How to change default Python version?

Question:

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?

Asked By: Zhu Shengqi

||

Answers:

[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.)

Answered By: ninjagecko

According to a quick google search, this update only applies to the current shell you have open. It can probably be fixed by typing python3, as mac and linux are similar enough for things like this to coincide. Link to the result of google search.

Also, as ninjagecko stated, most programs have not been updated to 3.x yet, so having the default python as 3.x would break many python scripts used in applications.

Answered By: Matt Habel

Change the “default” Python by putting it ahead of the system Python on your path, for instance:

export PATH=/usr/local/bin:$PATH
Answered By: Adam Vandenberg

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*
Answered By: Ned Deily

I am using OS X 10.7.5 and Python 3.4.2. If you type python3 and what you want to run it will run it using python 3. For example
pyhton3 test001.py. That ran a test program I made called test001. I hope this helps.

Answered By: ITGM

Old question, but alternatively:

virtualenv --python=python3.5 .venv
source .venv/bin/activate
Answered By: FreshPow

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
Answered By: Ryosuke Hujisawa

Navigate to:

My Computer -> Properties -> Advanced -> Environment Variables -> System Variables

Suppose you had already having python 2.7 added in path variable and you want to change default path to python 3.x

then add path of python3.5.x folder before python2.7 path.

open cmd: type “python –version”

python version will be changed to python 3.5.x

Answered By: Ankit Agrawal

Update 11 May 2022 Wed EST PM 06:36

Thanks @Aditya Deshpande! Great suggestion!

I’ve just fixed my answer.

After taking ~/.bashrc or ~/.zshrc effect, there is no need to
quit and restart the terminal.


Do right thing, do thing right!

—>Zero Open your terminal,

–Firstly input python -V, It likely shows:

Python 2.7.10

-Secondly input python3 -V, It likely shows:

Python 3.7.2

–Thirdly input where python or which python, It likely shows:

/usr/bin/python

—Fourthly input where python3 or which python3, It likely shows:

/usr/local/bin/python3

–Fifthly add the following line at the bottom of your PATH environment variable file in ~/.profile file or ~/.bash_profile under Bash or ~/.zshrc under zsh.

alias python='/usr/local/bin/python3'

OR

alias python=python3

-Sixthly input source ~/.bash_profile under Bash or source ~/.zshrc under zsh.

–Seventhly then checkout the version of Python




input python -V, It likely shows:

Python 3.7.2

I had done successfully try it.

Others, the ~/.bash_profile under zsh is not that ~/.bash_profile.

The PATH environment variable under zsh instead ~/.profile (or ~/.bash_file) via ~/.zshrc.

Help you guys!

Answered By: Vittore Marcas
sudo mv /usr/bin/python /usr/bin/python2
sudo ln -s $(which python3) /usr/bin/python

This will break scripts, but is exactly the way to change python. You should also rewrite the scripts to not assume python is 2.x. This will work regardless of the place where you call system or exec.

Answered By: gnkdl_gansklgna

In short: change the path in Environment Variables!

For Windows:

  • Advanced System Settings > Advance (tab). On bottom you’ll find 'Environment Variables'

  • Double-click on the Path. You’ll see path to one of the python installations, change that to path of your desired version.

Answered By: amalik2205

Check the execution path of python3 where it has libraries

$ which python3
/usr/local/bin/python3  some OS might have /usr/bin/python3

open bash_profile file and add an alias

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

Reload bash_profile to take effect of modifications

source ~/.bash_profile

Run python command and check whether it’s getting loading with python3

$ python --version
Python 3.6.5
Answered By: Javeed Shakeel

you can change temporarily or switch between different versions using following commands:

set path=C:UsersShainaAppDataLocalProgramsPythonPython35-32;%PATH%
python --version

enter image description here

Answered By: Shaina Raza

Set Python 3.5 with higher priority

sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.5 2

Check the result

sudo update-alternatives --config python
python -V
Answered By: Topper

In my case, on my Mac OSX, with Python 2.7.18 installed via mac ports, I was able to set the python version to 2.7 with:

$ sudo port select --set python python27

So:

$ python -V
Python 2.7.18
Answered By: bitfox

After installing the newer version of python to your computer…

When you want to run a python program (e.g. ‘program.py’) from the terminal (using the latest version of python on your system); instead of running ‘python program.py’ run ‘python3 program.py’

Similarly, if you want to use python in the terminal (using the latest version of python on your system) run ‘python3’ instead of ‘python’

As a test try to run ‘python3 –v’ in the terminal…

Answered By: D_H

It should be noted that recent versions of Homebrew/MacOS will require a different entry for the PATH as the location where Homebrew installs Python has changed. Add this like to your .zshrc:

export PATH="/opt/homebrew/opt/python/libexec/bin:$PATH"

This will ensure that the appropriate unversioned python link installed by Homebrew appears before that of version 2.x and will protect you from being impacted by any version updates to python as brew will update the unversioned links whenever it updates the version.

Answered By: AfroRick

In order to change the python version context switching without exporting environment variable. Please use the below video link to see:
https://youtu.be/jTN4MHNhJZs

Answered By: VISHAL AGGARWAL

Starting with macOS Catalina the default shell is zsh. Therefore, all those ~/.bash_profile changes are not going to change the default when you open a new terminal since the new terminal is a zsh shell and not a bash shell.

You can confirm your terminal is a zsh shell by typing echo $SHELL and you should see a response of: zsh.

What should you do? You should use a solution that works for zsh shell and bash shell. Therefore, do the following:

  1. enter: vi ~/.bash_profile
  2. enter: alias python='python3'
  3. close and save your bash_profile. Enter: :wq
  4. Now open/create your zsh profile. Enter: vi ~/.zshrc
  5. source your bash inside your zshrc. Enter: source ~/.bash_profile

Now python will be aliased to python3 in your zsh shells (and in bash if you switch the default) automatically.

Test:
python --version

Python 3.8.9

Answered By: kvothe__
Categories: questions Tags: ,
Answers are sorted by their score. The answer accepted by the question owner as the best is marked with
at the top-right corner.