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In this video we will cover functions. You will learn how to use some of Python’s built-in

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functions as well as how to build your own functions.

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Functions take some input then produce some output or change.

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The function is just a piece of code you can reuse.

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You can implement your own function, but in many cases, you use other people’s functions.

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In this case, you just have to know how the function works and in some cases how to import

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the functions. Let the orange and yellow squares represent

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similar blocks of code. We can run the code using some input and get

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an output. If we define a function to do the task we

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just have to call the function. Let the small squares represent the lines

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of code used to call the function. We can replace these long lines of code by

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just calling the function a few times. Now we can just call the function; our code

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is much shorter. The code performs the same task.

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You can think of the process like this: when we call the function f1, we pass an input

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to the function. These values are passed to all those lines of code you wrote.

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This returns a value; you can use the value. For example, you can input this value to a

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new function f2. When we call this new function f2, the value

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is passed to another set of lines of code. The function returns a value.

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The process is repeated passing the values to the function you call.

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You can save these functions and reuse them, or use other people’s functions.

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Python has many built-in functions; you don't have to know how those functions work internally,

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but simply what task those functions perform. The function len takes in an input of type

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sequence, such as a string or list, or type collection, such as a dictionary or set, and

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returns the length of that sequence or collection. Consider the following list.

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The len function takes this list as an argument, and we assign the result to the variable L.

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The function determines there are 8 items in the list, then returns the length of the

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list, in this case, 8. The function sum takes in an iterable like

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a tuple or list and returns the total of all the elements.

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Consider the following list. We pass the list into the sum function and

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assign the result to the variable S. The function determines the total of all the

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elements, then returns it, in this case, the value is 70.

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There are two ways to sort a list. The first is using the function sorted.

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We can also use the list method sort. Methods are similar to functions.

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Let's use this as an example to illustrate the difference.

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The function sorted Returns a new sorted list or tuple.

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Consider the list album ratings. We can apply the function sorted to the list

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album ratings and get a new list sorted album rating.

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The result is a new sorted list. If we look at the list album ratings, nothing

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has changed. Generally, functions take an input, in this

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case, a list. They produce a new output, in this instance, a sorted list.

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If we use the method sort, the list album ratings will change and no new list will be

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created. Let's use this diagram to help illustrate

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the process. In this case, the rectangle represents the

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list album ratings. When we apply the method sort to the list,

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the list album rating changes. Unlike the previous case, we see that the

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list album rating has changed. In this case, no new list is created.

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Now that we have gone over how to use functions in Python, let’s see how to build our own

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functions. We will now get you started on building your

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own functions in python. This is an example of a function in python

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that returns its input value + 1. To define a function, we start with the keyword

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def. The name of the function should be descriptive

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of what it does. We have the function formal parameter "A"

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in parentheses. Followed by a colon.

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We have a code block with an indent, for this case, we add 1 to "A" and assign it to B.

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We return or output the value for b. After we define the function, we can call

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it. The function will add 1 to 5 and return a

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6. We can call the function again; this time

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assign it to the variable "c" The value for 'c' is 11.

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Let's explore this further. Let's go over an example when you call a function.

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It should be noted that this is a simplified model of Python, and Python does not work

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like this under the hood. We call the function giving it an input, 5.

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It helps to think of the value of 5 as being passed to the function.

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Now the sequences of commands are run, the value of "A" is 5.

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"B" would be assigned a value of 6. We then return the value of b, in this case,

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as b was assigned a value of 6, the function returns a 6.

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If we call the function again, the process starts from scratch; we pass in an 8.

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The subsequent operations are performed. Everything that happened in the last call

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will happen again with a different value of "A"

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The function returns a value, in this case, 9.

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Again, this is just a helpful analogy. Let’s try and make this function more complex.

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It's customary to document the function on the first few lines; this tells anyone who

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uses the function what it does. This documentation is surrounded in triple

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quotes. You can use the help command on the function

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to display the documentation as follows. This will printout the function name and the

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documentation. We will not include the documentation in the

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rest of the examples. A function can have multiple parameters.

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The function mult multiplies two numbers; in other words, it finds their product.

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If we pass the integers 2 and 3, the result is a new integer.

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If we pass the integer 10 and the float 3.14, the result is a float 31.4.

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If we pass in the integer two and the string “Michael Jackson,” the string Michael

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Jackson is repeated two times. This is because the multiplication symbol

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can also mean repeat a sequence. If you accidentally multiply an integer and

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a String instead of two integers, you won’t get an error.

