CS50: no output in the terminal [closed]

this will be my first stack overflow contribution, so i hope it helps. I pasted your code snippet in my editor and had some syntax errors. it was all due to indentation, which may just be a result of it being copy-pasted a couple of times. I also had to add your main() function call to the end to start the program. Once i did that, your code ran, but my output was nothing to the console as well.

I first added some print statements throughout your function to see if it was breaking at any particular point. No output still. Then I tried to print the value of the function itself with print(is_valid(plate)) - no output still. Then i assigned it to a variable and still got no output. it was clear the function was never terminating. In other words it never reached True or False. It had to be stuck in an infinite loop somewhere.

i = 2
while i < len(s):
    if s[i].isdigit() == True: 
        if s[i + 1].isalpha() == True: 
            return False
    else:
        break
i = i + 1

this 'i = i + 1' is outside of the while loop, meaning it will never execute until you exit the while loop. And the while loop will never terminate if i never reached len(s)

Fix that and your code will run, but you'll also get an index error for that loop. this is because index and length are different. You can have 6 letters(a length of 6). but since your index (plate[0]) starts at 0, your last letter will be index[5] 'i' will continue to increment to 6 though since its being compared to len(plate). Easy fix though - Just make sure you change it to:

len(plate)-1

I didn't fix your second while loop with the j variable, so you'll want to do that. its good debugging practice. Here's a snippet with your mostly working code. You can delete my print statements and fix the j while loop. I moved your main function to the bottom for my own readability, but that's just personal, nothing wrong with how you had it. Your control logic seems to work well, and a tried a few plates that seem to work, but you'll want to try a few more, because i think i still see some bugs.

    def is_valid(s):
    if len(s) < 2 or len(s) > 6:
        print('failure1')
        return False

    elif s[0].isalpha() == False or s[1] == False:   < call .isalpha() on the other side of this statement to actually check it, instead of checking the value of s[1] for False
        print('failure2')
        return False
    i = 2
    while i < len(s)-1:
        if s[i].isdigit() == True:
            if s[i + 1].isalpha() == True:
                print('failure3')
                return False
        else:
            break
        i = i + 1


    j = 0
    while j < len(s):
        if s[j].isalpha() == False:
            if s[j] == "0":
                print('failure4')
                return False

        else:
            break
    j = j + 1
    for char in s:
        if char in [" ", "?", "!", "."]:
            return False

    return True

def main():
    plate = input("Plate: ")
    print(plate)
    returned_value =is_valid(plate)
    print(returned_value)
    if is_valid(plate):
        print("Valid")
    else:
        print("Invalid")
main()

One more thing I'd like to point out. The bug you had - an indentation error- is an easy mistake to make and a common symptom of having many nested for/while loops and if/else statements. Once you get your code working, look at refactoring some of it if possible to make it really stand out. If you do this, it will help with debugging in the future. Once again, hope that helps!