import pip

command = 'install'
parameter = 'selenium'
second_param = 'numpy' # You can give as many package names as needed
switch = '--upgrade'

pip.main([command, parameter, second_param, switch])

Only needed parameters are obligatory, so both pip.main(['freeze']) and pip.main(['freeze', '', '']) are aceptable.

Batch install

It is possible to pass many package names in one call, but if one install/upgrade fails, whole installation process stops and ends with status '1'.

import pip

installed = pip.get_installed_distributions()
list = []
for i in installed:
    list.append(i.key)

pip.main(['install']+list+['--upgrade'])

If you don't want to stop when some installs fail, call installation in loop.

for i in installed:
        pip.main(['install']+i.key+['--upgrade'])

When you use python file as module there is no need always check if package is installed but it is still useful for scripts.

if __name__ == '__main__':
    try:
        import requests
    except ImportError:
        print("To use this module you need 'requests' module")
        t = input('Install requests? y/n: ')
        if t == 'y':
            import pip
            pip.main(['install', 'requests'])
            import requests
            import os
            import sys
            pass
        else:
            import os
            import sys
            print('Some functionality can be unavailable.')
else:
    import requests
    import os
    import sys

Many packages for example on version 3.4 would run on 3.6 just fine, but if there are no distributions for specific platform, they can't be installed, but there is workaround. In .whl files (known as wheels) naming convention decide whether you can install package on specified platform. Eg. scikit_learn‑0.18.1‑cp36‑cp36m‑win_amd64.whl[package_name]-[version]-[python interpreter]-[python-interpreter]-[Operating System].whl. If name of wheel file is changed, so platform does match, pip tries to install package even if platform or python version does not match. Removing platform or interpreter from name will rise an error in newest versoin of pip module kjhfkjdf.whl is not a valid wheel filename..

Alternativly .whl file can be unpacked using an archiver as 7-zip. - It usually contains distribution meta folder and folder with source files. These source files can be simply unpacked to site-packges directory unless this wheel contain installation script, if so, it has to be run first.