Functions

Objectives

• Understand the concept of functions and procedures in Python.
• Learn why functions are useful in programming.
• Discover when and how to use functions.
• Understand the roles of arguments and parameters in functions.
• Practice writing and using functions with simple examples.

Most problems we encounter can be broken down in to a series of steps. Earlier we saw the following series of steps to make a sandwich:

• Go to the kitchen.
• Open the refrigerator.
• Take out the bread, cheese, and lettuce.
• Put two slices of bread on a plate.
• Put some cheese and lettuce on one slice of bread.
• Put the other slice of bread on top.
• Close the sandwich.
• Bring it to me.

Here's we've decomposed the larger problem into a series of smaller problems each can be tackled on their own but when put together will result in a sandwich being made (hopefully).

Example 4

Read the following code and predict what the output will be using the questions below as a prompt.

def convert_celsius_to_fahrenheit(celsius):
    return (celsius * 9/5) + 32

def calculate_average(temperatures):
    return sum(temperatures) / len(temperatures)

def greet(name):
    return f"Hello, {name}! Welcome to Climate Quest."

weekly_celsius_temps = [22, 24, 20, 26, 23, 25, 21]
average_temp = calculate_average(weekly_celsius_temps)
print("Average Weekly Temperature (Celsius):", average_temp)

name = "Student"
greeting = greet(name)
print(greeting)

temp_in_fahrenheit = convert_celsius_to_fahrenheit(average_temp)
print("Average Weekly Temperature (Fahrenheit):", temp_in_fahrenheit)

• "What will convert_celsius_to_fahrenheit(25) return?"
• "What does the calculate_average function do with the list of temperatures?"
• "What will be stored in greeting?"
• "What will be printed for the average temperature in Fahrenheit?"

Answers

1. What will convert_celsius_to_fahrenheit(25) return? 77
2. What does the calculate_average function do with the list of temperatures? Calculates the average, which is 23.0 for the provided list.
3. What will be stored in greeting? "Hello, Student! Welcome to Climate Quest."
4. What will be printed for the average temperature in Fahrenheit? Average Weekly Temperature (Fahrenheit): 73.4

Copy and paste the example code into your code editor. Run the code to check your predictions.

Syntax of a functions

To create a function in Python, we use the def keyword, followed by the function name, pair of parentheses, (), followed by a colon, :.

For example:

def my_function():
    # Function body
    pass

Note

The keyword pass is a statement that does not do anything. It is useful when scaffolding code as in the previous example. We know we want a function that calculates the area of a rectangle but are either unsure of how to complete this function or want to return to it later so we enter pass. Our main program will still call that function but nothing will happen.

Naming the function

Similar to the naming of variables and constants, the name we give to our function should be self-documenting. In our example program one of the functions is named convert_celcius_to_fahrenheit(). This name describes exactly what this function will do! It might be more to type than say conv_cel to fahr() of c_to_f() but being more precise will result in your code being easier to read and maintain.

The syntax required can be summarised as:

• Function Name: Descriptive and meaningful (follow naming conventions), the identifier for the function
• Parentheses: Used to pass arguments (inputs) to the function.
• Colon: Indicates the beginning of the function body.
• Indentation: All code inside the function must be indented.
• Statement(s): The body of the function, statements that get executed every time the function is called.

Using functions are highly recommended. They provide:

• Modularity: Divide your code into smaller, self-contained modules (functions) for easier development and debugging.
• Reusability: Write code once and use it multiple times, reducing duplication.
• Readability: Functions make code more understandable and concise.
• Maintenance: Changes are easier to implement and test within a function.

Indenting the function body

When defining a function the body of the code in that function must be indented to show the code belongs to that function.

def convert_celsius_to_fahrenheit(celsius):
    return (celsius * 9/5) + 32

celsius = 24
print(convert_celsius_to_fahrenheit(celsius)

The function body here only has one line (line 2), but it has been indented (by four spaces). Your code editor will probably indent this line automatically.

Lines 4 and 5 are not part of the function, they are lined up with the left-hand margin.

Note

The indentation feature of Python is really important to understand:

Code statements can be grouped into blocks. The function is an example of a block of code, indicated by the colon at the end of the function header. As we will see late, we can also create a block of code to take a decision (an if statement), or to repeat a block of code (a for loop, or a while loop). These also end with a colon and statements belonging to these blocks must also be indented.

