ATM machines allow 4 or 6 digit PIN codes and PIN codes cannot contain anything but exactly 4 digits or exactly 6 digits. If the function is passed a valid PIN string, return true, else return false

Sure, here's a Python function that checks if a PIN string is valid according to the given conditions:

def validate_pin(pin):
    if len(pin) == 4 or len(pin) == 6:
        if pin.isdigit():
            return True
    return False

You can use this function by passing a PIN string to it. It will return `True` if the PIN is valid (contains exactly 4 or 6 digits only), and `False` otherwise.