Accepting and responding to user input

What you'll learn

How Python 3 programs accept user input using input().
Why input is stored as text first, and when you need to convert it.
How to respond differently using if, elif and else.
How to make programs more robust by checking input before using it.

The basic idea: input, process, output

A program often follows an input → process → output pattern.

Input: data goes into the program.
Process: the program uses instructions to do something with that data.
Output: the program shows or returns a result.

Definition

User input

User input is data entered by the person using the program, usually by typing at the keyboard or choosing an option.

For this topic, you need to be able to write programs that accept user input and respond appropriately. “Appropriately” means the program should use the input sensibly: store it, convert it if needed, check it if needed, and produce a suitable output.

Key Idea

The core pattern

Most interactive programs do this: ask for input, store it in a variable, process it, then output a response.

Reading input in Python

In Python 3, the built-in function input() pauses the program and waits for the user to type something.

Definition

Function

A function is a named block of code that performs a particular task. Python’s input() function gets text from the user.

A prompt is the message shown to the user to tell them what to enter.

name = input("What is your name? ")
print(f"Hello, {name}!")

Here, the user’s answer is stored in the variable name.

Definition

Variable

A variable is a named storage location in memory that holds a value while a program is running.

Example

Making a personalised greeting

The program needs one piece of input: the user’s name, so a meaningful variable name is name.
The prompt should make the expected input clear: input("What is your name? ").
The response should use the stored input, so an f-string can insert the value of name into the output.

name = input("What is your name? ")
print(f"Hello, {name}!")

Tip

Prompt clearly

A good prompt tells the user exactly what to type, for example Enter your age: is better than just Age.

Input is text first

A very important Python rule is that input() always returns a string.

Definition

String

A string is a sequence of characters, such as "Maya", "17" or "Y".

Even if the user types 17, Python receives it as the string "17", not the integer 17. If you want to do arithmetic or numerical comparisons, you usually need to convert it.

Definition

Type conversion

Type conversion, also called casting, means changing data from one data type to another, such as from a string to an integer.

Common conversions include:

Purpose	Python example
Convert to integer	`age = int(age_text)`
Convert to real number	`height = float(height_text)`
Convert to string	`message = str(score)`

age_text = input("Enter your age: ")
age = int(age_text)

The variable age_text stores the original string. The variable age stores the converted integer.

Example

Calculating with typed-in numbers

The user types the number of tickets, but input() stores it as text, so the program first receives something like "3".
The program converts the text to an integer using int() so it can be used in multiplication.
The program calculates the total price and outputs a response using the result.

tickets_text = input("How many tickets do you want? ")
tickets = int(tickets_text)

total_price = tickets * 5

print(f"That will cost £{total_price}.")

Common Mistake

Forgetting to convert numeric input

If you write tickets = input("How many tickets? "), then tickets is a string. You cannot reliably use it as a number until you convert it with int() or float().

Responding with selection

Programs often need to respond differently depending on what the user enters. This uses selection.

Definition

Selection

Selection is when a program chooses which block of code to run based on a condition. In Python, selection is written using if, elif and else.

A condition is an expression that is either true or false, such as age >= 13.

This flow shows the common pattern: input is accepted, converted if necessary, then used in a decision.

Flow diagram showing a Python program accepting an age, converting it to an integer, then choosing an output based on whether the age is at least 13

In Python:

age = int(input("Enter your age: "))

if age >= 13:
    print("You can create an account.")
else:
    print("Ask an adult for help.")

Example

Choosing a message from an age

The input must be treated as a number because the program compares it with 13, so int() is needed.
The condition age >= 13 separates the two possible cases: age 13 or above, and age below 13.
The if branch handles the true case, while the else branch handles every remaining case.

age = int(input("Enter your age: "))

if age >= 13:
    print("You can create an account.")
else:
    print("Ask an adult for help.")

