python: padding punctuation with white spaces (keeping punctuation)

Question

What is an efficient way to pad punctuation with whitespace?

input:

s = \'bla. bla? bla.bla! bla...\'

desired output:

Answer 1

This will add exactly one space if one is not present, and will not ruin existing spaces or other white-space characters:

s = re.sub('(?

This works by finding a zero-width position between a punctuation and a non-space, and adding a space there.
Note that is does add a space on the beginning or end of the string, but it can be easily done by changing the look-arounds to (?<=[^ ]) and (?=[^ ]).

See in in action: http://ideone.com/BRx7w