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I have installed Python 3.2 on 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 Python 2.6.1 which is not what I expected.
How can I change the 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
python3leaves yourpython2installation intact (due to the above compatibility reasons); thus you can start python3 with thepython3command.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 expectpython=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 python2or#!/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
pythoncommand 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 (.bashrc, .zshrc, etc). 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 makeThis is acceptable nowadays, but PEP 394 suggests encouraging users to use a virtualenv instead.pythonexecutepython3; this is poor form.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 uninstallif you did a manual installation). If you require two versions, then reconfigure your$PATHvariable so the ‘default’ version you want is in front; or if you are using most Linux distros, the command you are looking for is sudoupdate-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 varPATHin 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 $PATHon Linux and Mac) environment variable. You can always run a program with the full path e.g./usr/bin/ some args, orcd /usr/binthen./ some args(replace blank with the ‘echo’ program I mentioned above for example), but otherwise typingsome argshas no meaning withoutPATHenv variable which declares the directories we implicitly may search-then-execute files from (if/usr/binwas not inPATH, then it would say: command not found). The first matching command in the first directory is the one which is executed (thewhichcommand 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/pythonwhich is a symlink to other symlinks to the final version somewhere, e.g.:sidenote2: (In the rarer case a python program invokes a sub-program with the
subprocessmodule, to specify which program to run, one can modify the paths of subprocesses withsys.pathfrom the sys module or thePYTHONPATHenvironment 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.)