Evaluate string from user to comma separated words string

Let's say I have a string given by user in the following fo开发者_开发百科rm:

"single, single, double word, single  word, also      single"

How I can parse this string to obtain string of comma separated words, where each not single whitespace is treated as a separator and replaced with ',', and each single is replaced with %20 ? So the result would be:

"single,single,double%20word,single,word,also,single" 

? can I do it in one step ?

This does what you ask:

def repl(m):
  if m.group(0) == ' ':
    return '%20'
  else:
    return ','

re.sub(',? +', repl, "single, single, double word, single  word, also      single")

However, if your goal is to end up with proper URL escaping I'd advise that you actually use a library function designed to do that. eg: urllib.quote

You probably could do it in a single step, but it would be safer to do it in two.
First use a regex to replace \s\s+ with ,
Then replace \s with %20