asked for the sorting function, I was told a function has no name. Is this

possible? Doesn't every function need a name? The answer is no. There are

functions that have no name at all. These nameless functions are known as

anonymous functions or lambda expressions. Today, we will learn how to

write and used lambda expressions in Python.

Get ready for some pithy anonymity.

Suppose you want to write a function that will compute the value of 3x plus 1.

The standard approach would be to define a function. Let's call it f with a single

input X. Next, you would return the value 3x plus 1. If you input 2, you get the

value 7, so it works nicely. Let us now do this using anonymous

functions. Before we get started, a quick note. Throughout this video we will use

the terms "anonymous functions" and "lambda expressions" interchangeably. They both

mean the same thing. To create a lambda expression, you type the keyword lambda,

followed by your inputs. Next, type a colon. Finally, enter an expression that

will be the return value. This anonymous function will take the input X and

return 3x plus 1, just like the earlier function f. There is a problem, however. We

cannot use this function because it does not have a name. It is, after all

anonymous. lambda is not the name of the function.

It is a Python keyword that says what follows is an anonymous function. So how

do you use it? One way is to give it a name. Let us call this lambda expression

G. Now, you can use this like any other function. If you input 2, you still get 7.

Let us now see a lambda expression with more than one input. Suppose you are

processing user data from a web registration form, and would like to

combine the first and last names into a single full name for displaying on the

user interface. We will call this lambda expression full name. This anonymous

function will have two inputs: first name and last name.

For both the first and last names, we will remove the leading and trailing

whitespace with the strip function. We will also ensure that only the first

letter of each string is capitalized with the title function. This is

necessary because humans are sloppy when typing. Notice we separated the first and

last names with a space. Let us now test this lambda expression.

You use it just like any other function. Outstanding. We should not judge Euler's

typing skills. This is the first time he has ever used a computer. Here is the

general way to create a lambda expression: you type the keyword lambda

followed by zero or more inputs. Just like functions, it is perfectly

acceptable to have anonymous functions with no inputs. Next, type a colon. Then

finally, you enter a single expression. This expression is the return value. You

cannot use lambda expressions for multi-line functions. Let us now see a

common use of lambda expressions where we do not give it a name. Suppose we have

a list of science fiction authors. We would like to sort this list by last

name. Notice that some of these authors have a middle name, while others have

initials. Our strategy will be to create an anonymous function that extracts the

last name, and uses that as the sorting value. Lists have a built-in method

called sort. To see how to use it call the help function on the method name. The

key argument is a function that will be used for sorting. We will pass it a

lambda expression. To access the last name, split the string into pieces

wherever it has a space. Next, access the last piece by index negative 1. As a

final precaution, convert the string to lowercase. This way, the sorting is not

case-sensitive. Trust me - some people do not know how to

use the shift key. The list is now in alphabetical order.

These names are a pleasure to read.

We must go deeper. Next we will write a function that makes functions.

Suppose you are working with quadratic functions. Perhaps you are

computing the trajectories of cannonballs - something you should know

how to do before becoming a pirate. To do this, let's write a function called build

quadratic function. The inputs are the three coefficients A, B, and C. Naturally,

we write a docstring. And with a single line we return an

anonymous quadratic function with these coefficients. Let's test this by creating

the function 2x squared plus 3x minus 5. If you test this for the input 0 1 & 2

you can see this function works correctly. And just for bites and giggles,

let's make and use a quadratic function without ever giving it a name. Let's

create a different function and then pass in the value 2. This code creates

the function 3x squared plus 1 and passes in the value 2 which should give

us 13... and it does. This is a useful demonstration, but it is not the most

readable code. Sometimes, an extra line is perfectly fine.

Lambda expressions are quite useful when you need a short, throwaway function.

Something simple that you will only use once.

Common applications are sorting and filtering data.

And while we are on the subject, did you know that Socratica has a sorted

list of Python videos? Unlike lambda expressions, we would prefer NOT to

remain anonymous... so if you know someone who is learning Python or SHOULD learn

Python, please, send them our way.

We will upgrade their knowledge banks as best we can...