Python Functions | Python Tutorial for Absolute Beginners #1

so far you have learned how to use some of the built-in functions in Python such as print round and so on in this section you're gonna learn how to write your own functions now you might ask but why do you even need to write our own functions well when you build a real program that program is going to consist hundreds or thousands of lines of code you shouldn't write all that code in one file like we have done so far you should break that code into smaller more maintainable and potentially more reusable chunks we refer to these chunks as functions so let me show you how to create your own custom functions we start with the DEF keyword which is short for define next we need to give our function and name so let's call this greet all the best practices you'll learn about naming your variables also apply to naming your functions so make sure your function names are meaningful descriptive use lowercase letters to name your functions and an underscore to separate multiple words now after the name we need to add parentheses you will see why shortly and then we'll add a colon now what is going to happen you know it you're gonna get indentation which means the following statements will belong to this function so here I'm gonna add two statements hi there and welcome aboard both designs belong to this function because they're indented now we're done with this function we need to call it so we remove the indentation and we add two line breaks after this function this is what Papp 8 recommends to keep our code clean and maintainable now if you forget to add two line breaks don't worry as soon as you save the changes auto-pay of eight will automatically add these line breaks for you let me show you so I'm gonna remove these line breaks and call this function create with parentheses just like how we call the built-in functions now save the changes there you go so we get two line breaks after our function now let's run this program so we get these two messages on a terminal now here is a question for you what is the difference between the greed and trent functions the difference is that this print function takes an input whereas our greet function doesn't take any inputs so let me show you how to pass inputs like first name and last names of this function when defining a function in between parentheses will list our parameters so here we add two parameters like first under like name and last underline name now when calling this function we need to supply two values for those parameters we refer to them as arguments so maash Hamadan e these are the arguments to the greet function that's one of the terms that a lot of developers out there don't know they don't know the difference between parameters and arguments a parameter is the input that you define for your function whereas an argument is the actual value for a given parameter okay now let's change line two and instead of saying hi there we can greet a person by their full name so we can convert this to a formatted string and has two fields here first name as well as last name save the changes run the program and this is what we get in terminal now this function is more useful we can reuse it and call it with different arguments so let's greet John Smith as well run the program so we get hi Muhammad on e and hi John Smith now note that by default all the parameters that you define for a function are required so here our greet function takes two parameters if I exclude one of these arguments and save the changes you can see we have this red underline so pilant is complaining and saying there is no value for argument last name also if we run the program we get this type error greet missing one required positional argument so let's put this back now later I will show you how to define optional parameters so this is the simplified version of this greet function with created earlier now in programming we have two types of functions functions that perform a task and functions that calculate and return a value here are some examples both the print and greet functions are examples of type 1 the performing a task which is printing something on the terminal in contrast the brown function is an example of a function that calculates and returns a value so the functions that you create fall into these two categories now let me show you how to rewrite this great function but in the second form so instead of printing this string on the terminal we simply return it let me show you so I'm gonna delete all this code define a new function but call it get underline greeting we add the name parameter and simply return this formatted string high name that's all we have to do so we use the return statement to return a value from this function now we can call this function get underlined greeting pass a name like Maj because it returns a value we can store that value and a separate variable like message now you might be curious which form of this greeting functions is better well with this first implementation we are locked to printing something in the terminal in the future if you want to write that message in a file or send it in an email we have to create another function so we cannot reuse this greet function in other scenarios in contrast this second form is not tied to printing something on the terminal it simply returns a value now we get this value and we can do whatever we want with it we can print it on the terminal or we can use the built-in open function to write this message to a file so we can create a I like Content txt open it for writing this returns a file object and then we can call file the right message now don't worry about these two lines later in the course I'm gonna talk about working with files but what I want you to take away here is that we have this message variable and we can do whatever we want with it we can print it on the terminal write it to a file send it in an email and so on and one more thing before we finish this lecture so here's our greet function and as you can see we're simply printing a string now if we call great give it a name run the program we get this message hi Maj but what if we put this inside of a call to the print function let's see what we get we get high Marsh followed by none what is this none is the return value of the greet function so in Python all functions by default return the non value none is an object that represents the absence of a value later in the course you're gonna learn more about not what matters now is that all functions return non by default unless you specifically return a value so here if we return some string none will no longer be returned now I just want to clarify something earlier I told you that we have two types of functions in programming functions that carry out the tasks or functions that calculate and return a value so back to the code we previously had so even though this function returns none by default it is still classified as a function that carries out a task let's create another function we call it increment you want to use this function to increment a number by a given value so here we simply return number plus by now we can call this function like this increment two and one this returns a value so we can store it in a variable like result and then print it on the terminal let's run the program we get three beautiful now we can simplify this code we have used this result variable only in a single place that is line six so we don't really need it so on line six we can replace result with a call to increment function like this so when python interpreter executes this code first it will call the increment function it will get the result and temporary store it in a variable for us we don't see that variable and then it will pass that variable as an argument to the print function now if we run this program we get the exact same result beautiful now we can make this code more readable if someone else looks at line five they may not know exactly what these arguments are for we can use a keyword argument to make this code more readable so this one here is the value of this by parameter we can prefix it with the name of the