Python Lambda, Map, and Filter

What Is A Lambda Function?

A lambda function is a small anonymous function written as an expression.

double = lambda number: number * 2
print(double(5))

This prints 10. A normal def function is usually better for important or multi-line logic, but lambda is useful for short functions passed into another function.

The map() Function

map() applies a function to every item in an iterable.

numbers = [1, 2, 3]
doubled = map(lambda number: number * 2, numbers)

print(list(doubled))

Output:

[2, 4, 6]

In Python 3, map() returns an iterator, so convert it with list() if you want to see all results at once.

The filter() Function

filter() keeps items where the function returns true.

numbers = [1, 2, 3, 4]
evens = filter(lambda number: number % 2 == 0, numbers)

print(list(evens))

Output:

[2, 4]

Readability Matters

Many map() and filter() examples can also be written as comprehensions.

doubled = [number * 2 for number in numbers]
evens = [number for number in numbers if number % 2 == 0]

These versions are often easier for Python learners to read.

Sorting With Lambda

Lambda is common with sorted() when choosing a key.

students = [("Asha", 88), ("Riya", 92)]
ordered = sorted(students, key=lambda student: student[1])

The lambda tells Python to sort by the score.

Common Mistakes

A common mistake is writing too much logic in a lambda. Lambda functions are limited to a single expression and quickly become hard to read.

Another mistake is forgetting that map() and filter() return iterators. If you print the object directly, you will not see the final list of values.

Quick Summary

• lambda creates a small anonymous function.
• Lambda syntax is lambda arguments: expression.
• map() transforms each item.
• filter() keeps items that pass a test.
• map() and filter() return iterators in Python 3.
• Comprehensions are often clearer for simple transformations.
• Use normal def functions for complex logic.

Practice Quiz

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