14-lambdas

14 · Lambdas & map/filter

Part 3 — Functions & Modules · Estimated time: 25–35 min · Prerequisite: 13 · Functions

A lambda is a tiny, one-line function with no name. You reach for one when you need a quick function to hand to another function — most often map, filter, or sorted. These tools let you transform and filter whole collections with very little code.

What you'll learn

• Writing a lambda and how it relates to a normal def function
• map() — apply a function to every item
• filter() — keep only the items that pass a test
• sorted(..., key=...) — sort by a custom rule

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1. Lambda basics

def square(x):       # a normal function
    return x * x

square2 = lambda x: x * x   # the same idea, as a lambda

print(square(4))     # 16
print(square2(4))    # 16

add = lambda a, b: a + b    # lambdas can take several arguments
print(add(2, 3))     # 5

A lambda is just lambda arguments: expression. It automatically returns the expression — no return keyword. If a function needs more than one line, use a normal def instead.

▶️ Run it: python examples/01_lambda_basics.py

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2. map() — transform every item

nums = [1, 2, 3, 4, 5]

squared = map(lambda x: x * x, nums)   # apply the lambda to each item

# map returns an iterator — wrap it in list() to see the values.
print(list(squared))   # [1, 4, 9, 16, 25]

▶️ Run it: python examples/02_map.py

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3. filter() — keep some items

filter keeps only the items for which the function returns True.

nums = [1, 2, 3, 4, 5, 6, 7, 8]

evens = filter(lambda x: x % 2 == 0, nums)
print(list(evens))     # [2, 4, 6, 8]

▶️ Run it: python examples/03_filter.py

💡 Try it yourself: use filter to keep only the words longer than 3 letters from ["hi", "hello", "hey", "yo"].

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4. sorted(key=...) — sort your way

Pass key= a function that returns the value to sort by.

words = ["banana", "kiwi", "apple"]

print(sorted(words))                       # ['apple', 'banana', 'kiwi'] (alphabetical)
print(sorted(words, key=lambda w: len(w))) # ['kiwi', 'apple', 'banana'] (by length)
print(sorted([3, 1, 2], reverse=True))     # [3, 2, 1] (descending)

▶️ Run it: python examples/04_sorted_key.py

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Common mistakes

Mistake	What happens	Fix
print(map(...))	prints <map object ...>	Wrap it: print(list(map(...))).
Putting a statement in a lambda	SyntaxError	Lambdas hold one expression; use def for logic.
Re-using a map/filter result twice	it's empty the second time (iterators exhaust)	Save it as a list: result = list(map(...)).
sorted(x, key=len())	calls len with no args — error	Pass the function itself: key=len or key=lambda w: len(w).

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Recap / cheat-sheet

f = lambda x: x * 2          # one-line function
lambda a, b: a + b           # several arguments

list(map(fn, items))         # transform every item
list(filter(fn, items))      # keep items where fn(item) is True
sorted(items, key=fn)        # sort by what fn returns
sorted(items, reverse=True)  # descending

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Exercises

Run a file with python exercises/<file>.py, then compare with the matching file in solutions/.

1. exercises/01_double_all.py — double every number with map.
2. exercises/02_filter_evens.py — keep the even numbers with filter.
3. exercises/03_sort_by_length.py — sort words by length with sorted.

Try each yourself before opening the solution.

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Next → 15 · Modules & the Standard Library (coming soon)