Functions return a value

def convert_celsius_to_fahrenheit(celsius):
    return (celsius * 9/5) + 32

celsius = 24
fahrenheit = convert_celsius_to_fahrenheit(celsius)

Note

All functions, by definition, will return a value. Here the fahrenheit equivalent of the given celsius value is returned from our function (line 2) and assigned to the variable 'fahrenheit' (line 5)

Procedures vs Functions

Look at the following code, it assumes the functions get_value(), calculate_area() but these are not shown, also print_result():

def main():
    length = get_value()
    height = get_value()
    area = calculate_area(length, height)
    print_result(area)

The statements in the first three lines are assignment statements. An item of data is being returned from that block of code and immediately assigned to a variable.

Line 5 is different, print_result(area). Any value being returned here is not assigned to a variable. This is known as a procedure.

We might expect print_result() to be:

def print_result(area):
    print(area)

Notice there is no return statement here. This is what makes it a procedure and not a function.

Warning

This is not strictly true in Python, as a procedure will always return None when it is called, but we generally ignore this and it is not used.

To return a value from a function we have to use the return statement. Here's how it might be used for the get_value() function:

def get_value():
    print("Enter a value: ")    # prompt for a value
    value = int(input())        # cast the input to an integer
    return value                # return the input back to the main program

Similarly to calculate the area the result is being returned:

def calculate_area(length, height):
    return length * height

That return value needs to be stored somewhere, yes, in a variable: area = calculate_area(length, height).

Parameters and Arguments

def convert_celsius_to_fahrenheit(celsius):
    return (celsius * 9/5) + 32

celsius = 24
fahrenheit = convert_celsius_to_fahrenheit(celsius)

Line 1 declares the function using the def keyword, followed by the name of the function and then a pair of brackets. To calculate the fahrenheit value we need to know the celsius value. This is passed in as a parameter to the function.

Think of this parameter as a placeholder. We do no know the value yet, it could be any value as defined by the user or our program.

Line 5 calls our function, using the function name followed by the argument to pass to the function. In this instance it is the value \(24\).

When the function is executed Python will create a new local variable and copy the value of the argument to that new variable. When the function finishes that local variable will no longer be available.

In this instance we happen to use the same name for our argument and our parameter - this can aid readability but they could have been different e.g.

def convert_celsius_to_fahrenheit(celsius):
    return (celsius * 9/5) + 32

celsius_input = 24
fahrenheit = convert_celsius_to_fahrenheit(celsius_input)

Remember:

• Parameters: Variables declared in the function header.
• Arguments: Values passed to the function when it's called.

Calling a function

To call a function we enter the name for the function and any arguments that need to be passed to the function from another part of our program. The function call must come after the function definition in Python.

def calculate_area(rec_length, rec_height):     # function definition
    return rec_length * rec_height)

length = 10
height = 5
area = calculate_area(length, height)           # function call
print(area)

Default parameters

Consider the following example:

def greet(name, message="Hello"):
    print(f"{message} {name})

Python allows for functions (and procedures) to have default arguments. The procedure greet() can now be called with only one argument:

greet("Sally")          # "Hello Sally"

Or we can override the default parameter replacing the default message with one of our own:

greet("Sally", "Hi")    # "Hi Sally"

Why bother with functions?

It can be a lot of trouble to organise your code into these blocks, functions, so why bother? There are a number of good reasons:

• Providing a name for a group of statements that are responsible for a task will make the program easier to read and debug
• Using functions can eliminate the need to duplicate code. One of the guiding rules of good programming style is the acronym DRY. This stands for Don't Repeat Yourself
• With a long program decomposed into smaller chunks it becomes easier to locate where there might be a bug. It can also be easier to start with a high level description (even as code) and then progressively break each of these down into as many functions as might be required.
• Well designed functions can be used in other programs. Once its written and tested you can use it elsewhere.

Keyword parameters

When you call a function, you usually pass values as arguments in the same order as the function's parameters are defined. However, sometimes it's more convenient to specify which argument corresponds to which parameter by using their names. These are called keyword parameters.

Let's say we have our simple function that calculates the area of a rectangle, we can call this function using keyword parameters:

area = calculate_area(length=5, width=10)

The names of the parameters are explicitly mentioned with their values. It makes the code more readable, especially when you have a function that takes in several arguments. If the keyword parameter is used the order of the variables do not matter.

Summary

Functions are fundamental to programming in Python. They promote code organization, reusability, and readability. As you continue your programming journey, you'll find that functions become an essential tool for solving complex problems efficiently.

