Anonymous functions

One last type of function that we want to talk about are anonymous functions. These functions, which are called lambdas in Python, are usually used when a fully fledged function with its own name would be overkill, and all we want is a quick, simple one-liner that does the job.

Imagine that we wanted a list of all the numbers up to a certain value of N that are also multiples of five. We could use the filter() function for this, which will require a function and an iterable as input. The return value is a filter object that, when you iterate over it, yields the elements from the input iterable for which the function returns True. Without using an anonymous function, we might do something like this:

# filter.regular.py
def is_multiple_of_five(n):
    return not n % 5
def get_multiples_of_five(n):
    return list(filter(is_multiple_of_five, range(n)))

Note how we use is_multiple_of_five() to filter the first n natural numbers. This seems a bit excessive...