Not the simplest but it works:

print ''.join([i.lower() if i.isupper() else i.upper() for i in raw_input()])

Use inbuilt Libraries

def swap_case(s):   
    return s.swapcase()

Here is a simple solution that's very readable and avoids using the built in swapcase() function.

def swap_case(s):
    result = ""
    for letter in s:
        if letter == letter.upper():
            result += letter.lower()
        else:
            result += letter.upper()
    return result

Even though i solved it in first attempt within seconds i feel like noob watching people giving 1 liners :/ s=raw_input() x=[] for i in s: if i.isupper(): i=i.lower() x.append(i) else: i=i.upper() x.append(i) str1=''.join(x) print str1

Came up with this solution before realizing swapcase() exists:

print(*map(lambda ch : ch.lower() if ch.isupper() else ch.upper(), input()), sep="")