Using more than two possible responses

Sometimes two branches are not enough. Python uses elif, meaning “else if”, to test another condition.

mark = int(input("Enter the test mark: "))

if mark >= 80:
    print("Excellent")
elif mark >= 50:
    print("Pass")
else:
    print("Try again next time")

The order matters. Python checks from top to bottom and stops when it finds the first true condition.

Example

Tracing multiple responses

Suppose the user enters 72.

The program converts "72" into the integer 72, so numerical comparisons can be made.
It tests mark >= 80; 72 is not at least 80, so that branch is skipped.
It tests mark >= 50; 72 is at least 50, so the program outputs Pass and does not run the else.

Common Mistake

Putting conditions in the wrong order

If you test mark >= 50 before mark >= 80, then a mark of 85 would match the first condition and never reach the “Excellent” branch.

Responding to menu choices

Not all input has to be numeric. Many programs ask the user to choose from a menu.

choice = input("Choose A to add or S to subtract: ")

if choice == "A":
    print("You chose addition.")
elif choice == "S":
    print("You chose subtraction.")
else:
    print("That was not a valid choice.")

For string comparisons, exact characters matter. "A" and "a" are different strings.

Tip

Make case easier to handle

You can convert input to upper case with .upper(), so "a" and "A" can be treated the same way.

choice = input("Choose A to add or S to subtract: ").upper()

Checking input before using it

A program should often check whether input is acceptable before processing it. This is called validation.

Definition

Validation

Validation checks whether data is sensible or allowed. It does not prove the data is true; it only checks whether it follows the rules set by the program.

Examples of validation include:

Presence check: making sure something has been entered.
Length check: making sure input is the correct length.
Range check: making sure a number is between allowed limits.
Pattern check: making sure input follows a required format.

A program can respond to invalid input by showing an error message, asking again, or stopping safely.

Definition

Iteration

Iteration means repeating a set of instructions. In Python, a while loop can repeat until the input is valid.

Example

Repeating until a valid menu choice

The valid inputs are only "S" and "P", so any other value should be rejected.
The loop condition must stay true while the choice is not "S" and not "P".
Once the loop ends, the program can safely respond to the valid choice.

choice = input("Choose S for sandwich or P for pasta: ").upper()

while choice != "S" and choice != "P":
    print("Please enter S or P.")
    choice = input("Choose S for sandwich or P for pasta: ").upper()

if choice == "S":
    print("You chose sandwich.")
else:
    print("You chose pasta.")

Common Mistake

Conversion can fail

If the user types hello and your program tries int("hello"), Python will produce an error. In GCSE questions, you may sometimes be told to assume sensible input, but if validation is required you should handle invalid input.

Good responses are specific

A program is responding appropriately when its output matches the input and the purpose of the program.

Weak response:

print("Error")

Better response:

print("Please enter a number from 1 to 10.")

A useful response tells the user what happened and, if needed, what to do next.

Key Idea

Appropriate response

A good interactive program does not just accept input. It uses selection, iteration and clear output to respond in a way that helps the user complete the task.

Mini checklist for writing input code

When you write a program that accepts input, ask yourself:

What data does the user need to enter?
What data type should it become: string, integer, real number or Boolean-style choice?
Does it need validation?
Which output should be shown for each possible case?
Are the variable names meaningful?

Exam technique

In the exam

Identify whether the input should stay as a string or be converted using int() or float() before processing.
Use if, elif and else when the program must respond differently to different inputs.
If the question asks for validation, repeat the input using a loop and give a clear message when the input is invalid.

Self review

Check yourself

Why does input() need int() before you can do arithmetic with a typed-in whole number?
What is the difference between validation and proving that data is correct?
How could a program respond differently to the inputs "Y" and "N"?

Accepting and responding to user input

What you'll learn

How Python 3 programs accept user input using input().
Why input is stored as text first, and when you need to convert it.
How to respond differently using if, elif and else.
How to make programs more robust by checking input before using it.

The basic idea: input, process, output

A program often follows an input → process → output pattern.

Input: data goes into the program.
Process: the program uses instructions to do something with that data.
Output: the program shows or returns a result.

