How to randomize uppercases of an string?

i want a way of grab an string variable and randomize which letters are uppercase, like so:

upperRandomizer("HelloWorld") == "HeLLoWORlD"

I've tried this so far:

for p in strVar:
        result = ""
        if random.choice((True, False)):
            result += p.upper()
        else:
            result += p

But the code only spits out the last letter of the string variable. I tried to use the join() method without success. Any help would be appreciated.