-- Python:  ",".join(lst)
-- either...
import Data.List
intercalate "," lst
-- or...
import Data.List
concat (intersperse "," lst)

-- a.split(ss)
-- You might need to get Data.List.Split from Cabal
import Data.List.Split
splitOn ss a 
-- using regexes
import Text.Regex
splitRegex (mkRegex ss) a

-- Python: a.startswith(b)
import Data.List
isPrefixOf b a
-- ByteString
import Data.ByteString
isPrefixOf b a

-- Python: a.endswith(b)
import Data.List
isSuffixOf b a
-- ByteString
import Data.ByteString
isSuffixOf b a

-- Python: a.find(b) != -1
import Data.List
isInfixOf b a
-- ByteString
import Data.ByteString
isInfixOf b a 
-- ByteString
import Data.ByteString
findSubstring b a -- returns Just int or Nothing

-- Python: a[::-1]
reverse a
-- ByteString
import Data.ByteString
reverse bs

-- Python: a[pos]
a !! pos    -- slow O(n)
-- ByteString
import Data.ByteString
index a pos

-- Python: a.find(b)
import Data.ByteString
findSubstring a b

-- Python: b = a.replace(src,dest)
import Text.Regex
b = subRegex (mkRegex src) a dest

-- Python: b = re.sub("[0-9]+", lambda x: str(int(x.group(0)) + 1), a)
import Text.Regex (matchRegexAll, mkRegex)
 
subRegexFn fn re s = concat $ reverse $ sub s []
    where sub s accum = case matchRegexAll re s of
                            Nothing -> s:accum
                            Just (pre, mid, post, _) -> sub post $ (fn mid):pre:accum 
 
b = subRegexFn (read . (+ 1) . show) "[0-9]+" a

-- Python: a.strip()
import Data.Char (isSpace)
trim      :: String -> String
trim      = f . f
   where f = reverse . dropWhile isSpace

-- Python:  loop through every line in stdin, applying a function
-- for line in sys.stdin:
--      print fn(line)
let main = interact (unlines . map fn . lines) 
-- or...
let main = do { a <- stdin; mapM_ print (map fn (lines a)); }