Lambda and Built-in Functions

Use short lambda functions and Python built-ins to sort, transform, filter, and check data.

12 min read 10 quiz questions

Small one-line functions

A lambda is a small anonymous function written on one line. It takes inputs before the colon and has one expression after it.

Lambdas are useful when another function needs a quick rule.

lambda parameters: expression

Example 1: Sort with a lambda key

words = ["pear", "fig", "banana"]
by_length = sorted(words, key=lambda word: len(word))
print(by_length)

sorted calls the lambda for each word. The returned length becomes the sort key, so shorter words appear first.

Map and filter

Example 2: Transform and filter values

numbers = [1, 2, 3, 4, 5]
squares = list(map(lambda number: number * number, numbers))
evens = list(filter(lambda number: number % 2 == 0, numbers))
print(squares)
print(evens)

map applies a function to every item. filter keeps only items for which its function returns True. Converting them to a list lets you see the results.

Useful built-in functions

Function	What it does	Example
sum	Adds values	sum([2, 3]) -> 5
min / max	Finds the smallest / largest value	max([2, 7]) -> 7
abs	Gets distance from zero	abs(-4) -> 4
round	Rounds a number	round(3.14159, 2) -> 3.14
any / all	Checks whether any / all values are true	all([True, False]) -> False
sorted	Returns a new sorted list	sorted([3, 1]) -> [1, 3]
reversed	Gives items in reverse order	list(reversed([1, 2])) -> [2, 1]
zip	Pairs items from iterables	list(zip([1, 2], ['a', 'b'])) -> [(1, 'a'), (2, 'b')]
len	Counts items	len('hi') -> 2

Example 3: Combine built-ins

values = [-3, 2, 5]
print(sum(values))
print(min(values), max(values))
print(abs(-8), round(3.14159, 2))
print(any([False, True]), all([True, True]))
print(list(reversed(sorted(values))))
print(list(zip([1, 2], ["a", "b"])))

Key points

A lambda has parameters, a colon, and one expression.
Use sorted(key=...) to sort by a custom rule.
map transforms every item and filter keeps matching items.
Built-ins solve common tasks with short, readable code.
sorted returns a new list rather than changing the original.

Worked example

Sort students by marks without writing a def — a lambda supplies the sort key inline.

Example

students = [("Mim", 78), ("Rafi", 64), ("Joe", 91)]
top_first = sorted(students, key=lambda pair: pair[1], reverse=True)
print(top_first[0])

How it works, step by step

sorted() orders pairs using whatever key() returns.
lambda pair: pair[1] says 'compare by the marks slot'.
reverse=True puts the highest first.

References & Further Reading

Related entries from PyLabs's own reference library.

sorted() (Make a new list containing items in sorted order.)
sort() (Sort the elements of the list in ascending or custom order.)
filter() (Keep items that pass a test.)

Your notes

Practice exercise

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Use filter with a lambda to keep numbers greater than 3 from [1, 4, 2, 5]. Convert the result to a list and print it.

Expected output: [4, 5]

# Write your code below

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