Curriculum Python input() and Type Conversion for KidsValidating input

Validating Python input() with a while Loop

Re-ask until the answer is actually valid — using a while loop wrapped around input().

Lesson 5 of 5PROAges 10+

Validating input

So far, every level trusted the player's FIRST answer. Real programs can't always do that.

You already know while loops repeat until a condition changes. Wrap one around input(), and you can keep asking until the answer is actually good.

while True:
    answer = input("Password? ")
    if answer == "dragon":
        break
    print("Nope, try again.")
print("The gate opens!")

Output

Nope, try again.
The gate opens!

while True: means 'repeat forever' — the ONLY way out is the break inside the if. That's the whole pattern: ask, check, break if good, otherwise loop back and ask again.

while True:
    answer = input("Password? ")
    if answer == "dragon":
        break
    print("Nope, try again.")

This works for numbers too — not just wrong answers, but numbers that convert fine but break a GAME rule, like gold that shouldn't be negative.

int('-5') converts perfectly well — it's a real, valid number. The while loop re-asks because -5 fails YOUR rule (gold > 0), not because Python refused to convert it.

while True:
    gold = int(input("Gold? "))
    if gold > 0:
        break
    print("Try again.")

But some text can't be converted AT ALL — and that's a different, real crash.

int('abc') raises a ValueError. Not the TypeError from before — this is Python saying 'abc' was never digits, so there's nothing to convert.

int('abc')

Output

ValueError: invalid literal for int() with base 10: 'abc'

answer.isdigit() checks whether text is made ENTIRELY of digits, without converting it — True or False, and it never crashes, even on 'abc'.

"abc".isdigit()

Output

False

Put it together: check .isdigit() BEFORE calling int(), inside a while loop. Bad text gets re-asked instead of ever reaching int() at all.

while True:
    age_text = input("Age? ")
    if age_text.isdigit():
        age = int(age_text)
        break
    print("Not a whole number — try again.")

Check your understanding

A level re-asks "Password? " until the player types "dragon", using while True: with a break inside an if. The player types "wrong" first, then "dragon". How many times does the loop ask?

while True:
    answer = input("Password? ")
    if answer == "dragon":
        break
    print("Nope, try again.")

Twice — once for the wrong guess, once for the right one
Once — checking the answer once is enough to know if it's valid
Forever — while True: never stops

Why: Twice. The first answer (wrong) fails the if, so the loop asks again; the second answer (dragon) passes the if and break ends the loop. while True: plus a break is exactly how you repeat 'ask again' an unknown number of times, however many wrong guesses it takes.

gold = int(input("Gold? ")) and the player types "-5". Does this crash?

gold = int(input("Gold? "))

No — int('-5') converts fine, it's a real (negative) number
Yes — int() can't handle negative numbers

Why: No crash — int('-5') is a completely valid conversion. Rejecting -5 as bad gold is a rule YOUR code has to check with an if, same as any other re-ask loop, not something int() does automatically.

The player types "abc" where the code does int(input("Age? ")). What happens?

age = int(input("Age? "))

A ValueError — 'abc' isn't digits, so there's nothing to convert
A TypeError, same as forgetting int() entirely
Nothing — int() just returns 0 for text it can't convert

Why: A ValueError — Python's honest way of saying 'abc' was never digits, so int() has nothing valid to convert. Checking answer.isdigit() BEFORE calling int() is how you avoid ever triggering it.

What you'll practice

The dragon's gate only opens for the password "dragon". Keep asking "Password? " until the answer is exactly "dragon" — print "Nope, try again." for every wrong guess, then once the right password arrives, break out and print "The gate opens!". The player will answer wrong once, then right.

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Validating input

Check your understanding

What you'll practice

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