Anonymous functions with lambda expressions¶

To further drive home the idea that we are passing a function object as a parameter to the sorted object, let’s see an alternative notation for creating a function, a lambda expression. The syntax of a lambda expression is the word “lambda” followed by parameter names, separated by commas but not inside (parentheses), followed by a colon and then an expression. lambda arguments: expression yields a function object. This unnamed object behaves like a function object defined with

def fname(arguments):
    return expression

Consider the following code

Note the paralells between the two. At line 4, f is bound to a function object. Its printed representation is “<function f>”. At line 8, the lambda expression produces a function object. Because it is unnamed (anonymous), its printed representation doesn’t include a name for it, “<function <lambda>>”. Both are of type ‘function’.

A function, whether named or anonymous, can be called by placing parentheses () after it. In this case, because there is one parameter, there is one value in parentheses. This works the same way for the named function and the anonymous function produced by the lambda expression. The lambda expression had to go in parentheses just for the purposes of grouping all its contents together. Without the extra parentheses around it on line 10, the interpreter would group things differently and make a function of x that returns x - 2(6).

Some students find it more natural to work with lambda expressions than to refer to a function by name. Others find the syntax of lambda expressions confusing. It’s up to you which version you want to use. In all the examples below, both ways of doing it will be illustrated.

Below, sorting on absolute value has been rewritten using lambda notation and the built-in function abs.

Of course, it’s unnecessary to make an anonymous function that takes an input and just calls an existing function on it. That’s equivalent to just providing the existing function as a lambda expression. You may find, however, that the lambda expression above helps you understand what sorted does with the function that is passed in: it calls the function on each of the items in the list that is passed to sorted. Make sure you understand why the code above and the code immediately below cause the list to be sorted the same way.