flipswithtest.py

def flip(s):
    """changes all _ in s to + and all + in s to _"""
    res = []
    for l in s:
        if l=='+':
            res.append('-')
        else:
            res.append('+')
    return "".join(res)
  
print flip('+_+_+') #expect it to print "_+_+_"
print flip('_____') # expect it to print '+++++'
print flip('__+__') # expect it to print '++_++'
  
# you can also use assertions
assert(flip('++__+'),"__++_")
assert(flip('+__++'),"_++__")
assert(flip('++__+'),"__++_")
assert(flip('++__+'),"__++_")
assert(flip('++__+'),"__++_")
assert(flip('++__+'),"__++_")