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.