Lambda

A small anonymous function written in one line with the lambda keyword.

A lambda is a tiny function with no name, written inline: lambda x: x * 2. It lists its arguments before the colon and returns the single expression after it — no def or return needed.

Lambdas shine as a quick argument to functions like sorted(), map(), or filter() when a full named function would be overkill. For anything longer, use def.

Example 
double = lambda x: x * 2
print(double(5))

pairs = [("a", 3), ("b", 1), ("c", 2)]
pairs.sort(key=lambda pair: pair[1])   # sort by the number
print(pairs)
Output 
10
[('b', 1), ('c', 2), ('a', 3)]

Where this shows up in real Python

Lambdas shine as short, throwaway functions passed to other functions — most often the key= for sorting or finding a max.

Commonly used Lambda tools

  • lambda x: x * 2 — an inline, unnamed function
  • sorted(items, key=lambda r: r['age']) — sort by a computed key
  • max(items, key=lambda x: len(x)) — pick by a computed value

Official documentation: Python Tutorial: Lambda Expressions

Related lessons

Related terms