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How to change default Python version?

191

I have installed Python 3.2 in my Mac. After I run /Applications/Python 3.2/Update Shell Profile.command, it’s confusing that when I type Python -V in Terminal it says that Python 2.6.1.

How can I change the default Python version?

2

166

[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.)

5

  • 142

    This is helpful information, but the question was “how can I change the default python version.” Can we add something to help people trying to do that?

    – dmeyerson

    Sep 2, 2016 at 14:27

  • 1

    On Mac installing Python3 leaves Python2 intact as well. But it seems to me there is a way to change the default that is run when just “python” is typed. I came across the option once. I have chosen to leave it as is where “python –version” shows 2.7.12 and “python3 –version” shows 3.7.0. I agree with dmeyerson above that the question is how to change it. Your advice is good but there is a way to change it if you must (or really, really want to).

    Aug 15, 2018 at 3:41

  • 4

    > The answer you are probably looking for is You should not change this. And here I thought it was “how do I change this”!

    May 6, 2019 at 19:18

  • 2

    This answer is correct. Do not change what python points to at the system level as you can break important components of you OS if you do. Instead, use a separate Python 3 installation and virtualen to give yourself a “local” default Python.

    – Martijn Pieters

    Sep 24, 2019 at 23:20


  • It’s mac that has python 2 as default.

    Jun 21, 2021 at 5:37

120

Check the location of python 3

$ which python3
/usr/local/bin/python3

Write alias in bash_profile

vi ~/.bash_profile  
alias python='/usr/local/bin/python3'

Reload bash_profile

source ~/.bash_profile

Confirm python command

$ python --version
Python 3.6.5

3

  • 8

    this is a bad solution. As mentioned in the accepted answer, $ python is reserved for python 2. Any program you run on your computer that expects python 2 would then receive python 3 due to the alias, which could have unforeseen negative consequences.

    – rykener

    Mar 5, 2019 at 3:42

  • Apple’s intentions are irrelevant to the question.

    May 6, 2019 at 19:19

  • I think that source is only compatible with the Bourne command line. For greater compatibility it would be . instead

    Sep 15, 2021 at 19:11

49

On Mac OS X using the python.org installer as you apparently have, you need to invoke Python 3 with python3, not python. That is currently reserved for Python 2 versions. You could also use python3.2 to specifically invoke that version.

$ which python
/usr/bin/python
$ which python3
/Library/Frameworks/Python.framework/Versions/3.2/bin/python3
$ cd /Library/Frameworks/Python.framework/Versions/3.2/bin/
$ ls -l
total 384
lrwxr-xr-x  1 root  admin      8 Apr 28 15:51 [email protected] -> 2to3-3.2
-rwxrwxr-x  1 root  admin    140 Feb 20 11:14 2to3-3.2*
lrwxr-xr-x  1 root  admin      7 Apr 28 15:51 [email protected] -> idle3.2
-rwxrwxr-x  1 root  admin    138 Feb 20 11:14 idle3.2*
lrwxr-xr-x  1 root  admin      8 Apr 28 15:51 [email protected] -> pydoc3.2
-rwxrwxr-x  1 root  admin    123 Feb 20 11:14 pydoc3.2*
-rwxrwxr-x  2 root  admin  25624 Feb 20 11:14 python3*
lrwxr-xr-x  1 root  admin     12 Apr 28 15:51 [email protected] -> python3.2-32
lrwxr-xr-x  1 root  admin     16 Apr 28 15:51 [email protected] -> python3.2-config
-rwxrwxr-x  2 root  admin  25624 Feb 20 11:14 python3.2*
-rwxrwxr-x  1 root  admin  13964 Feb 20 11:14 python3.2-32*
lrwxr-xr-x  1 root  admin     17 Apr 28 15:51 [email protected] -> python3.2m-config
-rwxrwxr-x  1 root  admin  25784 Feb 20 11:14 python3.2m*
-rwxrwxr-x  1 root  admin   1865 Feb 20 11:14 python3.2m-config*
lrwxr-xr-x  1 root  admin     10 Apr 28 15:51 [email protected] -> pythonw3.2
lrwxr-xr-x  1 root  admin     13 Apr 28 15:51 [email protected] -> pythonw3.2-32
-rwxrwxr-x  1 root  admin  25624 Feb 20 11:14 pythonw3.2*
-rwxrwxr-x  1 root  admin  13964 Feb 20 11:14 pythonw3.2-32*

If you also installed a Python 2 from python.org, it would have a similar framework bin directory with no overlapping file names (except for 2to3).

$ open /Applications/Python\ 2.7/Update\ Shell\ Profile.command
$ sh -l
$ echo $PATH
/Library/Frameworks/Python.framework/Versions/2.7/bin:/Library/Frameworks/Python.framework/Versions/3.2/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin
$ which python3
/Library/Frameworks/Python.framework/Versions/3.2/bin/python3
$ which python
/Library/Frameworks/Python.framework/Versions/2.7/bin/python
$ cd /Library/Frameworks/Python.framework/Versions/2.7/bin
$ ls -l
total 288
-rwxrwxr-x  1 root  admin    150 Jul  3  2010 2to3*
lrwxr-x---  1 root  admin      7 Nov  8 23:14 [email protected] -> idle2.7
-rwxrwxr-x  1 root  admin    138 Jul  3  2010 idle2.7*
lrwxr-x---  1 root  admin      8 Nov  8 23:14 [email protected] -> pydoc2.7
-rwxrwxr-x  1 root  admin    123 Jul  3  2010 pydoc2.7*
lrwxr-x---  1 root  admin      9 Nov  8 23:14 [email protected] -> python2.7
lrwxr-x---  1 root  admin     16 Nov  8 23:14 [email protected] -> python2.7-config
-rwxrwxr-x  1 root  admin  33764 Jul  3  2010 python2.7*
-rwxrwxr-x  1 root  admin   1663 Jul  3  2010 python2.7-config*
lrwxr-x---  1 root  admin     10 Nov  8 23:14 [email protected] -> pythonw2.7
-rwxrwxr-x  1 root  admin  33764 Jul  3  2010 pythonw2.7*
lrwxr-x---  1 root  admin     11 Nov  8 23:14 [email protected] -> smtpd2.7.py
-rwxrwxr-x  1 root  admin  18272 Jul  3  2010 smtpd2.7.py*

2

  • Regarding “currently reserved for Python 2 versions,” do you have a link for that?

    – Wildcard

    Jul 2, 2016 at 4:15

  • 1

    If i have python 3.4 and python 3.7, when I type python3 from terminal, it executes python 3.4 not 3.7, how to fix this?

    – Ysh

    Jul 10, 2019 at 18:19