On Linux, Mac OS X and other POSIX systems, use the versioned Python commands in combination with the -m switch to run the appropriate copy of pip:
python2.7 -m pip install SomePackage python3.4 -m pip install SomePackage python2.7 -m pip install SomePackage python3.4 -m pip install SomePackage (appropriately versioned pip commands may also be available)
On Windows, use the py Python launcher in combination with the -m switch:
py -2.7 -m pip install SomePackage # specifically Python 2.7 py -3.4 -m pip install SomePackage # specifically Python 3.4 py -2.7 -m pip install SomePackage # Specifically Python 2.7 py -3.4 -m pip install SomePackage # Specifically Python 3.4 if you get an error for py -3.4 then try:
pip install SomePackage pip install SomePackage