len() function in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of len() with nested lists

What is the output of the following code?

Python

lst = [1, [2, 3], 4]
print(len(lst))

A 3

B 4

C 2

D 5

💡 Hint

len() counts the top-level items in the list.

The list has three top-level elements: 1, [2, 3], and 4. So len(lst) is 3.

❓ Predict Output

intermediate

Length of a string with spaces

What will be printed by this code?

Python

text = 'Hello World'
print(len(text))

A 9

B 10

C 12

D 11

💡 Hint

Spaces count as characters in strings.

The string 'Hello World' has 11 characters including the space.

❓ Predict Output

advanced

Length of dictionary keys

What is the output of this code?

Python

d = {"a": 1, "b": 2, "c": 3}
print(len(d.keys()))

A TypeError

B 0

C 3

D 1

💡 Hint

len() counts how many keys are in the dictionary.

d.keys() returns a view of the keys, which has length 3.

❓ Predict Output

advanced

len() on a generator expression

What happens when you run this code?

Python

gen = (x for x in range(5))
print(len(gen))

A 5

B TypeError

C 0

D AttributeError

💡 Hint

Generators do not have a length.

Calling len() on a generator raises a TypeError because generators do not support len().

❓ Predict Output

expert

Length of a custom class with __len__

What will this code print?

Python

class Box:
    def __init__(self, items):
        self.items = items
    def __len__(self):
        return sum(len(item) for item in self.items)

b = Box([[1, 2], [3, 4, 5], []])
print(len(b))

A 5

B 3

C 0

D TypeError

💡 Hint

The __len__ method sums lengths of inner lists.

The Box contains three lists with lengths 2, 3, and 0. Their sum is 5.