Introduction to functions in python

introduction_functions.md

Functions are programming constructs that helps us package common operations. In addition they make our code that much more readable.

They have a format that looks like this:

def first_function():
        pass

output = first_function()

OR

def first_function():
        some_string = "An example string"
        return some_string

output = first_function()

Basically we start by telling python that we are about to define a function by using the keyword def and then give the name of the function followed by round brackets with any arguments and then the body of the function.

A function in python must have a body. If none is available, say if you are still sketching out the overall system, then use the keyword pass

If you don't explicitly return an output. Python will return a None object. See the reference.

Note that Method is another way of referring to Function. This terminology is more common in OOP conversations.

##Step 1

Let us implement this requirement

    As a user I want to see welcome message from fairmont when I run the welcome_message.py script

Let us start by creating our lesson folder

mkdir 6lesson
cd 6lesson
touch welcome_message.py

Now open the welcome_message.py in the 6lesson folder and type in the following code.

def welcome_message():
    message =  "Welcome guest to Fairmont hotels. It is a pleasure to serve you"
    return message
    
output = welcome_message()
print(output)

Run the code from the bash terminal

python3 welcome_message.py

What did you see on the screen?

###Assignment

Can you make the script print out a welcome message for you instead of generic guest

##Step 2

Now that we are able to print out a string, let us make it more useful by having it accept an argument. An argument is simply a value that you pass to a function or method.

Suppose the requirement is now.

As a guest I want the script welcome_message to first ask for my name and then give me a personalized message

Modify your welcome script so that it now looks like this.

def welcome_message(name):
    message =  "Welcome {} to Fairmont hotels. It is a pleasure to serve you".format(name)
    return message
name = input("What is your name?")

output = welcome_message(name)
print(output)

Run the script.

python3 welcome_message.py

What do you now see as the output?

##Step 3

Now that we are able to work with strings, how would a function help us with numbers.

Suppose we now have a requirement.

    As a user I want the script area_triangle.py to first ask me for a base and height and then give me an area

We can now create a new file called area_triangle.py.

touch area_triangle.py

And then type in the following code into your python file

def area_triangle(base,height):
    int_base = int(base)
    int_height = int(height)
    area = 0.5 * int_base * int_height
    return area
base = input("What is the base of your triangle?:")
height = input ("What is the height of your triangle?:")
area = area_triangle(base,height)
print(area)

You notice that we moved the operations necessary to clean up the data into the function itself.

What was the result?

##Assignment

Create two more scripts, area of rectangle and area of circle. When the files are run the area of rectangle should ask for a base and height and then give the area. The area of a circle should ask for radius and then give the area

##References

• http://www.learnpython.org/en/Functions
• http://introtopython.org/introducing_functions.html
• https://docs.python.org/3/library/constants.html
• http://blog.chenchatech.com/2016/01/name-well-or-go-home/