Handling User Input

State changes need a trigger. In a real program, that trigger is often a user typing something. We connect input() to the controlled methods from Session 21.

1. Learning Objective

2. Pre-Coding Quiz

Answer these before reading the concept explanation or writing any code. It is fine to get these wrong — that is the point. You need 4/5 to proceed to the lab.

3. The Concept — Reading and Validating User Input

input() always returns a string

The built-in input() function pauses the program, waits for the user to type a line and press Enter, and returns exactly what they typed — always as a string, never automatically converted.

name = input("Enter a country name: ")
print(type(name))  # <class 'str'> — always, even if they typed "54000000"

Converting and validating before using the value

A numeric field like population needs explicit conversion, and that conversion can fail on bad input — exactly the kind of case Session 11's try/except exists for.

raw = input("Enter population: ")

try:
    population = int(raw)
except ValueError:
    print(f"'{raw}' is not a valid number.")
    population = None

if population is not None:
    print("Parsed population:", population)

Connecting validated input to a controlled method

Once input is converted and confirmed valid, it can safely flow into the Session 21 methods, which add their own domain-specific validation (like rejecting negative numbers) on top.

def prompt_population(country):
    raw = input(f"New population for {country.name}: ")
    try:
        value = int(raw)
    except ValueError:
        print(f"'{raw}' is not a valid number. No change made.")
        return
    try:
        country.set_population(value)
        print("Updated.")
    except ValueError as e:
        print("Rejected:", e)

A simple menu loop

Combining a loop, input(), and conditional branching gives us an interactive text menu — a small but real interactive program.

def run_menu(country):
    while True:
        choice = input("(g)row population, (s)how summary, (q)uit: ").strip().lower()
        if choice == "q":
            print("Goodbye.")
            break
        elif choice == "g":
            prompt_population(country)
        elif choice == "s":
            print(country.summary())
        else:
            print("Unrecognised option, try again.")

4. Lab

What you will build

A file called input_lab.py.

Step-by-step instructions

# input_lab.py
class Country:
    def __init__(self, name, region, population):
        self.name = name
        self.region = region
        self.population = population

    def summary(self):
        return f"{self.name} ({self.region}): pop. {self.population:,}"

    def set_population(self, value):
        if value < 0:
            raise ValueError(f"population must be non-negative, got {value}")
        self.population = value

    def grow_population(self, amount):
        if amount < 0:
            raise ValueError(f"amount must be non-negative, got {amount}")
        self.population += amount

def read_int(prompt):
    raw = input(prompt)
    try:
        return int(raw)
    except ValueError:
        print(f"'{raw}' is not a valid number.")
        return None

def prompt_set_population(country):
    value = read_int(f"New population for {country.name}: ")
    if value is None:
        return
    try:
        country.set_population(value)
        print("Updated:", country.summary())
    except ValueError as e:
        print("Rejected:", e)

def run_menu(country):
    while True:
        choice = input("(g)row, (s)et, (v)iew, (q)uit: ").strip().lower()
        if choice == "q":
            print("Goodbye.")
            break
        elif choice == "s":
            prompt_set_population(country)
        elif choice == "g":
            amount = read_int("Amount to grow by: ")
            if amount is not None:
                try:
                    country.grow_population(amount)
                    print("Updated:", country.summary())
                except ValueError as e:
                    print("Rejected:", e)
        elif choice == "v":
            print(country.summary())
        else:
            print("Unrecognised option.")

if __name__ == "__main__":
    kenya = Country(name="Kenya", region="Africa", population=54000000)
    run_menu(kenya)

5. Expected Files Changed

6. Commit Checkpoint

Once the lab is complete and you can explain every line, make this exact commit:

git add input_lab.py docs/sessions/session-22/index.html
git commit -m "session-22: read and validate user input, connect it to controlled state methods"

7. Code Review Checklist

Go through your code line by line and check each item:

8. Post-Coding Quiz

Same 5 questions. Take it again now that you have written and run the code. You need 4/5 to mark this session complete.

9. Reflection Questions

Think through these after the post-quiz. No right answer — they are for discussion.

10. What Breaks If This Knowledge Is Missing?

11. What We Learned