Remove characters except digits from string using Python?

Use re.sub, like so:

>>> import re
>>> re.sub('\D', '', 'aas30dsa20')
'3020'

\D matches any non-digit character so, the code above, is essentially replacing every non-digit character for the empty string.

Or you can use filter, like so (in Python 2):

>>> filter(str.isdigit, 'aas30dsa20')
'3020'

Since in Python 3, filter returns an iterator instead of a list, you can use the following instead:

>>> ''.join(filter(str.isdigit, 'aas30dsa20'))
'3020'