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Input

Note: If you’re using Python 2.7, replace all input() functions in the code below with raw_input(). You can check your version by running python --version in the command line.

Python allows you to take input directly from the user using the input function. Let’s use it to improve our weather application by asking for the weather before displaying the output.

weather = input("What is the weather like today? ")

if weather == "sunny":
    print("Bring your shades")
elif weather == "rainy":
    print("Bring your umbrella")
elif weather == "snowy":
    print("Bring your wooly muffler")
else:
    print("I don't know what you should bring! I'm just a little program...")

When you run this program, Python should ask you for some input with the prompt What is the weather like today? (The space before the second " makes the prompt look more tidy in the console.) It will then return some advice based on the input. Try running it now.

Asking repeatedly

What if we want Python to keep asking for input instead of exiting after the first question is answered? For that, we can use something called a while loop.

Remember our for loop? Instead of iterating through a list like the for loop, our while loop will continue to execute as long as a certain condition is true. Here’s a very simple while loop that will run forever until you quit it manually.

while True:
	print("Oh no! I'm stuck...")

In the terminal, you can escape from this endless loop by pressing Control-c on your keyboard.

Let’s apply the while loop to our weather app:

while True:
    weather = input("What is the weather like today? ")

    if weather == "sunny":
        print("Bring your shades")
    elif weather == "rainy":
        print("Bring your umbrella")
    elif weather == "snowy":
        print("Bring your wooly muffler")
    else:
        print("I don't know what you should bring! I'm just a little program...")

Notice that we had to shift everything over one tab to fit it in the while block. Now our program will ask us for input again and again, and give us different answers each time.

Let’s add one more feature: an elif statement that will break us out of the loop and end the program:

while True:
    weather = input("What is the weather like today? ")

    if weather == "sunny":
        print("Bring your shades")
    elif weather == "quit":
        break
    elif weather == "rainy":
        print("Bring your umbrella")
    elif weather == "snowy":
        print("Bring your wooly muffler")
    else:
        print("I don't know what you should bring! I'm just a little program...")

The break command ends the current loop early, ending the program when “quit” is given as input.

Challenge

1. How much of the code above do you understand? Even if you do kind of understand it, do you “grok” it—that is, really understand it?

Open up your REPL (type python at the $ prompt). Play around with input() a bit until you understand it’s behavior really well. Write a two-line program in the REPL or in a script that takes some input and echoes it back to the user.

Alternatively, mess around with while. Try using things other than True and see if the code in the loop runs. If you can, write a while loop that prints out the numbers from 1 to 10 and stops.

2. (optional) Read a little about the weird word grok.

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