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39. Default and Keyword Arguments

1. Introduction​

Default values and keyword arguments let you write flexible functions with sensible defaults and clear calls.

2. Default Arguments Example​

def greet(name='Guest'):
    print('Hello,', name)

greet()          # Hello, Guest
greet('Alice')   # Hello, Alice

3. Keyword Arguments Example​

def describe(name, age):
    print(f"{name} is {age} years old")

describe(age=25, name='Bob')

4. Mixing Positional and Keyword Arguments​

Positional arguments must come before keyword arguments when calling a function:

def display(a, b, c):
    print(a, b, c)

display(1, c=3, b=2)

5. Mutable Default Argument Pitfall​

Avoid using mutable defaults (lists, dicts) directly.

Bad example:

def add_item(item, collection=[]):
    collection.append(item)
    return collection

Better pattern:

def add_item(item, collection=None):
    if collection is None:
        collection = []
    collection.append(item)
    return collection

6. Best Practices​

  • Use immutable default values when possible.
  • Use keyword arguments to improve readability for functions with many parameters.

7. Next Steps​

✅ You now understand default and keyword arguments.
Next: *args and **kwargs.