How could I make an infinite "For" loop able to save values, without using "while"?

Question

I'm learning Python (using 3.6.2) and on my last class, they asked me to do something where I need to make an infinite for loop. For some reason, the teacher doesn't want us to use while for the entire practice. This is where it gets complicated...

So, I've been looking for a way to do it. But, it's also difficult because the teacher doesn't want us to use any commands we haven't seen in class. So I can't use .append, sys functions, well, I can't even use a break. I must find a way to do with "simple" commands.

I thought I could do it this way;

x=1
    for i in range(x):
    do_something()
    x += 1

However, it didn't seemed to work. I think that's because Python doesn't read the value for the range again?

I couldn't find a way, but after hours of thinking I found myself a small workaround I could use:

def ex():
    print("Welcome")
    for i in range(1):
        math = int(input("Please insert grades obtained at Math, or insert 666 to exit" ))
        if(math > 0 and math < 60):
            print("Sorry. You failed the test")
            return ex():
        elif(math >= 60 and math <= 100):
            print("Congratulations. You passed the test")
            return ex():
        elif(math == 666):
            return exit()
        else:
            print("ERROR: Please insert a valid number")
            return ex():

def exit():
     pass

As you can see, what makes it "infinite" is that it returns to the function once and once again, until you tell the program to "exit", by entering "666". I'd also like to have a more proper way to exit the function.

I'm still wondering if there's a better way to make my for loop infinite until the user calls it to stop. However, one way or another I got this exercise working. The problem came when I started with the second one, which is more or less like this:

Imagine the same past program, but this time, it will not just show you if you passed the test or not. It wants to collect as many grades you enter through the input, and then calculate the average of all the grades. I'm not able to save those values (the grades) because I kind of "restart" my own function every time.

And according to my teacher's instructions, I can't ask the user how many grades he wants me to calculate. It must be infinite and keep asking for inputs until the user choses not to.

I'm really stuck and lost on it. It's very hard and frustrating because it'd be way easier if we could just use while's :( And also harder because we can't use any options we haven't seen...

So, I have 3 questions:

• How do I make an appropiate "infinite" for loop?
• How do I make a proper way to "finish" it?
• How do I make it able to save values?

A lot of thanks in advance for anyone willing to help, and sorry for my ignorance.
I'm new to the community, so any advice about my problems, the question formatting or anything is well received :)

EDIT: I talked to my teacher and he allowed me to use either itertools or just a range big enough to not be reached. Now i'm wondering, how can I save those values inside the for for later manipulation?

Answer 1

I hate "trick" questions like this that have very little to do with how you'd use Python in the real world. But anyway...

The trick here is to iterate over a list that you modify inside the for loop. This is generally regarded as a bad practice, but we can exploit it here for this contrived assignment.

We ask for user input inside a function so we can escape from the loop by using return, since you aren't permitted to use break.

def get_data(prompt):
    lst = [None]
    for i in lst:
        s = input(prompt)
        if not s:
            return lst[1:]
        lst += [int(s)]
        print(lst)

print('Enter data, one number at a time. Enter an empty line at the end of the data')
lst = get_data('Number: ')
print('Data:', lst)

demo

Enter data, one number at a time. Enter an empty line at the end of the data
Number: 3
[None, 3]
Number: 1
[None, 3, 1]
Number: 4
[None, 3, 1, 4]
Number: 1
[None, 3, 1, 4, 1]
Number: 5
[None, 3, 1, 4, 1, 5]
Number: 9
[None, 3, 1, 4, 1, 5, 9]
Number: 
Data: [3, 1, 4, 1, 5, 9]

Answer 2

Can you try something like this:

for i in iter(int, 1):
    print("Infinite for loop executing")

Refer to this question regarding infinte iterator without while for more info.