How to customize python-mode

Customize default Python shell as `py-shell-name'

`py-shell-name' might be an installed executable as
shell command `type' would display, but also a
PATH/TO/(I)PYTHON, of a virtualenv for example

You might run code by another (I)Python-version than default.

Normally this would happen if a shebang in buffer specifies it.
Shebang precedes default py-shell-name.

In case you will run code by default-shell, ignoring shebang, set
`py-force-py-shell-name-p' to. This might be done also via menu
Python/.../Switches

Another way to enforce a different shell than default is using
commands which end in version like `py-execute-statement-python3'
See menu Python/Interpreter/Other/Ignoring Defaults