checking if string only contains certain letters in Python
<p>i'm trying to write a program that completes the MU game
<a href="https://en.wikipedia.org/wiki/MU_puzzle" rel="nofollow">https://en.wikipedia.org/wiki/MU_puzzle</a></p>

<p>basically i'm stuck with ensuring that the user input contains ONLY M, U and I characters.</p>

<p>i've written</p>

<pre><code>alphabet = ('abcdefghijklmnopqrstuvwxyz')

string = input("Enter a combination of M, U and I: ")

if "M" and "U" and "I" in string:
    print("This is correct")
else:
    print("This is invalid")
</code></pre>

<p>i only just realised this doesnt work because its not exclusive to just M U and I. can anyone give me a hand?</p>