Split and join are useful in many places ... But here I found use of "replace" apt.

print raw_input().replace(" ","-")

print can do joining too.

print(*input().split(), sep='-')

the task misleadingly requests to print rather than return

Kind of useless challenge, since the solution is provided already in the description... ;)

Unclear question. It asks to "print" but in actuality it's looking for a return value. If you are seeing a none in stdout, replace your print with return