Anonymous Functions

The act of defining a function using the def keyword binds that function to a name. However, some functions can be defined without giving them a name. Such functions are called “anonymous” and are defined using the lambda keyword.

lambda <parameter_list> : <function_body>

The following two definitions are equivalent.

def add(a, b):
  return a + b


add = lambda a, b: a + b

The expression to the right of the assignment operator is called a “lambda expression”. The Python interpreter takes this expression and defines a function object which can be bound to an identifier (in this case, add). There is no difference between binding a function to a name using the assignment operator or by using the def keyword.

Parameters are optional when defining an anonymous function. However, a function body must be present, and it must only contain a single return expression.

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

odd_numbers = filter(lambda n : n % 2 == 1, numbers)

print(list(odd_numbers))
# Output: [1, 3, 5, 7, 9]

Anonymous functions can also be evaluated immediately after they are defined, similar to an immediately-invoked function expression (IIFE) in JavaScript.

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Anonymous functions are useful when the function can be written in a single line. Otherwise, if the function is more complex, it is recommended to use the def keyword.