A function is a block of code which only runs when it is called.
A function can return data as a result.
A function helps avoiding code repetition.
Creating a Function
In Python, a function is defined using the def keyword, followed by a function name and parentheses:
Example
def my_function():
print("Hello from a function")This creates a function named my_function that prints "Hello from a function" when called.
The code inside the function must be indented. Python uses indentation to define code blocks.
Calling a Function
To call a function, write its name followed by parentheses:
Example
def my_function():
print("Hello from a function")
my_function()You can call the same function multiple times:
Example
def my_function():
print("Hello from a function")
my_function()
my_function()
my_function()Function Names
Function names follow the same rules as variable names in Python:
- A function name must start with a letter or underscore
- A function name can only contain letters, numbers, and underscores
- Function names are case-sensitive ( myFunction and myfunction are different)
Valid function names
calculate_sum()
_private_function()
myFunction2()It's good practice to use descriptive names that explain what the function does.
Why Use Functions?
Imagine you need to convert temperatures from Fahrenheit to Celsius several times in your program. Without functions, you would have to write the same calculation code repeatedly:
Example
temp1 = 77
celsius1 = (temp1 - 32) * 5 / 9
print(celsius1)
temp2 = 95
celsius2 = (temp2 - 32) * 5 / 9
print(celsius2)
temp3 = 50
celsius3 = (temp3 - 32) * 5 / 9
print(celsius3)With functions, you write the code once and reuse it:
Example
def fahrenheit_to_celsius(fahrenheit):
return (fahrenheit - 32) * 5 / 9
print(fahrenheit_to_celsius(77))
print(fahrenheit_to_celsius(95))
print(fahrenheit_to_celsius(50))Return Values
Functions can send data back to the code that called them using the return statement.
When a function reaches a return statement, it stops executing and sends the result back:
Example
def get_greeting():
return "Hello from a function"
message = get_greeting()
print(message)You can use the returned value directly:
Example
def get_greeting():
return "Hello from a function"
print(get_greeting())If a function doesn't have a return statement, it returns None by default.
The pass Statement
Function definitions cannot be empty. If you need to create a function placeholder without any code, use the pass statement:
Example
def my_function():
passThe pass statement is often used when developing, allowing you to define the structure first and implement details later.