6.4.3 – Validate input

What validation establishes

Validation checks whether data satisfies the rules a program requires before it accepts the data. A booking program might allow a party size from 1 to 6. Rejecting 0 prevents a booking with no people; rejecting 9 prevents a booking too large for the table.

Passing the check means valid according to those rules. It does not prove that the data is true: someone can enter 4 when their party actually contains 3 people. Validation also does not establish a user's identity; checking a supplied ID and password against stored details is authentication.

Start with the requirement, then choose the check. Not every field needs all four. A three-digit suffix in an identifier is text: 007 may be permitted and must keep its leading zeros. Do not invent a numeric range for it. If a task guarantees valid whole-number input and says no validation is required, use that assumption rather than adding unrelated rejection rules.

Presence, length and fresh input

Keyboard input is a string. An empty submission is "". If spaces-only input must also be rejected, remove spaces from both ends before checking for emptiness. This does not change the original string unless the result is assigned back to it.

name = input("Name: ")
missing = name.strip(" ") == ""

A length check uses the number of characters returned by len(). Spaces count. For example, "A B" has three characters. The rule must specify whether it measures the entered text or a cleaned version.

A display name must contain 3 to 12 entered characters and at least one non-space character:

name = input("Display name: ")
while name.strip(" ") == "" or len(name) < 3 or len(name) > 12:
    print("Enter 3 to 12 characters, not spaces only.")
    name = input("Display name: ")
print("Accepted: " + name)

The condition lists ways the input can be invalid. Join these failures with or: any one is enough to reject it. The loop reads and stores fresh input each time. Without that assignment, an invalid name would never change and the loop could not finish.

Display name: AI
Enter 3 to 12 characters, not spaces only.
Display name: Nova7
Accepted: Nova7

Range checks and boundaries

An inclusive range includes both endpoints. A rating from 1 to 5 is valid at 1 and at 5. Validity needs both limits to hold; invalidity needs only one limit to fail.

valid = rating >= 1 and rating <= 5
invalid = rating < 1 or rating > 5

A value cannot be below 1 and above 5 simultaneously. Joining the two invalid comparisons with and would reject nothing.

In this focused example the user is guaranteed to enter text that converts to an integer. It checks the numeric range only. Do not use this version when letters or empty input are possible.