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If, else and elif

Using if statements

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 executed and the prompt went back to >>>.

An important thing to notice is that the line with a print is indented. You can press the tab key, or if it doesn't work just press space a few times.

But why is that if its_raining instead of if(its_raining)?

Earlier we learned that if is a keyword.

>>> if = 123
  File "<stdin>", line 1
    if = 123
       ^
SyntaxError: invalid syntax
>>>

Functions like print need () after their name to work. But if is a keyword, not a function, so it doesn't need (). Python has separate functions and keywords because it's possible to create custom functions, but it's not possible to create custom keywords. That's why keywords are usually used for "magic" things that would be difficult to do with just functions.

Also note that if statements check the condition once only, so if we set it to false later the if statement won't notice it.

>>> its_raining = True
>>> if its_raining:
...     its_raining = False
...     print("It's not raining, but this runs anyway.")
...
It's not raining, but this runs anyway.
>>>

Using else

What if we want to print a different message if it's not raining? We 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.")

Note that this code example doesn't start with >>>, so you should save it to a file and run the file.

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

We 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 we can make it even better by using else.

its_raining = True

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

The else part simply runs when the if statement doesn't run. It doesn't check the condition again.

>>> its_raining = True
>>> if its_raining:
...     its_raining = False
... else:
...     print("It's not raining, but this still doesn't run.")
...
>>>

By combining else 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 we enter:

Hello!
Enter your password: secret
Welcome!

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.

Avoiding many levels of indentation with elif

If we have more than one condition to check, we could do this:

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.")

This code is a mess. We need to indent more every time we want to check for more words. Here we check for 5 different words, so we have 5 levels of indentation. If we would need to check 30 words, the code would become really wide and it would be hard to work with.

Instead of typing else, indenting more and typing an if we 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.")

Now the program is shorter and much easier to read.

Note that the elif parts only run if nothing before them matches, and the else runs only when none of the elifs match. If we would have used if instead, all possible values would be always checked and the else part would run always except when word is "gday m8". This is why we use elif instead of if.

For example, this program prints only hello...

if 1 == 1:
    print("hello")
elif 1 == 2:
    print("this is weird")
else:
    print("world")

...but this prints hello and world:

if 1 == 1:
    print("hello")
if 1 == 2:
    print("this is weird")
else:
    print("world")

Now the else belongs to the if 1 == 2 part and it has nothing to do with the if 1 == 1 part. On the other hand, the elif version grouped the multiple ifs together and the else belonged to all of them. Adding a blank line makes this obvious:

if 1 == 1:
    print("hello")

if 1 == 2:
    print("this is weird")
else:
    print("world")

In general, adding blank lines to appropriate places is a good idea. If you are asked to "fix code", feel free to add missing blank lines.

Summary

  • If a code example starts with >>> run it on the interactive prompt. If it doesn't, write it to a file and run that file.
  • Indentation is important in Python.
  • Indented code under an if statement runs if the condition is true.
  • We can also add an else statement. Indented code under it will run if the code under the if statement does not run.
  • elif is short for else if.

Exercises

  1. This program contains several problems. Copy-paste it to a file, then try to run it, fix the errors you got, try to run it again and keep going until it works.

    print(Hello!)
something == input('Enter something: )
print('You entered:' something)

  2. Fix this program the same way:

    print('Hello!')
something = input("Enter something: ")
if something = 'hello':
    print("Hello for you too!")

elif something = 'hi'
    print('Hi there!')
else:
    print("I don't know what," something, "means.")

  3. Write a program into a file that asks the user to write a word and then prints that word 1000 times. For example, if the user enters hi the program would reply hihihihihihihihi ...

  4. Add spaces between the words, so the output is like hi hi hi hi ...

  5. Make something that asks the user to enter two words, and prints 1000 of each with spaces in between. For example, if the user enters hello and hi the program would print hello hi hello hi hello hi hello hi hello hi ...

  6. 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.

The answers are here.

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