Default arguments in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are default arguments in Python functions?

Default arguments are values given to function parameters that are used if no argument is provided when the function is called.

beginner

How do you define a default argument in a Python function?

You assign a value to a parameter in the function definition, like def greet(name='Friend'):.

beginner

What happens if you call a function with a default argument but provide a value for that argument?

The provided value overrides the default argument value for that call.

intermediate

Why should mutable objects be avoided as default argument values?

Because the same mutable object is shared across calls, which can cause unexpected behavior if it is modified.

intermediate

Example: What is the output of this code?

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

print(add_item('apple'))
print(add_item('banana'))

Output:

['apple']
['apple', 'banana']

This happens because the default list is shared between calls.

What is the correct way to set a default argument for a function parameter in Python?

A def func(x): default=10

B def func(x): x=10

C def func(x=10):

D def func(x:10):

Default arguments are set by assigning a value in the function definition, like def func(x=10):.

If a function has a default argument, what happens when you call it without that argument?

A The argument becomes None.

B The default value is used.

C An error occurs.

D The function ignores the parameter.

When a default argument is not provided in the call, Python uses the default value.

Why can using a mutable object like a list as a default argument cause problems?

A Because it creates a new object every call.

B Because Python does not allow lists as defaults.

C Because mutable objects cannot be default arguments.

D Because the same object is shared across calls.

Mutable default arguments are shared between calls, so changes persist and can cause bugs.

How can you avoid issues with mutable default arguments?

A Use None as the default and create a new object inside the function.

B Use an empty list as default anyway.

C Avoid default arguments altogether.

D Use global variables instead.

Using None as default and creating a new object inside the function avoids sharing the same mutable object.

What will this code print?

def greet(name='Friend'):
    print(f'Hello, {name}!')

greet()
greet('Alice')

A Hello, Friend!<br>Hello, Alice!

B Hello, !<br>Hello, Alice!

C Hello, Friend!<br>Hello, Friend!

D Error

The first call uses the default 'Friend', the second call uses the provided 'Alice'.

Explain what default arguments are and how they work in Python functions.

Think about function parameters that have a value ready if you don't give one.

Describe the problem with using mutable objects as default arguments and how to fix it.

Consider what happens if you change a list used as a default argument.