How To: If and Else


If statements

So far, we haven't been able to do anything conditionally; we cannot tell Python "do this in some cases and do this other thing" in others. But this is the sort of logic we use everyday when making simple decisions.

Programming languages like Python have tools to mimic those real-world choices.

Many of the decisions we make throughout the day – "it's cold, so put on a coat"; "you're running late, so text your friend"; "you're hungry, so eat something" – have an if-then structure. Just like you, Python can make if-then decisions.

If statements

If statements are how you direct Python to do something if something else is true. For example – run the code below. What do you think it'll do?

Seems pretty reasonable, right?

What about this code?

Nothing prints. That's because those two lines of code, converted to English, say "if False, print 'Goodbye'" – but since the statement is false, nothing happens.

Parts of an if statement

An if statement has two parts: the condition and the action(s). Here's what if statements look like generally:

if some_condition_thats_True_or_False:
    action

The condition – the part after the if and before the semicolon – should be a True or False statement. Python can cope with things that don't come in True/False format, though – perhaps somewhat confusingly – it'll only print when the condition's answer isn't 0:

If you try to use a string for a condition, Python'll assume it's True – unless the string is empty:

For the sake of your sanity, as well as those who read your code: keep the things in your if statements to those that become either True or False.

Else

If statements only allow you one course of action; what happens, you might wonder, if you've got more than one instruction to give? Something like:

if its_cold_outside:
    put_on_a_coat
else:
    put_on_sandles

Or in terms that we can run in a Trinket:

if 5 > 0:
    print "Hello there!"
else:
    print "Goodbye"

What's the code above print when run? ("Hello there!", right?) What change can you make to get it to print "Goodbye"?

Generally, these look like

if some_condition_thats_True_or_False:
    action
else:
    another_action

The else gets printed if nothing else happens above it. You can think of it as a sort of last-resort escape hatch.

One last example, this time using a variable container that holds a boolean:

Elif

Sometimes you might want to put in a few conditions. Imagine the somewhat real world example of deciding how to dress for the weather. Let's say your thinking goes like this:

  • if it's raining and less than 60 degrees Fahrenheit, you should wear a raincoat
  • if it's raining and more than 60 degrees Fahrenheit, you should bring an umbrella
  • if it's not raining and is less than 60 degrees Fahrenheit, you should wear sandles
  • if it's not raining and more than 60 degrees Fahrenheit, you should wear shoes

We could convert those instructions to Python with some code that looks like this:

if temperature < 60 and is_raining:
    wear_a_raincoat
elif temperature > 60 and is_raining:
    bring_an_umbrella
elif temperature > 60:
    wear_shoes
else:
    wear_sandles

Order with conditions

Notice that, in the pseudocode above, we didn't need write is_raining for the third and fourth conditions. That's because Python goes through each if lines in order, stopping as soon as it gets to something that's True and then skipping all the rest.

For a more concrete depiction, take a look at the below:

Nested if statements

One neat thing Python lets you do: put if statements inside of other if statements. It can help clean up your code:

num = 8
if num > 6:
    if num > 10:
        print "The number is greater than 10"
    else:
        print "The number is greater than 6 but not greater than 10"
else:
    print "The number is less than or equal to 6."

The output would be:

The number is greater than 6 but not greater than 10

You could write that same logic without nested if statements, though it'd be a bit messier:

num = 8
if num > 6 and num > 10:
    print "The number is greater than 10"
elif num > 6:
    print "The number is greater than 6 but not greater than 10"
else:
    print "The number is less than or equal to 6."