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Instead, you will get a String, and your program will progress, potentially failing later because

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you have a String where you expected an integer. This property will make coding simpler, but

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you must test your code more thoroughly. In many cases a function does not have a return

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statement. In these cases, Python will return the special

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“None” object. Practically speaking, if your function has

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no return statement, you can treat it as if the function returns nothing at all.

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The function MJ simply prints the name 'Michael Jackson’.

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We call the function. The function prints “Michael Jackson.”

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Let's define the function “No work” that performs no task.

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Python doesn’t allow a function to have an empty body, so we can use the keyword pass,

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which doesn’t do anything, but satisfies the requirement of a non-empty body.

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If we call the function and print it out, the function returns a None.

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In the background, if the return statement is not called, Python will automatically return

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a None. It is helpful to view the function No Work

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with the following return statement. Usually, functions perform more than one task.

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This function prints a statement then returns a value.

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Let's use this table to represent the different values as the function is called.

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We call the function with an input of 2. We find the value of b.

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The function prints the statement with the values of a and b.

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Finally, the function returns the value of b, in this case, 3.

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We can use loops in functions. This function prints out the values and indexes

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of a loop or tuple. We call the function with the list album ratings

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as an input. Let's display the list on the right with its

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corresponding index. Stuff is used as an input to the function

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enumerate. This operation will pass the index to i and

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the value in the list to “s”. The function would begin to iterate through

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the loop. The function will print the first index and

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the first value in the list. We continue iterating through the loop.

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The values of i and s are updated. The print statement is reached.

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Similarly, the next values of the list and index are printed.

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The process is repeated. The values of i and s are updated.

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We continue iterating until the final values in the list are printed out.

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Variadic parameters allow us to input a variable number of elements.

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Consider the following function; the function has an asterisk on the parameter names.

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When we call the function, three parameters are packed into the tuple names.

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We then iterate through the loop; the values are printed out accordingly.

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If we call the same function with only two parameters as inputs, the variable names only

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contain two elements. The result is only two values are printed

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out. The scope of a variable is the part of the

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program where that variable is accessible. Variables that are defined outside of any

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function are said to be within the global scope, meaning they can be accessed anywhere

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after they are defined. Here we have a function that adds the string

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DC to the parameter x. When we reach the part where the value of

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x is set to AC, this is within the global scope, meaning x is accessible anywhere after

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it is defined. A variable defined in the global scope is

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called a global variable. When we call the function, we enter a new

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scope or the scope of AddDC. We pass as an argument to the AddDC function,

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in this case, AC. Within the scope of the function, the value

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of x is set to ACDC. The function returns the value and is assigned

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to z. Within the global scope, the value z is set

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to ACDC After the value is returned, the scope of

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the function is deleted. Local variables only exist within the scope

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of a function. Consider the function thriller; the local

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variable Date is set to 1982. When we call the function, we create a new

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scope. Within that scope of the function, the value

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of the date is set to 1982. The value of date does not exist within the

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global scope. Variables inside the global scope can have

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the same name as variables in the local scope with no conflict.

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Consider the function thriller; the local variable Date is set to 1982.

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The global variable date is set to 2017. When we call the function, we create a new

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scope. Within that scope, the value of the date is

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set to 1982. If we call the function, it returns the value

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of Date in the local scope, in this case, 1982.

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(click6) When we print in the global scope, we use the global variable value.

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The global value of the variable is 2017. Therefore, the value is set to 2017.

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If a variable is not defined within a function, Python will check the global scope.

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Consider the function "AC-DC“. The function has the variable rating, with no value assigned.

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If we define the variable rating in the global scope, then call the function, Python will

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see there is no value for the variable Rating. As a result, python will leave the scope and

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check if the variable Ratings exists in the global scope. It will use this value of Ratings

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in the global scope within the scope of "AC-DC“. In the function, will print out a 9.

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The value of z in the global scope will be 10, as we added one.

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The value of rating will be unchanged within the global scope.

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Consider the function Pink Floyd. If we define the variable Claimed Sales with

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the keyword global, the variable will be a global variable.

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We call the function Pink Floyd. The variable claimed sales is set to the string

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“45 million” in the global scope. When we print the variable, we get a value

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of “45 million.” There is a lot more you can do with functions.

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Check out the lab for more examples.

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(Music)