I'm wish to install Numpy for the www-data user, but I can not log into this user using login. How can I make www-data make us of the Numpy module?

To clarify. Numpy is available for root, and for my default user.

Running pip with sudo should install a package for all users

sudo pip install numpy

If it doesn't work, try

sudo -H -u www-data pip install --user numpy

which should install numpy only for www-data