Lambda, Map & Filter

Lambda functions

A lambda is a small anonymous function for short tasks. If the logic is long, use a normal def.

python
add = lambda a, b: a + b
print(add(2, 3))

Using map

map() applies a function to every item. It returns an iterator, so use list() if you want to see the results.

python
nums = ["1", "2", "3"]
converted = list(map(int, nums))
print(converted)

Using filter

filter() keeps only items that return True.

python
values = ["", "hi", "", "ok"]
non_empty = list(filter(bool, values))
print(non_empty)

Sorting with a lambda key

A lambda is handy for custom sorting.

python
pairs = [("a", 3), ("b", 1), ("c", 2)]
print(sorted(pairs, key=lambda p: p[1]))

When a list comprehension is clearer

Many map() and filter() cases are easier to read as comprehensions.

python
nums = [1, 2, 3, 4]
result = [n * 2 for n in nums if n % 2 == 0]
print(result)

Common mistakes to avoid

• Overusing lambdas for long logic.
• Forgetting to wrap map() or filter() with list() when you need a list.
• Using lambdas when a named function would be clearer.

What you should understand after this lesson

• What a lambda is and when to use it.
• How map() and filter() work.
• When a list comprehension is a better choice.