Definition

User input

User input is data entered by the person using the program, usually by typing at the keyboard or choosing an option.

Key Idea

The core pattern

Most interactive programs do this: ask for input, store it in a variable, process it, then output a response.

Reading input in Python

In Python 3, the built-in function input() pauses the program and waits for the user to type something.

Definition

Function

A function is a named block of code that performs a particular task. Python’s input() function gets text from the user.

A prompt is the message shown to the user to tell them what to enter.

name = input("What is your name? ")
print(f"Hello, {name}!")

Here, the user’s answer is stored in the variable name.

Definition

Variable

A variable is a named storage location in memory that holds a value while a program is running.

Example

Making a personalised greeting

The program needs one piece of input: the user’s name, so a meaningful variable name is name.
The prompt should make the expected input clear: input("What is your name? ").
The response should use the stored input, so an f-string can insert the value of name into the output.

name = input("What is your name? ")
print(f"Hello, {name}!")

Tip

Prompt clearly

A good prompt tells the user exactly what to type, for example Enter your age: is better than just Age.

Input is text first

A very important Python rule is that input() always returns a string.

Definition

String

A string is a sequence of characters, such as "Maya", "17" or "Y".

Even if the user types 17, Python receives it as the string "17", not the integer 17. If you want to do arithmetic or numerical comparisons, you usually need to convert it.

Definition

Type conversion

Type conversion, also called casting, means changing data from one data type to another, such as from a string to an integer.

Common conversions include:

Purpose	Python example
Convert to integer	`age = int(age_text)`
Convert to real number	`height = float(height_text)`
Convert to string	`message = str(score)`

age_text = input("Enter your age: ")
age = int(age_text)

The variable age_text stores the original string. The variable age stores the converted integer.

Example

Calculating with typed-in numbers

The user types the number of tickets, but input() stores it as text, so the program first receives something like "3".
The program converts the text to an integer using int() so it can be used in multiplication.
The program calculates the total price and outputs a response using the result.

tickets_text = input("How many tickets do you want? ")
tickets = int(tickets_text)

total_price = tickets * 5

print(f"That will cost £{total_price}.")

Common Mistake

Forgetting to convert numeric input

If you write tickets = input("How many tickets? "), then tickets is a string. You cannot reliably use it as a number until you convert it with int() or float().

Responding with selection

Programs often need to respond differently depending on what the user enters. This uses selection.

Definition

Selection

Selection is when a program chooses which block of code to run based on a condition. In Python, selection is written using if, elif and else.

A condition is an expression that is either true or false, such as age >= 13.

This flow shows the common pattern: input is accepted, converted if necessary, then used in a decision.

Flow diagram showing a Python program accepting an age, converting it to an integer, then choosing an output based on whether the age is at least 13

In Python:

age = int(input("Enter your age: "))

if age >= 13:
    print("You can create an account.")
else:
    print("Ask an adult for help.")

Example

Choosing a message from an age

The input must be treated as a number because the program compares it with 13, so int() is needed.
The condition age >= 13 separates the two possible cases: age 13 or above, and age below 13.
The if branch handles the true case, while the else branch handles every remaining case.

age = int(input("Enter your age: "))

if age >= 13:
    print("You can create an account.")
else:
    print("Ask an adult for help.")

Using more than two possible responses

Sometimes two branches are not enough. Python uses elif, meaning “else if”, to test another condition.

mark = int(input("Enter the test mark: "))

if mark >= 80:
    print("Excellent")
elif mark >= 50:
    print("Pass")
else:
    print("Try again next time")

The order matters. Python checks from top to bottom and stops when it finds the first true condition.

Example

Tracing multiple responses

Suppose the user enters 72.

The program converts "72" into the integer 72, so numerical comparisons can be made.
It tests mark >= 80; 72 is not at least 80, so that branch is skipped.
It tests mark >= 50; 72 is at least 50, so the program outputs Pass and does not run the else.

