Building Programs with Variables, Expressions, and Statements

From Fixed Data to User Data

A program can use values written directly in its code, but useful programs often need information from the person running them. A banking app may ask for an account number, a game may ask for a name, and a calculator may wait for numbers. Python provides the built-in function input() for this kind of interaction. It pauses program execution, waits for keyboard input, and then gives the program the user's entry.

The Waiting Point

The diagram shows a pause, not a permanent end. The program reaches input(), waits while the user responds, receives the typed entry as a string, and then moves to the next statement. Nothing after that input() call runs until the response has been provided.

Saving the Returned String

Writing name = input() creates a pipeline. First, input() reads from the keyboard. Next, it returns what the user typed as a string. Finally, the assignment operator stores that string in the variable name. From then on, name holds the returned value and later statements can use it.

name = input() print(name)

Alice

Following Multiple Input Calls

A program can contain more than one input() call. Execution reaches the first call in the program's order and waits there. After the user responds, the program continues with the statements between that call and the next one. When execution reaches the next input() call, it pauses again. This makes the locations of input() calls important when you trace a program.

To trace such a program, mark every input() call and read from top to bottom. At each marked point, ask what the user types, what string is returned, which variable receives it, and which statements run before the next input() call.

Strings That Look Like Numbers

input() always returns a string, regardless of what the user types. If a user types 25, the returned value is the string '25', not the integer 25. The characters may look like a number, but the value is still string data. Strings and integers behave differently, so arithmetic requires conversion with int() or float() first.

Mistakes with User Input

• Assuming input() returns a number when the user types digits.

  input() returns the user's entry as a string regardless of what they type.

  Fix: Use int() or float() before performing arithmetic.

• Using input() without assigning its result to a variable.

  The returned string is not available through a named variable for later use.

  Fix: Assign the result, as in name = input().

• Expecting statements after input() to run before the user responds.

  input() pauses execution at that point.

  Fix: Trace the pause first, then continue tracing after the user types and presses Enter.

Practice the Execution Trace

name = input()
age = input()
print(name)
print(age)

Key Takeaways

1. input() pauses program execution and waits for keyboard input.
2. The function returns the user's entry as a string, even when the entry looks like a number.
3. Assigning input() to a variable lets later statements use the returned data.
4. With multiple input() calls, execution reaches them in program order and pauses at each one.
5. Convert string input with int() or float() before performing arithmetic.