Default arguments in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of function with mutable default argument

What is the output of this code when calling append_number() three times in a row?

Python

def append_number(num, lst=[]):
    lst.append(num)
    return lst

print(append_number(1))
print(append_number(2))
print(append_number(3))

A

[1]
[2]
[3]

B TypeError

C

[1]
[1]
[1]

D

[1]
[1, 2]
[1, 2, 3]

💡 Hint

Think about how default arguments are evaluated once when the function is defined.

The default list lst is created once when the function is defined, not each time it is called. So all calls share the same list, accumulating the numbers.

❓ Predict Output

intermediate

Output with default argument and keyword argument

What will this code print?

Python

def greet(name, greeting="Hello"):
    return f"{greeting}, {name}!"

print(greet("Alice"))
print(greet("Bob", greeting="Hi"))

A Hello, Alice!\nHi, Bob!

B Hello, Alice!\nHello, Bob!

C Hi, Alice!\nHi, Bob!

D TypeError

💡 Hint

Check how the default argument is overridden by the keyword argument.

The first call uses the default greeting "Hello". The second call overrides it with "Hi".

❓ Predict Output

advanced

Default argument evaluated once

What is the output of this code?

Python

def counter(start=0):
    def inner():
        nonlocal start
        start += 1
        return start
    return inner

c1 = counter()
c2 = counter(10)
print(c1())
print(c1())
print(c2())
print(c2())

A

0
1
10
11

B

1
1
11
11

C

1
2
11
12

D TypeError

💡 Hint

Each call to counter creates a new closure with its own start value.

Each counter() call returns a new function with its own start. Calling c1() increments from 0, c2() from 10.

🔧 Debug

advanced

Identify the error with default argument

What error does this code raise when called as add_item(5)?

Python

def add_item(item, items=None):
    items.append(item)
    return items

add_item(5)

A AttributeError

B TypeError

C NameError

D No error, returns [5]

💡 Hint

Check what happens when items is None and you try to call append.

The default items is None. Calling None.append() causes an AttributeError.

🚀 Application

expert

Predict the dictionary content after function calls

Consider this function and calls. What is the content of data after all calls?

Python

def add_entry(key, value, data={}):
    data[key] = value
    return data

data = add_entry('a', 1)
data = add_entry('b', 2)
data = add_entry('a', 3)

A {'a': 1, 'b': 2, 'a': 3}

B {'a': 3, 'b': 2}

C {'a': 1, 'b': 2}

D TypeError

💡 Hint

Remember that the default dictionary is shared across calls.

The default dictionary data is shared. The last call updates key 'a' to 3, so the dictionary has keys 'a' and 'b' with values 3 and 2.

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