In what cases is data validation performed before using or saving data?

Data validation is performed in the following cases:

• Before using data in program logic to prevent errors and incorrect calculations.
• Before saving data to a database or file to ensure integrity and correctness.
• When receiving data from users (e.g., through forms) to prevent incorrect or malicious input.
• When processing external data (API, files, networks) to protect against errors and attacks.

In Python, this might look like:

user_input = input("Enter age: ")
if user_input.isdigit() and 0 < int(user_input) < 120:
    age = int(user_input)
else:
    print("Invalid input")

Thus, data validation helps prevent failures and enhances application security.