Cheat Sheet for functions¹

Activity

Using the following code:

def add(a, b):
    return a + b

def multiply(a, b):
    return a * b

num1 = 10
num2 = 5

sum_result = add(num1, num2)
print("Sum:", sum_result)

product_result = multiply(num1, num2)
print("Product:", product_result)

Modify this code:

• Change the variable values and observe the output.
• Add more functions that perform different basic tasks.
• Experiment with returning different types of values (e.g., strings, booleans).

Climate Quest Project

Throughout this topic we'll be working on a large scale project: Climate Quest. In this project a player embarks on a journey to combat the effects of climate change by making decisions that impact the environment. Each choice affects the outcome of the game, emphasizing the importance of individual actions in addressing climate change.

Go to task 4 - Functions

Questions

1. What is the main purpose of using functions in programming?

To make the code longer and more complex

To reduce duplication of code and make the program easier to maintain

To confuse other programmers

To slow down program execution

Functions help in organizing code by encapsulating repetitive tasks. This makes the code more readable, reusable, and easier to maintain.

Submit

2. In Python, how do you define a function?

Using the def keyword followed by the function name, parentheses, and a colon

Using the function keyword followed by the function name and parentheses

Using the define keyword followed by the function name and parentheses

Using the func keyword followed by the function name and a semicolon

In Python, a function is defined using the def keyword followed by the function name, parentheses (which may contain parameters), and a colon. The function body is indented underneath this line.

Submit

3. What is the difference between a function and a procedure in Python?

Functions return values, procedures do not

Functions do not use parameters, while procedures do

Functions are called with call(), and procedures are called with execute()

There is no difference; the terms are interchangeable.

The primary distinction is that functions return a value after execution, whereas procedures (often just another term for functions that don’t return a value) do not.

Submit

4. When defining a function, what are the components included in the function header?

Function name and indentation

Function name, parentheses for parameters, and a colon

Function name, parentheses for arguments, and a semicolon

Function name, parentheses for return values, and a comma

The function header in Python includes the function name, parentheses (which may contain parameters), and a colon. This is followed by the indented function body.

Submit

5. What is the purpose of function parameters in Python?

They are used to specify the return value of a function

They allow you to pass data into a function for processing

They determine the indentation level of a function

They are optional and not required for defining functions

Function parameters are variables that allow data to be passed into a function, making the function flexible and able to handle different inputs.

Submit

6. Which keyword is used to return a value from a function in Python?

result

output

return

response

The return keyword is used in Python to exit a function and return a value to the caller. This is essential when the function is supposed to provide a result.

Submit

7. What does it mean to have a 'default parameter' in a function?

It means the parameter cannot be changed

It means the parameter is mandatory

It means the parameter has a predefined value

It means the parameter is hidden from the function

A default parameter is one that has a predefined value in the function definition. If no argument is provided for that parameter when the function is called, the default value is used.

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8. When calling a function with keyword parameters, what advantage does it offer?

It allows you to call the function without any parameters

It specifies which argument corresponds to which parameter, making the code more readable

It allows you to call the function with a variable number of arguments

It prevents the function from being called

Keyword arguments improve readability by explicitly stating which value corresponds to which parameter, making the function call easier to understand.

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9. What is the purpose of using functions in a program's structure?

To make the program run faster

To make the program more complex

To organize and modularize the code

To eliminate the need for variables

Functions help in organizing and modularizing code, breaking down complex problems into smaller, manageable sections, and improving code clarity and reusability.

Submit

10. In Python, where should the function call be placed if you want it to be the entry point for your program?

Before defining the function

After defining the function

Inside the function definition

It doesn't matter where the function call is placed

In Python, a function must be defined before it is called. This ensures that the Python interpreter knows what the function is supposed to do when it encounters the function call.

Submit

Programming Tasks

For each of the following tasks write a program using Python.

Area of a triangle

Write a Python function called calculate_triangle_area(base, height) that calculates and returns the area of a triangle using the `formula area = 0.5 * base * height`. Then, call this function from your main program and print the result.

Read more ...

BMI Calculator Function

A previous task was to write a program to calculate the BMI of an individual from their height and weight. Use the program structure from this chapter to amend that program so the height and weight are entered via a function, so too the BMI calculation and the display of the result

Read more ...

String Reversal Function

Write a function that takes a string as a parameter and returns the reverse of that string. Use string slicing to help

Read more ...

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1. [https://ehmatthes.github.io/pcc_3e/cheat_sheets/(https://ehmatthes.github.io/pcc_3e/cheat_sheets/)] ↩