Conditionals in Python

In the last article ( "Lists and Loops in Python" ), we created a programme that outputted our name, age, profession, favoriteAnimal and a list of our hobbies.

In this article we're going to extend our programme to act differently depending on the age variable. Before we get started, let's remove some code we won't be needing in this article:

#!/bin/env python

# Start of our code

name = "Jordan"
# age = 31

hobbies = ["hiking", "playing music", "example1", "example2"]

print("Hello World. My name is {0}.".format(name))

print("I have {0} hobbies, and they are: ".format(len(hobbies)))

i = 0
while i < len(hobbies):
    print(hobbies[i])
    i+=1

# End of our code

Note: We've commented out the age parameter. While not a hard rule of Python, it is best practice that you not declare any unused variables. If we had declared it ( age = 29 ) but not referenced it - it is considered unused, but you will not see an error when running the programme like you would with a more strict language; rather, the interpreter will just clean up the memory at the end of the programme run.

Running the application now, you should see the following output:

$ python first.py
Hello World. My name is Jordan.
I have 4 hobbies, and they are: 
hiking
playing music
example1
example2

We're now going to bring back the age variable and, depending on its value, we'll print one of the following:

  • If age is less than 10, we will print "I am x years young".

  • If age is more than 10, we will print "I am x years old".

To do this we're going to use an if statement. The syntax for declaring an if statement in Python is simple:

if expression:
    // Run this code if expression evaluates to true

To perform an action dependant on age, we can simply add two if statements to our code ( Note: we have omitted all of the code not related to age, at this stage, to focus on what we're discussing )

#!/bin/env python

# Start of our code

age = 31

if age < 10:
    print("I am {0} years young".format(age))

if age > 10:
    print("I am {0} years old".format(age))

# End of our code

We are now performing two checks:

  • Is age less than ( < ) 10? If it is, we print out the first message.

  • Is age greater than ( > ) 10? If it is, we print out the second message.

Perfect! Actually, unfortunately not. There are two things wrong with this code snippet.

Problem 1

We're checking if age is less than ( < ) and greater than ( > ) 10, but what if age is 10? It doesn't call either Print statements.

python conditionals.py

To fix this we first need to decide what statement we should print if the persons age is 10. I think 10 is still pretty young so let's print out the first statement. If you disagree, see if you can make it print out the second statement.

We then need to change our "less than" ( < ) condition, to be "less than or equal to" ( <= ).

#!/bin/env python

# Start of our code

age = 31

if age <= 10:
    print("I am {0} years young".format(age))

if age > 10:
    print("I am {0} years old".format(age))

# End of our code

Now if we run our programme with an age of 10, we get our message printed!

$ python conditionals.py
I am 10 years young

Problem 2

Regardless of what value is given to age, we're always checking two things:

  • Is the value less than or equal to 10?

  • Is the value greater than 10?

In this situation age can only match one of these expressions, so we're asking the computer to do extra work. This might not seem like a big deal now, but if we grew our application to have thousands or millions of unnecessary checks we'd see a big performance hit.

Instead of two if statements, we can use an if/else statement. The syntax for declaring an if/else statement is very simple:

if expression:
    # Run this code if expression evaluates to true
else:
    # Run this code if expression evaluates to false

The age variable is only ever going to be less than or equal to 10, or greater than 10, as we've previously stated. So we only need to check for one of those conditions, everything else fits in the else statement:

#!/bin/env python

# Start of our code

age = 31

if age <= 10:
    print("I am {0} years young".format(age))
else:
    print("I am {0} years old".format(age))

# End of our code

Take some time to have a play with the code - we'll still be here when you're ready to continue.

Ready?

Excellent. Let's take this to the next level and make our programme more "age aware". This time we're going to check for four possible outcomes:

  • If the age variable is less than 13, we will print "I am considered a child"

  • If the age variable is between 13 and 19, we will print "I am considered a teenager"

  • If the age variable is between 20 and 67, we will print 'I am considered an adult'.

  • If the age variable is 68 or older, we will print "I am considered a senior".

We already know having four separate if statements isn't a good idea, but an if/else statement can only handle two outcomes so what are we to do now? We can use the if/elif/else statement!

The syntax for declaring an if/elif/else statement is very simple:

if expression:
    # Run this code if expression evaluates to true
elif anotherExpression:
    # Run this code if anotherExpression evaluates to true
elif yetAnotherExpression:
    # Run this code if yetAnotherExpression evaluates to true
else:
    # Run this code if none of the above expressions have evaluated to true

The difference between else and elif is the latter allows us to specify another expression.

If the first expression hasn't evaluated to true, then the next expression is checked. This continues until an expression does evaluate to true, an else statement or the conditional statement ends.

If an expression does evaluate to true, it skips the rest of the elif and else statements.

We can easily use this logic in our programme, without needing to learn many new concepts:

package main

# Start of our code

age = 70

if age < 13:
    print("I am considered a child")
elif age >= 13 and age < 20:
    print("I am considered a teenager")
elif age >= 20 and age < 68:
    print("I am considered an adult")
else:
    print("I am considered a senior")

# End of our code

Let's run through these expressions, one by one:

  • if age < 13 - we check if age is less than ( < ) 13. If it is, we print out the "child" statement and don't have to perform any more checks.

  • elif age >= 13 and age < 20 - we check if age is greater than or equal to ( >= ) 13 and if age is less than 20. This gives us a range of 13 - 19. If the expression evaluates to true we print out the "teenager" statement and don't have to perform any more checks.

  • elif age >= 20 and age < 68 - we check if age is greater than or equal to ( >= ) 20 and if age is less than 68. This gives us a range of 20 - 67. If the expression evaluates to true we print out the "adult" statement, and don't have to perform any more checks.

  • else - if age is any other value (i.e. 68 or above) we print out the "senior" statement.

Now let's add back the code from the beginning of this article:

#!/bin/env python

# Start of our code

name = "Jordan"
age = 31
hobbies = ["hiking", "playing music", "example1", "example2"]

print("Hello World. My name is {0}.".format(name))

if age < 13:
    print("I am considered a child")
elif age >= 13 and age < 20:
    print("I am considered a teenager")
elif age >= 20 and age < 68:
    print("I am considered an adult")
else:
    print("I am considered a senior")

print("I have {} hobbies, and they are: ".format(len(hobbies)))

i = 0
while i < len(hobbies):
    print(hobbies[i])
    i+=1

# End of our code

That's it - we've now added an if/elif/else statement to our Python programme!

Before you go, see if you can complete the following challenge:

  • It is possible to optimise the last if/elif/else statement further, see if you can figure out how.
    Hint: we're still asking the computer to do more work than is necessary.

If you need another hint, or want to show us your solution, get in touch with us on our Contact Us page.

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