A recursive solution to symmetirc tree
Possibly the most obvious part is here

    left = self.isMirror(l.left, r.right)
    right = self.isMirror(l.right, r.left)
    return left and right


there's no need to perform the second test if the first one returns False:

    if not self.isMirror(l.left, r.right): return False
    if not self.isMirror(l.right, r.left): return False

    return True