Now we know what True and False are.

>>> 1 == 1
True
>>> 1 == 2
False
>>>
>>> its_raining = True
>>> its_raining
True
>>>

But what if we want to execute different code depending on something? That's when if comes in.

>>> its_raining = True
>>> if its_raining:
...     print("It's raining!")
... 
It's raining!
>>> its_raining = False
>>> if its_raining:
...     print("It's raining!")        # nothing happens
... 
>>> 

The prompt changed from >>> to .... It meant that Python was expecting me to keep typing. When I was done, I just pressed Enter twice. My code was execute and the prompt was restored to >>>. IDLE does this a bit differently, so if you use IDLE, running this example code looks more like this:

>>> its_raining = True
>>> if its_raining:
        print("It's raining!")

        
It's raining!
>>> 

At this point it's easier to put your code into a file and use it there. If you use IDLE, go to File at top left and select New File, or just press Ctrl+N.

If you don't use IDLE, please take the time to set up your editor correctly.

Create a file called rain.py, and type the following content into it:

its_raining = True
if its_raining:
    print("It's raining!")

The line with a print is indented by four spaces. Indentation is important in Python. If the indentation is not consistent, we may get an error or our program may do something else than we wanted it to.

Now run the rain program. Most editors (including IDLE) will run your code when you press F5. If your editor doesn't, run it from PowerShell, command prompt or terminal. You probably need to first go to wherever you saved your file. with cd. For example, if the file is on your desktop, type cd Desktop before running the file.

Running from IDLE looks like this:

>>> 
========================= RESTART: /home/you/rain.py =========================
It's raining!
>>> 

And running from the Windows PowerShell or command prompt looks like this:

C:\Users\You> cd Desktop
C:\Users\You\Desktop> py rain.py
It's raining!
C:\Users\You\Desktop> 

Running from a terminal looks like this:

you@YourComputer:~$ cd Desktop
you@YourComputer:~/Desktop$ python3 rain.py
It's raining!
you@YourComputer:~/Desktop$ 

What if you want to print a different message if it's not raining? You could do something like this:

its_raining = True                  # you can change this to False
its_not_raining = not its_raining   # False if its_raining, True otherwise

if its_raining:
    print("It's raining!")
if its_not_raining:
    print("It's not raining.")

Now your program will print a different value depending on what the value of its_raining is.

You can also add not its_raining directly to the second if statement:

its_raining = True

if its_raining:
    print("It's raining!")
if not its_raining:
    print("It's not raining.")

But you can make it even better by using else.

its_raining = True

if its_raining:
    print("It's raining!")
else:
    print("It's not raining.")

By combining that with the input function we can make a program that asks for a password and checks if it's correct.

print("Hello!")
password = input("Enter your password: ")

if password == "secret":
    print("That's correct, welcome!")
else:
    print("Access denied.")

The program prints different things depending on what you enter.

>>> ================================ RESTART ================================
>>>
Hello!
Enter your password: secret
Welcome!
>>> ================================ RESTART ================================
>>>
Hello!
Enter your password: lol
Access denied.
>>>

Using the input function for passwords doesn't work very well because we can't hide the password with asterisks. There are better ways to get a password from the user, but you shouldn't worry about that just yet.

If you have more than one condition to check, your code will end up looking a bit messy.

print("Hello!")
word = input("Enter something: ")

if word == "hi":
    print("Hi to you too!")
else:
    if word == "hello":
        print("Hello hello!")
    else:
        if word == "howdy":
            print("Howdyyyy!")
        else:
            if word == "hey":
                print("Hey hey hey!")
            else:
                if word == "gday m8":
                    print("Gday 4 u 2!")
                else:
                    print("I don't know what", word, "means.")

Instead of typing else, indenting more and typing an if you can simply type elif, which is short for else if. Like this:

print("Hello!")
word = input("Enter something: ")

if word == "hi":
    print("Hi to you too!")
elif word == "hello":
    print("Hello hello!")
elif word == "howdy":
    print("Howdyyyy!")
elif word == "hey":
    print("Hey hey hey!")
elif word == "gday m8":
    print("Gday 4 u 2!")
else:
    print("I don't know what", word, "means.")

While loops are similar to if statements. The only difference is that at the end of the indented block they jump back to the line with the word while.

its_raining = True
while its_raining:
    print("It's raining!")
    # we'll jump back to the second line from here

When you run the program, it keeps printing It's raining! forever:

It's raining!
It's raining!
It's raining!
It's raining!
It's raining!
It's raining!
It's raining!
(many more lines of raining)

This program will not destroy or crash your computer. It just repeats the same thing quickly, but not quickly enough to damage anything. You can interrupt the program by hitting Ctrl+C.

The indented while block ran, just like an if block. Then, at the end of the block we moved back to the beginning of the while loop. its_raining was still True, so the indented block ran again, and so on.

While loops are often used for repeating things and endless number of times with a while True. These are called infinite loops. You can actually stop an infinite loop, just add a break into it. If you have two loops inside each other, break will break the outermost loop.

while True:
    answer = input("Is it still raining? (y=yes/n=no/q=quit) ")
    if answer == 'y':
        print("It's raining!")
    elif answer == 'n':
        print("It's not raining.")
    elif answer == 'q':
        break
    else:
        print("Please enter 'y', 'n' or 'q'.")

• Indentation is important in Python.
• Indented code under an if statement runs if the condition is true.
• While works just like if, but it jumps back to the line with the word while when it gets to the end of the indented block.
• while True: runs the loop until you interrupt it, for example with Ctrl+C or a break.

1. Make a program that asks for a password and prints Welcome!, Access denied or You didn't enter anything depending on whether the user entered the correct password, a wrong password, or nothing at all by pressing Enter without typing anything.
2. Make a program that asks for username and password and checks them. Make users "foo" and "bar" with passwords "biz" and "baz".
3. Make a program that asks for a username and a password and gives the user an infinite number of attempts, so the user can always try again if he mistypes the password.
4. Can you limit the number of attempts to 3?

The answers are here.

You may use this tutorial freely at your own risk. See LICENSE.

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