If you have both Python 3 and Python 2 installed, you can specify which version of Python you would like pip to use. This is useful when packages only support Python 2 or 3 or when you wish to test with both.
If you want to install packages for Python 2, run either:
pip install [package]
pip2 install [package]
If you would like to install packages for Python 3, do:
pip3 install [package]
You can also invoke installation of a package to a specific python installation with:
\path\to\that\python.exe -m pip install some_package # on Windows OR /usr/bin/python25 -m pip install some_package # on OS-X/Linux
On OS-X/Linux/Unix platforms it is important to be aware of the distinction between the system version of python, (which upgrading make render your system inoperable), and the user version(s) of python. You may, depending on which you are trying to upgrade, need to prefix these commands with
sudo and input a password.
Likewise on Windows some python installations, especially those that are a part of another package, can end up installed in system directories - those you will have to upgrade from a command window running in Admin mode - if you find that it looks like you need to do this it is a very good idea to check which python installation you are trying to upgrade with a command such as
python -c"import sys;print(sys.path);" or
py -3.5 -c"import sys;print(sys.path);" you can also check which pip you are trying to run with
On Windows, if you have both python 2 and python 3 installed, and on your path and your python 3 is greater than 3.4 then you will probably also have the python launcher
py on your system path. You can then do tricks like:
py -3 -m pip install -U some_package # Install/Upgrade some_package to the latest python 3 py -3.3 -m pip install -U some_package # Install/Upgrade some_package to python 3.3 if present py -2 -m pip install -U some_package # Install/Upgrade some_package to the latest python 2 - 64 bit if present py -2.7-32 -m pip install -U some_package # Install/Upgrade some_package to python 2.7 - 32 bit if present
If you are running & maintaining multiple versions of python I would strongly recommend reading up about the python
venv virtual enviroments which allow you to isolate both the version of python and which packages are present.