Very simple python question

>>> prefixes("overall")
['', 'o', 'ov', 'ove', 'over', 'overa', 'overal', 'overall']

I need to be able to return the statement above, so far all I have is:

def prefixes(x):
    return [v for v in

I don't know where to go from here or if what I have so far is wrong, but I know that the statement should only be about 1-2 lines.  I was told that there should be a slicing statement

is anyone able to help me with this program?

prefixes = scanl (++) [] . map (: [])