Common Mistake

Putting conditions in the wrong order

If you test mark >= 50 before mark >= 80, then a mark of 85 would match the first condition and never reach the “Excellent” branch.

Responding to menu choices

Not all input has to be numeric. Many programs ask the user to choose from a menu.

choice = input("Choose A to add or S to subtract: ")

if choice == "A":
    print("You chose addition.")
elif choice == "S":
    print("You chose subtraction.")
else:
    print("That was not a valid choice.")

For string comparisons, exact characters matter. "A" and "a" are different strings.

Tip

Make case easier to handle

You can convert input to upper case with .upper(), so "a" and "A" can be treated the same way.

choice = input("Choose A to add or S to subtract: ").upper()

Checking input before using it

A program should often check whether input is acceptable before processing it. This is called validation.

Definition

Validation

Validation checks whether data is sensible or allowed. It does not prove the data is true; it only checks whether it follows the rules set by the program.

Examples of validation include:

Presence check: making sure something has been entered.
Length check: making sure input is the correct length.
Range check: making sure a number is between allowed limits.
Pattern check: making sure input follows a required format.

A program can respond to invalid input by showing an error message, asking again, or stopping safely.

Definition

Iteration

Iteration means repeating a set of instructions. In Python, a while loop can repeat until the input is valid.

Example

Repeating until a valid menu choice

The valid inputs are only "S" and "P", so any other value should be rejected.
The loop condition must stay true while the choice is not "S" and not "P".
Once the loop ends, the program can safely respond to the valid choice.

choice = input("Choose S for sandwich or P for pasta: ").upper()

while choice != "S" and choice != "P":
    print("Please enter S or P.")
    choice = input("Choose S for sandwich or P for pasta: ").upper()

if choice == "S":
    print("You chose sandwich.")
else:
    print("You chose pasta.")

Common Mistake

Conversion can fail

Good responses are specific

A program is responding appropriately when its output matches the input and the purpose of the program.

Weak response:

print("Error")

Better response:

print("Please enter a number from 1 to 10.")

A useful response tells the user what happened and, if needed, what to do next.

Key Idea

Appropriate response

A good interactive program does not just accept input. It uses selection, iteration and clear output to respond in a way that helps the user complete the task.

Mini checklist for writing input code

When you write a program that accepts input, ask yourself:

What data does the user need to enter?
What data type should it become: string, integer, real number or Boolean-style choice?
Does it need validation?
Which output should be shown for each possible case?
Are the variable names meaningful?

Exam technique

In the exam

Identify whether the input should stay as a string or be converted using int() or float() before processing.
Use if, elif and else when the program must respond differently to different inputs.
If the question asks for validation, repeat the input using a loop and give a clear message when the input is invalid.

Self review

Check yourself

Why does input() need int() before you can do arithmetic with a typed-in whole number?
What is the difference between validation and proving that data is correct?
How could a program respond differently to the inputs "Y" and "N"?

Accepting and responding to user input

What you'll learn

The basic idea: input, process, output

Reading input in Python

Input is text first

Responding with selection

Using more than two possible responses

Responding to menu choices

Checking input before using it

Good responses are specific

Mini checklist for writing input code

You've reached the end

Accepting and responding to user input

What you'll learn

The basic idea: input, process, output

Reading input in Python

Input is text first

Responding with selection

Using more than two possible responses

Responding to menu choices

Checking input before using it

Good responses are specific

Mini checklist for writing input code

You've reached the end

Accepting and responding to user input

What you'll learn

The basic idea: input, process, output

Reading input in Python

Input is text first

Responding with selection

Using more than two possible responses

Responding to menu choices

Checking input before using it

Good responses are specific

Mini checklist for writing input code

You've reached the end

Accepting and responding to user input Revision Guide

Accepting and responding to user input

What you'll learn

The basic idea: input, process, output

Reading input in Python

Input is text first

Responding with selection

Using more than two possible responses

Responding to menu choices

Checking input before using it

Good responses are specific

Mini checklist for writing input code

You've reached the end

Accepting and responding to user input Revision Guide