Substitute environment variables in a string:

>>> import os
>>> from envsubst import envsubst

>>> del os.environ['PS1']
>>> print(envsubst('$USER@$HOST ${PS1:-$}:'))
ashafer01@github.com $:

>>> os.environ['PS1'] = ''
>>> print(envsubst('$USER@$HOST ${PS1:-$}:'))
ashafer01@github.com $:

>>> print(envsubst('$USER@$HOST ${PS1-foo}:'))
ashafer01@github.com :

>>> os.environ['DEFAULT_PROMPT'] = '$'
>>> print(envsubst('$USER@$HOST ${PS1:-$DEFAULT_PROMPT}:'))

Also supports $0, $1, etc. from argv.