Summary  
This chapter covers using default parameter values to create optional function arguments in Python, ensuring that missing arguments don’t cause errors and that optional parameters follow required ones.

General domain of usage  
Robust function interface design in Python applications.

What happens if one of the positional arguments is missing? Examine the following example:


# Function with two positional arguments
def greet(name, age):
    print(f'Hello, {name}! You are {age} years old.')

# Calling the `greet()` function with one missing argument
greet(name='Alex')

An **error occurs** if you forget to specify one or more **positional arguments**. In **real projects**, this can cause multiple **bugs** and reduce the system’s **fault tolerance**. To prevent this, you can use **default arguments**.


```python
def function_name(optional_argument_name=default_value):
    ...
```
  
These arguments are optional when calling the function, as the default value will be used if no value is specified for that argument.

To define an optional argument, you can assign a default value to the corresponding parameter in the function definition. Here's an example:

def greet(name, age=0):
    print(f'Hello, {name}! You are {age} years old.')

# Calling the `greet()` function with missing optional argument
greet(name='Alex')

However, there is an important rule when using optional arguments: **they must be specified after all non-optional arguments**. If this rule is not followed, an error will occur.

def greet(name='Alex', age):
    print(f'Hello, {name}! You are {age} years old.')

# Calling the `greet()` function with optional argument before non-optional 
greet(age=35)

What happens if a function has both optional and non-optional arguments?

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Optional Arguments in Python