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5.6. Lambda Expressions

Lambda expressions are small anonymous functions that can be defined in a single line. They are called anonymous because they don't have a name on their own.

Syntax

Anonymous functions are defined using the lambda keyword followed by list of parameters (if any) and the expression inside the function.

adder = lambda a, b: a + b
print(adder(5, 2))

7

Here, adder is an anonymous function that adds two integers. If we look at the syntax:

  • The arguments function takes are defined before colon : and after lambda keyword
  • The expression that the function returns is written after colon :

Lambda can only contain one expression after the : but supports all features of parameters shown in section 5.2. Arguments including default value, variadic arguments and positional/keyword-only arguments.

Application

Lambda functions are generally used when there is a need of a small function temporarily in some code.

Another use of these functions are in closures that are defined using nested functions.

For example:

def factor(a):
    return lambda b: b * a

doubler = factor(2)
tripler = factor(3)

print(doubler(20))
print(tripler(20))

40
60