24. Default parameter values

Quiz: 0/5

In Python, default parameter values provide a way to make function arguments optional. You can specify default values for one or more parameters when defining a function. If the caller doesn’t provide a value for a parameter with a default, the function will use the default value instead. This makes functions more flexible and reduces the need for the caller to specify every argument.

How to use default parameter values:

To set a default value for a parameter, assign the value directly in the function definition. Here’s the syntax:

def function_name(parameter=default_value):
    # Function body

If the caller doesn’t pass a value for a parameter with a default, the function will use the default value. If a value is provided, it will override the default.

Example: simple greeting function

Let’s create a function that greets a user. The function will have a default parameter value for the greeting.

def greet_user(name, greeting="Hello"):
    print(f"{greeting}, {name}!")

In this example:

• greeting has a default value of Hello.
• name has no default value, so it’s required when calling the function.

Calling the function:

Without specifying greeting:

greet_user("Alice")  # Output: Hello, Alice!

Here, greeting uses its default value Hello, so the output is Hello, Alice!.

By specifying greeting:

greet_user("Bob", greeting="Hi")  # Output: Hi, Bob!

Here, we pass Hi for greeting, overriding the default value.

Multiple default parameters:

You can define multiple default parameters. Let’s modify the function to allow a customizable punctuation mark.

def greet_user(name, greeting="Hello", punctuation="!"):
    print(f"{greeting}, {name}{punctuation}")

Now:

• greeting defaults to Hello.
• punctuation defaults to !.

Calling the Function:

Using all defaults:

greet_user("Alice")  # Output: Hello, Alice!

Overriding some defaults:

greet_user("Bob", greeting="Hi")         # Output: Hi, Bob!
greet_user("Charlie", punctuation=".")    # Output: Hello, Charlie.
greet_user("Diana", greeting="Hey", punctuation="?")  # Output: Hey, Diana?

Important rules with default parameters

Rule 1: Default parameters must come after non-defaults

In a function definition, parameters with default values must appear after parameters without default values. This ensures clarity when arguments are assigned.

Incorrect:

def greet_user(greeting="Hello", name):  # ❌ SyntaxError
    # Function body

Correct:

def greet_user(name, greeting="Hello"):  # ✔️
    # Function body

Rule 2: Mutable default values caution

Be cautious when using mutable objects (like lists or dictionaries) as default parameter values, as they retain their state across multiple function calls. This can lead to unexpected behavior.

Example with mutable default values:

def add_to_list(item, my_list=[]):
    my_list.append(item)
    return my_list

print(add_to_list("apple"))  # Output: ['apple']
print(add_to_list("banana"))  # Output: ['apple', 'banana']

In this example, my_list retains its state between calls. To avoid this, use None as the default and initialize the list inside the function:

def add_to_list(item, my_list=None):
    if my_list is None:
        my_list = []
    my_list.append(item)
    return my_list

Summary:

• Default parameter values let you set a default for a parameter, making it optional for the caller.
• Order matters: Parameters with defaults must come after required parameters.
• Care with mutable defaults: Using mutable objects (e.g., lists, dictionaries) as default values can lead to unexpected behavior, as they retain their state between calls.

Default parameters add versatility to functions, allowing for simpler code that is more flexible by only requiring values when necessary.