Two Versions of Python on Linux. How to Make 2.7 the Default

Two versions of python on linux. how to make 2.7 the default

You probably don't actually want to change your default Python.

Your distro installed a standard system Python in /usr/bin, and may have scripts that depend on this being present, and selected by #! /usr/bin/env python. You can usually get away with running Python 2.6 scripts in 2.7, but do you want to risk it?

On top of that, monkeying with /usr/bin can break your package manager's ability to manage packages. And changing the order of directories in your PATH will affect a lot of other things besides Python. (In fact, it's more common to have /usr/local/bin ahead of /usr/bin, and it may be what you actually want—but if you have it the other way around, presumably there's a good reason for that.)

But you don't need to change your default Python to get the system to run 2.7 when you type python.

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First, you can set up a shell alias:

alias python=/usr/local/bin/python2.7

Type that at a prompt, or put it in your ~/.bashrc if you want the change to be persistent, and now when you type python it runs your chosen 2.7, but when some program on your system tries to run a script with /usr/bin/env python it runs the standard 2.6.

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Alternatively, just create a virtual environment out of your 2.7 (or separate venvs for different projects), and do your work inside the venv.

Dealing with multiple Python versions and PIP?

The current recommendation is to use python -m pip, where python is the version of Python you would like to use. This is the recommendation because it works across all versions of Python, and in all forms of virtualenv. For example:

# The system default python:
$ python -m pip install fish

# A virtualenv's python:
$ .env/bin/python -m pip install fish

# A specific version of python:
$ python-3.6 -m pip install fish

Previous answer, left for posterity:

Since version 0.8, Pip supports pip-{version}. You can use it the same as easy_install-{version}:

$ pip-2.5 install myfoopackage
$ pip-2.6 install otherpackage
$ pip-2.7 install mybarpackage

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EDIT: pip changed its schema to use pipVERSION instead of pip-VERSION in version 1.5. You should use the following if you have pip >= 1.5:

$ pip2.6 install otherpackage
$ pip2.7 install mybarpackage

Check https://github.com/pypa/pip/pull/1053 for more details

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References:

• https://github.com/pypa/pip/issues/200
• http://www.pip-installer.org/docs/pip/en/0.8.3/news.html#id4
  
  https://pip.pypa.io/en/stable/news/#v0-8 or
  
  https://web.archive.org/web/20140310013920/http://www.pip-installer.org:80/docs/pip/en/0.8.3/news.html#id4

Unable to set default python version to python3 in ubuntu

EDIT:

I wrote this when I was young and naive, update-alternatives is the better way to do this. See @Pardhu's answer.

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Outdated answer:

Open your .bashrc file nano ~/.bashrc. Type alias python=python3
on to a new line at the top of the file then save the file with ctrl+o
and close the file with ctrl+x. Then, back at your command line type
source ~/.bashrc. Now your alias should be permanent.

Change default Python version from 2.4 to 2.6

As root:

ln -sf /usr/bin/python2.6 /usr/local/bin/python

This will make a symbolic link from /usr/local/bin/python --> /usr/bin/python2.6
(replacing the old hardlink).

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 specify the version of python (2 or 3, or finer-grained under specific circumstances) 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.

Different 3.* versions, or 2.* versions:

In the extremely unlikely case that if someone comes to this question with two python3 versions e.g. 3.1 vs 3.2, and you are confused that you have somehow installed two versions of python, this is possibly because you have done manual and/or manual installations. You can use your OS's standard package/program install/uninstall/management facilities to help track things down, and perhaps (unless you are doing dev work that surprisingly is impacted by the few backwards-incompatible changes between minor versions) delete the old version (or do make uninstall if you did a manual installation). If you require two versions, then reconfigure your $PATH variable so the 'default' version you want is in front; or if you are using most Linux distros, the command you are looking for is sudo update-alternatives. Make sure any programs you run which need access to the older versions may be properly invoked by their calling environment or shell (by setting up the var PATH in that environment).

