Lambda with sorted() in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the sorted() function do in Python?

It returns a new list containing all items from the iterable in ascending order by default.

beginner

What is a lambda function in Python?

A lambda function is a small anonymous function defined with the lambda keyword, which can take any number of arguments but has only one expression.

intermediate

How do you use a lambda function with sorted()?

You pass the lambda function to the key parameter of sorted() to specify the value to sort by for each item.

intermediate

Example: Sort a list of tuples by the second item using sorted() and lambda.

sorted_list = sorted([(1, 3), (2, 1), (3, 2)], key=lambda x: x[1]) # Result: [(2, 1), (3, 2), (1, 3)]

intermediate

How to sort a list of strings by their length using sorted() and lambda?

Use sorted(strings, key=lambda s: len(s)) to sort strings by length from shortest to longest.

What does the key parameter in sorted() do?

A Specifies a function to extract a comparison key from each element.

B Changes the original list in place.

C Reverses the list before sorting.

D Filters elements before sorting.

The key parameter takes a function that returns a value used to compare elements during sorting.

Which of these is a valid lambda function to sort a list of numbers in descending order using sorted()?

A sorted(numbers, key=lambda x: -x)

B sorted(numbers, key=lambda x: x)

C sorted(numbers, reverse=False)

D sorted(numbers, key=lambda x: x+1)

Using key=lambda x: -x sorts numbers by their negative value, effectively sorting descending.

What will this code output? sorted(['apple', 'banana', 'cherry'], key=lambda x: x[0])

A ['cherry', 'banana', 'apple']

B ['banana', 'apple', 'cherry']

C ['apple', 'cherry', 'banana']

D ['apple', 'banana', 'cherry']

Sorting by the first letter keeps the list in the same order because 'a' < 'b' < 'c'.

How can you sort a list of dictionaries by the value of the key 'age' using lambda?

A sorted(list_of_dicts, key=lambda d: age)

B sorted(list_of_dicts, key=lambda d: d.age)

C sorted(list_of_dicts, key=lambda d: d['age'])

D sorted(list_of_dicts, key=lambda d: d.get('name'))

Use lambda d: d['age'] to access the 'age' value in each dictionary for sorting.

What is the output of sorted([3, 1, 2], key=lambda x: x % 2)?

A [1, 2, 3]

B [2, 3, 1]

C [3, 1, 2]

D [1, 3, 2]

Sorting by x % 2 groups even numbers (0) before odd numbers (1), so 2 comes before 3 and 1.

Explain how to use a lambda function with sorted() to sort a list of complex items.

Think about how to tell Python what part of each item to sort by.

Describe a real-life example where sorting with a lambda function would be useful.

Consider sorting a list of people by age or sorting products by price.