Reading user input
input() reads a line from standard input and returns a string. Even if the user enters 25, the result is still text. Convert it explicitly when a number, date or other type is required.
Formatting output
print() can display multiple values, control separators and line endings, and work naturally with f-strings. F-strings are usually the clearest way to insert calculated values into user-facing text.
Validate at the boundary
Console input is untrusted data. Convert it inside appropriate error handling and reject values that do not meet the program's rules. The same principle applies to data received from forms, APIs and files.
Practice: create a bill calculator that accepts price and quantity, handles invalid input and prints a correctly formatted subtotal.
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