Mastering User Input Validation in Python: Loops, Exceptions, and Custom Rules

The simplest way to achieve this is to place the input call inside a while loop, using continue when the input is wrong and break when you are satisfied.

When your input may raise exceptions

Use try and except to detect when the user enters data that cannot be parsed.

while True:
    try:
        # Note: Python 2.x users should use raw_input, the equivalent of 3.x's input
        age = int(input("Please enter your age: "))
    except ValueError:
        print("Sorry, I didn't understand that.")
        # better try again... Return to the start of the loop
        continue
    else:
        # age was successfully parsed!
        # we're ready to exit the loop.
        break
if age >= 18:
    print("You are able to vote in the United States!")
else:
    print("You are not able to vote in the United States!")

Implement your own validation rules

If you want to reject values that Python can parse, add your own validation logic.

while True:
    data = input("Please enter a loud message (must be all caps): ")
    if not data.isupper():
        print("Sorry, your response was not loud enough.")
        continue
    else:
        # we're happy with the value given.
        # we're ready to exit the loop.
        break

while True:
    data = input("Pick an answer from A to D:")
    if data.lower() not in ('a', 'b', 'c', 'd'):
        print("Not an appropriate choice.")
    else:
        break

Combine exception handling and custom validation

The two techniques can be combined into a single loop.

while True:
    try:
        age = int(input("Please enter your age: "))
    except ValueError:
        print("Sorry, I didn't understand that.")
        continue

    if age < 0:
        print("Sorry, your response must not be negative.")
        continue
    else:
        # age was successfully parsed, and we're happy with its value.
        # we're ready to exit the loop.
        break
if age >= 18:
    print("You are able to vote in the United States!")
else:
    print("You are not able to vote in the United States!")

Wrap everything in a function

If you need to ask the user many different values, placing this code in a function avoids repetition.

def get_non_negative_int(prompt):
    while True:
        try:
            value = int(input(prompt))
        except ValueError:
            print("Sorry, I didn't understand that.")
            continue

        if value < 0:
            print("Sorry, your response must not be negative.")
            continue
        else:
            break
    return value

age = get_non_negative_int("Please enter your age: ")
kids = get_non_negative_int("Please enter the number of children you have: ")
salary = get_non_negative_int("Please enter your yearly earnings, in dollars: ")

Putting it all together

You can extend this idea to create a very generic input function:

def sanitised_input(prompt, type_=None, min_=None, max_=None, range_=None):
    if min_ is not None and max_ is not None and max_ < min_:
        raise ValueError("min_ must be less than or equal to max_.")
    while True:
        ui = input(prompt)
        if type_ is not None:
            try:
                ui = type_(ui)
            except ValueError:
                print("Input type must be {0}.".format(type_.__name__))
                continue
        if max_ is not None and ui > max_:
            print("Input must be less than or equal to {0}.".format(max_))
        elif min_ is not None and ui < min_:
            print("Input must be greater than or equal to {0}.".format(min_))
        elif range_ is not None and ui not in range_:
            if isinstance(range_, range):
                template = "Input must be between {0.start} and {0.stop}."
                print(template.format(range_))
            else:
                template = "Input must be {0}."
                if len(range_) == 1:
                    print(template.format(*range_))
                else:
                    print(template.format(" or ".join((", ".join(map(str, range_[:-1])), str(range_[-1])))))
        else:
            return ui

# Usage example:
age = sanitised_input("Enter your age: ", int, 1, 101)
answer = sanitised_input("Enter your answer: ", str.lower, range_=('a', 'b', 'c', 'd'))