1 “if” & “else”

Python has if, if-else, and if-elif-else similar to most languages. Suppose that we were to prompt a user for their favorite color.

def main():
    color = input("What is your favorite color? ")

    if color.lower() == "blue":
        print("Blue is the best color.")

    else:
        print(f"{color.title()} is a color.")


if __name__ == "__main__":
    main()

We could be a little more fair…

def main():
    color = input("What is your favorite color? ")
    color = color.lower()

    if color == "blue":
        print("Blue is the best color.")

    elif color == "purple" or color == "lavender":
        print(f"{color.title()} is a good choice.")

    else:
        print(f"{color.title()} is a color.")


if __name__ == "__main__":
    main()

Take note of the elif and the compound conditional. C++ and Java use…

• else if instead of elif

• && instead of and

• || instead of or

2 While You Wait…

Let us look at something a little more… objective. Consider the number guessing game.

Pick a number between 1 and 100.

Let us start with some quick pseudocode

def main():

    # while guess is not correct:
        # prompt "Is your number {guess}? (y/n): "

        # if answer is yes:
            # print "I win!"
            # exit

        # else answer is no:
            # prompt "Was my guess too high or too low? (h/l): "

            # if too high:
                # upper_limit = guess

            # else too low:
                # lower_limit = guess

            # next_guess = lower_limit + (upper_limit - lower_limit) // 2


if __name__ == "__main__":
    main()

Before we move on… I would like to move the guess logic to a separate function…

def get_next_guess(lower_limit, upper_limit) -> int:
    return lower_limit + (upper_limit - lower_limit) // 2

and get the next guess at the start of the loop…

def main():

    lower_limit = 1
    upper_limit = 100

    # while guess is not correct:
        guess = get_next_guess(lower_limit, upper_limit)

        # prompt "Is your number {guess}? (y/n): "

        # if answer is yes:
            # print "I win!"
            # exit

        # else answer is no:
            # prompt "Was my guess too high or too low? (h/l): "

            # if too high:
                # upper_limit = guess

            # else too low:
                # lower_limit = guess


if __name__ == "__main__":
    main()

The user prompts can be added fairly quickly (based on previous lectures).

def main():

    lower_limit = 1
    upper_limit = 100

    # while guess is not correct:
        guess = get_next_guess(lower_limit, upper_limit)

        answer = input("Is your number {guess}? (y/n):") 
        answer = answer.lower()[0]

        # if answer is yes:
            # print "I win!"
            # exit

        # else answer is no:
            answer = input("Was my guess too high or too low? (h/l)")
            answer = answer.lower()[0]

            # if too high:
                # upper_limit = guess

            # else too low:
                # lower_limit = guess


if __name__ == "__main__":
    main()

Take note of how the answers were processed…

        answer = answer.lower()[0]

We want the first letter from the answer (i.e., [0]). We would also like to accept an uppercase or lowercase letters. In this case… we converted the letter to lowercase with .lower().

We can use the same answer variable for both prompts.

3 Adding the Loop and Conditions

We will start with a while True: (and replace it later). The conditional blocks can be written alongside the loop condition.

import sys


def get_next_guess(lower_limit, upper_limit) -> int:
    return lower_limit + (upper_limit - lower_limit) // 2


def main():

    lower_limit = 1
    upper_limit = 100

    while True:
        guess = get_next_guess(lower_limit, upper_limit)

        answer = input(f"Is your number {guess}? (y/n): ")
        answer = answer.lower()[0]

        if answer == "y":
            print("I win!")
            sys.exit(0)

        elif answer == "n":
            answer = input("Was my guess too high or too low? (h/l): ")
            answer = answer.lower()[0]

            if answer == "h":
                upper_limit = guess

            elif answer == "l":
                lower_limit = guess


if __name__ == "__main__":
    main()

Most of the pseudocode (e.g., lower_limit = guess) was actually valid Python.

Do not use sys.exit. A call to sys.exit should only occur if an error occurs, and never from within a loop. Let us replace sys.exitwithbreak` to exit the loop.

def get_next_guess(lower_limit, upper_limit) -> int:
    return lower_limit + (upper_limit - lower_limit) // 2


def main():

    lower_limit = 1
    upper_limit = 100

    while True:
        guess = get_next_guess(lower_limit, upper_limit)

        answer = input(f"Is your number {guess}? (y/n): ")
        answer = answer.lower()[0]

        if answer == "y":
            print("I win!")
            break

        elif answer == "n":
            answer = input("Was my guess too high or too low? (h/l): ")
            answer = answer.lower()[0]

            if answer == "h":
                upper_limit = guess

            elif answer == "l":
                lower_limit = guess


if __name__ == "__main__":
    main()

We should get rid of the break… by…

1. initializing guess to n

2. updating the loop condition

3. removing break

def get_next_guess(lower_limit, upper_limit) -> int:
    return lower_limit + (upper_limit - lower_limit) // 2


def main():
    lower_limit = 1
    upper_limit = 100

    answer = "n"
    while answer != "y":
        guess = get_next_guess(lower_limit, upper_limit)

        answer = input(f"Is your number {guess}? (y/n): ")
        answer = answer.lower()[0]

        if answer == "y":
            print("I win!")

        elif answer == "n":
            answer = input("Was my guess too high or too low? (h/l): ")
            answer = answer.lower()[0]

            if answer == "h":
                upper_limit = guess

            elif answer == "l":
                lower_limit = guess


if __name__ == "__main__":
    main()

I am much happier with that.

4 We are Done… For Now