Validate Before Converting

Before int() converts text, a program can check the form it expects. This lesson accepts ordinary keyboard digits from 0 through 9, and the string method .isdigit() is a simple first check for that deliberately narrow requirement.

Consider a seat-number prompt:

while True:
    answer = input("Seat number: ")

    if answer.isdigit():
        seat = int(answer)
        break

    print("Please enter digits only.")

print(f"Seat: {seat}")

The order matters:

1. input() produces a string.

2. .isdigit() validates that string.

3. int() runs only after validation succeeds.

4. break leaves the loop with a valid integer.

If the answer is invalid, the conversion is skipped. The message is printed, and the next loop iteration asks again.

Trace two attempts

If the answers are front and then 12, the flow is:

The program never sends "front" to int(), so that input cannot cause a ValueError.

Validate the rule you actually need

For the ordinary 0 through 9 input used here, .isdigit() accepts examples such as "0" and "27". It rejects:

• negative numbers such as "-2";

• decimals such as "3.5";

• signs such as "+4";

• words such as "many".

An empty string also fails this check. Surrounding spaces fail too, because this example does not strip the answer.

This exercise uses ordinary keyboard characters. Some other digit characters, such as "²", pass .isdigit() but still cause int() to raise ValueError. So .isdigit() is a useful first check here, not a guarantee that every possible string can be converted.

That is useful when the requirement really is digits only within this limited input set. It is not a complete test for every value that int() can understand. The next lessons introduce try and except, which can ask Python itself whether an integer conversion works.

Give actionable feedback

An error message should help the learner or user make a valid next attempt. “Invalid” is vague. “Please enter digits only.” states the expected format.

Keep the prompt inside the loop. Otherwise the program may repeat its check forever without collecting a new answer.

while True:
    text = input("Count: ")
    if text.isdigit():
        count = int(text)
        break
    print("Digits only.")