Python Default Parameters

Summary: in this tutorial, you’ll learn about the Python default parameters to simplify function calls.

Introduction to Python default parameters

When you define a function, you can specify a default value for each parameter.

To specify default values for parameters, you use the following syntax:

def function_name(param1, param2=value2, param3=value3, ...):Code language: Python (python)

In this syntax, you specify default values (value2, value3, …) for each parameter using the assignment operator (=).

When you call a function and pass an argument to the parameter that has a default value, the function will use that argument instead of the default value.

However, if you don’t pass the argument, the function will use the default value.

To use default parameters, you need to place parameters with the default values after other parameters. Otherwise, you’ll get a syntax error.

For example, you cannot do something like this:

def function_name(param1=value1, param2, param3):

This causes a syntax error.

Python default parameters example

The following example defines the greet() function that returns a greeting message:

def greet(name, message='Hi'):
    return f"{message} {name}"Code language: Python (python)

The greet() function has two parameters: name and message. And the message parameter has a default value of 'Hi'.

The following calls the greet() function and passes the two arguments:

def greet(name, message='Hi'):
    return f"{message} {name}"


greeting = greet('John', 'Hello')
print(greeting)Code language: Python (python)

Output:

Hello JohnCode language: Python (python)

Since we pass the second argument to the greet() function, the function uses the argument instead of the default value.

The following example calls the greet() function without passing the second argument:

def greet(name, message='Hi'):
    return f"{message} {name}"


greeting = greet('John')
print(greeting)Code language: Python (python)

Output:

Hi JohnCode language: Python (python)

In this case, the greet() function uses the default value of the message parameter.

Multiple default parameters

The following redefines the greet() function with the two parameters that have default values:

def greet(name='there', message='Hi'):
    return f"{message} {name}"Code language: Python (python)

In this example, you can call the greet() function without passing any parameters:

def greet(name='there', message='Hi'):
    return f"{message} {name}"


greeting = greet()
print(greeting)Code language: Python (python)

Output:

Hi thereCode language: Python (python)

Suppose that you want the greet() function to return a greeting like Hello there. You may come up with the following function call:

def greet(name='there', message='Hi'):
    return f"{message} {name}"


greeting = greet('Hello')
print(greeting)Code language: Python (python)

Unfortuntely, it returns an unexpected value:

Hi HelloCode language: Python (python)

Because when you pass the 'Hello' argument, the greet() function treats it as the first argument, not the second one.

To resolve this, you need to call the greet() function using keyword arguments like this:

def greet(name='there', message='Hi'):
    return f"{message} {name}"


greeting = greet(message='Hello')
print(greeting)Code language: Python (python)

Output:

Hello thereCode language: Python (python)

Summary

• Use Python default parameters to simplify the function calls.
• Place default parameters after the non-default parameters.