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.

Practice

1. What is the main purpose of default arguments in Python functions?

easy

A. To create variables inside the function

B. To provide preset values for parameters if no argument is given

C. To make functions run faster

D. To force the user to always provide all arguments

Solution

1. Step 1: Understand default arguments

   Default arguments allow a function to have parameters with preset values if the caller does not provide them.

2. Step 2: Identify the purpose

   This makes the function easier to call without needing all arguments every time.

3. Final Answer:

   To provide preset values for parameters if no argument is given -> Option B

4. Quick Check:

   Default arguments = preset values [OK]

Hint: Default arguments fill in missing inputs automatically [OK]

Common Mistakes:

• Thinking default arguments speed up code
• Believing default arguments force all inputs
• Confusing default arguments with variable creation

2. Which of the following function definitions uses default arguments correctly?

easy

A. def greet(name='Friend', age):

B. def greet(name=, age=30):

C. def greet(name, age=30):

D. def greet(name, age=):

Solution

1. Step 1: Check default argument placement

   Parameters with default values must come after parameters without defaults.

2. Step 2: Analyze each option

   def greet(name='Friend', age): places a default before a non-default parameter, which is invalid syntax. Options C and D have syntax errors. def greet(name, age=30): correctly places the default argument after a required parameter.

3. Final Answer:

   def greet(name, age=30): -> Option C

4. Quick Check:

   Defaults after required params = correct syntax [OK]

Hint: Put default parameters after required ones [OK]

Common Mistakes:

• Placing default arguments before required ones
• Leaving default values empty
• Using invalid syntax for defaults

3. What is the output of this code?

def multiply(a, b=2):
    return a * b

print(multiply(4))
print(multiply(4, 3))

medium

A. Error

B. 6 and 12

C. 8 and 6

D. 8 and 12

Solution

1. Step 1: Understand default argument usage

   When multiply(4) is called, b uses its default value 2, so 4 * 2 = 8.

2. Step 2: Evaluate second call

   When multiply(4, 3) is called, b is 3, so 4 * 3 = 12.

3. Final Answer:

   8 and 12 -> Option D

4. Quick Check:

   Default used first call, explicit second call = 8 and 12 [OK]

Hint: Default used if argument missing, else explicit value [OK]

Common Mistakes:

• Assuming default is ignored when argument is given
• Confusing order of arguments
• Expecting error when default is used

4. Find the error in this function definition:

def add_numbers(x=5, y):
    return x + y

medium

A. Default argument before non-default argument causes SyntaxError

B. Missing return statement

C. Function name is invalid

D. No error, code runs fine

Solution

1. Step 1: Check parameter order

   Parameters with default values must come after parameters without defaults in Python.

2. Step 2: Identify error

   Here, x has a default but y does not, so this causes a SyntaxError.

3. Final Answer:

   Default argument before non-default argument causes SyntaxError -> Option A

4. Quick Check:

   Default before required param = SyntaxError [OK]

Hint: Non-default params must come before default ones [OK]

Common Mistakes:

• Ignoring parameter order rules
• Thinking missing return causes error here
• Assuming function name is invalid

5. You want a function that greets a user with their name and an optional greeting word (default is 'Hello'). Which function definition is correct and prints "Hi, Alice!" when called as greet('Alice', 'Hi') and "Hello, Bob!" when called as greet('Bob')?

hard

A. def greet(name, greeting='Hello'):
print(f"{greeting}, {name}!")

B. def greet(greeting='Hello', name):
print(f"{greeting}, {name}!")

C. def greet(name, greeting):
print(f"{greeting}, {name}!")

D. def greet(name='User', greeting='Hello'):
print(f"{name}, {greeting}!")

Solution

1. Step 1: Check parameter order and defaults

   Parameters with defaults must come after those without. def greet(name, greeting='Hello'): print(f"{greeting}, {name}!") has name first (no default), greeting second (default 'Hello').

2. Step 2: Verify behavior

   Calling greet('Alice', 'Hi') prints "Hi, Alice!". Calling greet('Bob') uses default greeting "Hello" and prints "Hello, Bob!".

3. Final Answer:

   def greet(name, greeting='Hello'): print(f"{greeting}, {name}!") -> Option A

4. Quick Check:

   Defaults after required params, works as expected [OK]

Hint: Put required params first, defaults after [OK]

Common Mistakes:

• Placing default param before required param
• Not providing default for optional greeting
• Using default for name causing unexpected calls