altinstall" contain the major and minor
version and can thus live side-by-side. "make install" also creates
${prefix}/bin/python3 which refers to ${prefix}/bin/pythonX.Y. If you intend
to install multiple versions using the same prefix you must decide which
version (if any) is your "primary" version. Install that version using
"make install". Install all other versions using "make altinstall".

For example, if you want to install Python 2.5, 2.6 and 3.0 with 2.6 being
the primary version, you would execute "make install" in your 2.6 build
directory and "make altinstall" in the others.

Too many different Python versions on my system and causing problems

Why did it get messed up?

There're a couples of different way to install Python, as the update of OP says, and they locate files in different locations. For example, macports puts things into /opt/local/, while homebrew puts things into /usr/local/. Also, Mac OS X brings a few python versions with itself. So, if you install python many times via different ways, you will get many python versions existing independently on your system.

What problem does it cause?

I don't know exactly. I guess the problem is that if you have many versions of python, then which one to use and where to find packages will be determined by the path order in your system PATH and the PYTHONPATH respectively. So you may lose control of where to install python modules. Consider that if you run sudo python setup.py install to install a module (it finds python by the root's PATH) and then try to import the module by python -c "import it" (this time it finds python by your PATH), maybe something will go wrong. This is my guess, I didn't validate it. But in my own case, something did go wrong.

How to avoid this?

I think the principle would be that be aware of that different ways and tools install things independently to different locations, so use them mindfully.

• Unless you intend to, don't install the same thing twice via different
  ways. (If you intend to do it for python, you might want to check out virtualenv)
• Keep an eye on the path order in your PATH and consider if it's
  correct.
• When installing modules, be clear which python (or pip) is
  running and where the module is installed.

So, how did I solve my own case?

Since it had been messing up already and seemed to be very hard to cure, so finally I solved this question by a full OS re-installation, and started to follow the DOs-and-DONTs above. For the installation of the scientific environment with python (numpy/scipy/matplotlib, which had shown problems to make me ask this question), I found this tutorial was extremely helpful. So, problem solved finally.

How to switch Python versions in Terminal?

The simplest way would be to add an alias to python3 to always point to the native python installed. Add this line to the .bash_profile file in your $HOME directory at the last,

alias python="python3"

Doing so makes the changes to be reflected on every interactive shell opened.

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