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At the end of the Program Flow 
Control in Python section, 

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we finished with a challenge to 
create a simple menu program. 

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I called my program summarychallenge.py. If you've 
been experimenting with the code – and I hope you 

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have – you can download summarychallenge.txt 
from the resources for this video. 

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Pause the video, while you copy the text, 
and paste it into a new Python file. 

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I've called my file summarychallenge.py
Ok, I've got summarychallenge.py open in IntelliJ. 

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Before we continue, have a look 
at how simple this code is. 

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That code is very basic, but 
you probably struggled a bit 

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with the challenge, at the time.
Now you can understand it easily. 

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You've come a long way since we 
wrote that code, a few sections ago. 

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When you find yourself struggling and are 
getting frustrated, or thinking that you'll 

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never get the hang of this, take a break.
Have a look at some of the earlier 

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examples that we wrote.
Reviewing the earlier code, 

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which you probably also struggled with at the 
time, will show that you are learning a lot. 

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It's easy to lose sight of 
that, when you're stuck. 

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Focus on how far you've come, and 
you'll get the confidence to continue. 

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Alright, pep talk over .
Back in IntelliJ, quite a few students 

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spotted a problem with this code. Let's have a 
look at what the problem is, then we'll fix it. 

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Run the program,
and we get the menu displayed. 

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The program seems to work. When I enter 4, the 
program correctly tells me that I chose option 4. 

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The problem's with line 3 of the 
code. That's supposed to restrict 

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the input to one of the digits 1 to 5.
It works, as long as we only enter a single 

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character. If I enter 8, for example, 
the program doesn't print the choice. 

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It just displays the menu again.
"8" isn't in the string "12345", 

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and the condition on line 3 is False.
That's also working fine. Where there's a problem, 

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is if the user enters something like "123".
The program prints "You chose 123", 

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which isn't what we want.
The string "123" is in the string "12345", 

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so the condition evaluates to True.
In Python, the empty string is considered 

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to be in any other string. That means we also 
get the wrong result, when I just press ENTER. 

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We get "You chose ", with 
an empty string after it. 

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With what we knew at the time, 
this was the best we could do. 

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As I said, we've come a long way since then. 
We can now do better, and fix this bug. 

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If we check that the input is 
in a list instead of a string, 

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the bug goes away. I'll enter 0 first, 
to stop the program, then change line 3:

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If you're not sure what I've done there, 

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keep watching. I'll review what the 
list function is doing, in a moment. 

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Run the program again, and test that it 
now only allows the values we expect. 

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Entering 4 works, and the 
program prints the choice. 

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8 also works. The options are printed 
again, by the code on lines 6 to 12. 

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The problem was with longer sequences, 
so I'll enter "123" again. This time, 

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the program doesn't incorrectly identify that as a 
valid choice – we just get the menu printed again. 

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An empty string also isn't in the list 
– pressing ENTER prints the menu out. 

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Enter zero to terminate the program.
Ok, that's fixed the bug. The program 

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can now reliably check if an option is valid.
I used a list there, because you already know 

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about lists. You're only just learning about sets, 
and I didn't want to jump straight in with a set. 

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But a set is more appropriate here. We'll get 
it working, then I'll explain why a set is more 

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efficient than a list or tuple, when using in.
I'll change list to set, on line 3:

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I'll quickly test that, to make sure it works. 

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Option 1 is correctly identified as valid, 
and the program prints "You chose 1". 

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The same for the other options, 2 to 5. 

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Invalid input such as 8 causes the menu to 
be printed again. So does "123" and "345" . 

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ENTER also does the right 
thing – we get the menu again. 

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0 quits, so that's still working too.
Alright, let's review what we've done there. 

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Back in the Lists, Tuples and Ranges section, we 
saw that the list function can be used to create 

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a list, from any iterable type.
I'll review that now. 

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From the Tools menu, choose 
Python or Debug console.  

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We're going to type a few short snippets of code, 
and we don't need to create a new file for this. 

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There's no need for you to do this, 
but I'm going to use the cog icon, 

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and change the View Mode to Float. That's just to 
keep the Console window clear of the subtitles, 

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on the video.
In the console, type:

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and press ENTER. 

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We get a list containing 5 
strings — the characters '1' to '5'. 

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The set function does the same thing. It 
creates a set from any iterable. Enter: 

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The output, this time, is a set – notice the 
curly braces, instead of square brackets. 

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The items have probably appeared 
in a random order, as well. 

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Alright, that's two more 
things we've learnt about sets. 

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First, you can use the set 
function to create a new set. 

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You pass any iterable object to it, and you'll get 
a set containing all the items in the iterable. 

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For example – and still working in the Python 
Console – we can create a set of even numbers, 

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by passing a range to the set function. 
I'll type the code to do that: 

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Press ENTER, and confirm that we get 
a set containing all the even numbers, 

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from 0 to 19. That means the largest 
number in the set will be 18. 

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The second thing is, how to test set 
membership. We do that using the in keyword. 

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It works the same way as it does for lists 
and strings, which we've seen before. 

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If the item is in the set, in evaluates to True. 

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Otherwise, in evaluates to False.
We used that – and I'll close the Python Console, 

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to go back to summarychallenge.py – on line 3.
Just to be clear what's happening there, 

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I'll comment out that line, 
and use a set literal instead: 

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That does the same thing.
If the user's choice is in the set, 

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the condition evaluates to True. If it's not 
in the set, the condition evaluates to False. 

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On line 3, which is now commented out, we used 
the set function to create a set containing 

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the valid characters we'd accept.
On line 4, we use a set literal to 

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create a set containing the same characters.
In both cases, we check if the user's choice is 

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in the set, and only print the message if it is.
I still haven't explained why a set can give 

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better performance – although you might 
have guessed, when we talked about 

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accessing keys in a dictionary, earlier.
We'll look at that, in the next video.