A bit about $PATH

sidenote: To elaborate a bit on PATH: the usual ways that programs are selected is via the PATH (echo $PATH on Linux and Mac) environment variable. You can always run a program with the full path e.g. /usr/bin/ some args, or cd /usr/bin then ./ some args (replace blank with the 'echo' program I mentioned above for example), but otherwise typing some args has no meaning without PATH env variable which declares the directories we implicitly may search-then-execute files from (if /usr/bin was not in PATH, then it would say : command not found). The first matching command in the first directory is the one which is executed (the which command on Linux and Mac will tell you which sub-path this is). Usually it is (e.g. on Linux, but similar on Mac) something like /usr/bin/python which is a symlink to other symlinks to the final version somewhere, e.g.:

% echo $PATH
/usr/sbin:/usr/local/bin:/usr/sbin:usr/local/bin:/usr/bin:/bin

% which python
/usr/bin/python
% which python2
/usr/bin/python2
% ls -l /usr/bin/python
lrwxrwxrwx 1 root root 7 Mar  4  2019 /usr/bin/python -> python2*
% ls -l /usr/bin/python2  
lrwxrwxrwx 1 root root 9 Mar  4  2019 /usr/bin/python2 -> python2.7*
% ls -l /usr/bin/python2.7
-rwxr-xr-x 1 root root 3689352 Oct 10  2019 /usr/bin/python2.7*

% which python3         
/usr/bin/python3
% ls -l /usr/bin/python3
lrwxrwxrwx 1 root root 9 Mar 26  2019 /usr/bin/python3 -> python3.7*
% ls -l /usr/bin/python3.7
-rwxr-xr-x 2 root root 4877888 Apr  2  2019 /usr/bin/python3.7*

% ls -l /usr/bin/python*
lrwxrwxrwx 1 root root       7 Mar  4  2019 /usr/bin/python -> python2*
lrwxrwxrwx 1 root root       9 Mar  4  2019 /usr/bin/python2 -> python2.7*
-rwxr-xr-x 1 root root 3689352 Oct 10  2019 /usr/bin/python2.7*
lrwxrwxrwx 1 root root       9 Mar 26  2019 /usr/bin/python3 -> python3.7*
-rwxr-xr-x 2 root root 4877888 Apr  2  2019 /usr/bin/python3.7*
lrwxrwxrwx 1 root root      33 Apr  2  2019 /usr/bin/python3.7-config -> x86_64-linux-gnu-python3.7-config*
-rwxr-xr-x 2 root root 4877888 Apr  2  2019 /usr/bin/python3.7m*
lrwxrwxrwx 1 root root      34 Apr  2  2019 /usr/bin/python3.7m-config -> x86_64-linux-gnu-python3.7m-config*
lrwxrwxrwx 1 root root      16 Mar 26  2019 /usr/bin/python3-config -> python3.7-config*
lrwxrwxrwx 1 root root      10 Mar 26  2019 /usr/bin/python3m -> python3.7m*
lrwxrwxrwx 1 root root      17 Mar 26  2019 /usr/bin/python3m-config -> python3.7m-config*

sidenote2: (In the rarer case a python program invokes a sub-program with the subprocess module, to specify which program to run, one can modify the paths of subprocesses with sys.path from the sys module or the PYTHONPATH environment variable set on the parent, or specifying the full path... but since the path is inherited by child processes this is not remotely likely an issue.)

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.

I have python 2.7 and 3.8 both in my system, how can I use python 3.8 or remove 2.7?

Your first option would be to directly call python3 when running you python script
I.E:

python3 ~/MyProjects/main.py

instead of

python ~/MyProjects/main.py

If you want to remove python 2 all together - you could.

But as another user stated in the comments, there might be some program on your computer than still depends on python 2.

What I think would fit your needs best is to change the default python version to python3.

This still runs some risk that some program on your computer assumes python refers to the default python2.

But most programs call /usr/bin/python, and changing the default wont affect those programs.
So in the end changing the default is less risky than deleting python 2, while still giving you the comfortable python3 as the default.

I found this guide from a quick google search

basically - it instructs you to run the following command with root privillages:

update-alternatives --install /usr/bin/python python /usr/bin/python3 1

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