python - Need more elegant solution to even string length -

here's task got:

given string of length, return first half. string "woohoo" yields "woo".

first_half('woohoo') → 'woo' first_half('hellothere') → 'hello' first_half('abcdef') → 'abc' 

here's solution:

def first_half(str):     if len(str) % 2 == 0:         b = len(str) // 2         return str[:b] 

my question is:

can show me simpler solution in python of course doesn't require me use variable half of string length (b)?

in python:

str = "woohoo"    str = str[:-len(str)/2]