parameter like this now we can read this code almost like playing English increment two by one so if you're calling a function with multiple arguments and it's not quite clear what is arguments are for you can make your code more readable by using keyword arguments so here y equals one is a keyword argument earlier I told you that all the parameters that you define for a function are required by default in this lecture I'm gonna show you how to make the by parameter optional so let's say we don't want to explicitly pass by equals one every time we want to call this increment function you want to use this function to increment a value by one so we remove the second argument now we need to give this parameter a default value so we set it to one now if we call this function and don't supply the second argument this default value will be used otherwise the value that we specify here will be used let me show you so we run this program the result is three but if we pass the second argument here we'll increment two by five so we will get seven so you can see it's pretty easy to make a parameter optional just be aware that all these optional parameters should come after the required parameters in other words I cannot add another required parameter here let's call that another I cannot add that here if I save the changes you can see we get a red underline here so all the optional parameters should come after the required parameters now obviously in this case we don't need the second parameter so let's delete it there are times that you may want to create a function that takes a variable number of arguments here is an example let's define this function multiplied that takes two parameters x and y and simply returns x times y now we can call this function like this so far so good but what if you want to pass one or two more arguments here that doesn't work because our multiply function takes only two parameters to solve this problem we need to replace these two parameters with a single parameter we use a plural name here to indicate that this is a collection of arguments and then we prefix it with an asterisk this is the magical part let me show you what happens when you use an asterisk here so temporarily let's delete this line and simply print numbers let's see what we get here so run the program we can see all our arguments and they're packed in parentheses what is this well earlier you learned about lists I briefly mentioned that you can use square brackets to create a list of objects like 2 3 4 5 now later in the course we have a comprehensive section about lists so don't worry about the details of lists and how they work but what I want you to note here is that the only difference between this list and what we have here is in the notation so use square brackets to create lists and parentheses to create topples some people call it two poles or two poles so a top pole is similar to a list in that it's a collection of objects the difference is that we cannot modify this collection we cannot add a new object to this topple once again later in the course we're going to have a comprehensive section about lists tuples and other data structures what matters now is that these topples just like lists are each ruble so we can each rate over them which means we can use them in loops let me show you so let's write for number in numbers : let's just print one number at a time actually we don't need this line delete and run the program so we iterate over this topple and in each iteration we get one number and print it on the terminal so now with a simple change we can calculate the product of all numbers all we have to do is to define a variable like total initially we set it to 1 and then in each iteration we get total and multiplied by the current number or we can rewrite this statement using an Augmented assignment operator so total times equal number line 5 and 4 are exactly identical so I'm gonna use line 5 because it's shorter and cleaner delete and finally we'll return the total now one of the issues I see often in beginners code is that they don't use this indentation properly so they put the return statement here and then they wonder why their function doesn't work properly if you put the return statement here it will be part of the for loop so it will be executed in each iteration in this case after the first iteration because of this return statement we'll return from this multiply function so the total will not be calculated properly we need to put this at the same level of indentation as other statements in this function so here we have our for statement we loop over all the numbers we calculate the total and then finally return it so with this implementation we can get the result and printed on the terminal let's run the program and you can see the product of these numbers is 120 so in the last lecture you learned the syntax to pass a variable number of arguments to a function we have a variation of this syntax with double asterisk and that's what I'm going to show you in this lecture so for this demo let's rename this function to save underline user and rename the argument to user so let's imagine we're gonna use this function to save information about a user now in this function let's just print this user argument we're done with our function let's call it save underline user now instead of passing arbitrary arguments here we can pass arbitrary keyword arguments remember keyword arguments so we have name equals value so here we can add multiple keyword arguments like ID equals 1 name equals John age equals 22 and so on so we are passing 3 keyword arguments to this function now let's run this program this is what we get look at the syntax we have these curly braces and in between them we have multiple key value pairs so key colon value comma here's another key value pair and here is the last one this object you see here is called a dictionary it's another complex type or a data structure in Python and I'm going to talk about that in detail later in the course for now all I want you to take away is that when we use double asterisk here we can pass multiple key value pairs or multiple keyword arguments to a function and Python will automatically package them into a dictionary so this user object here is a dictionary now using the bracket notation we can get the value of various keys in this dictionary so we can print user square brackets we pass a string and this is the name of our key like ID let's run the program we get 1 or we can access the value of name key the program so name is John so this is how deeksha Mary's work in programming we have a very important concept called scope which refers to the region of the code where a variable is defined so in this example we have this message variable the scope of this variable is the greet function it only exists inside of this function so if we go outside of this function and try to print message see what happens as soon as I save the changes we get this red underlined undefined variable message and if we run our program we get this name error name message is not defined the same rule applies to the parameters of our functions so if you add a name parameter here and then try to print it outside of the greet function we get the same error so the scope of the name and message variables are the greet function and we refer to these variables as local variables in this function they're local in this function which means they don't exist anywhere else and that means we can have another function let's say send email with a parameter with the same name here we can have a message variable but this message variable is completely different from the message variable we have in the greet function and of course the same rule applies to the name parameters in these functions they are completely separate these local variables have a short lifetime so when we call let's say the greet function and pass a name Python interpreter will allocate some memory and have the name and message variables reference those memory locations when we finish executing the greet function because these variables are not referenced or used anywhere else eventually they get garbage collector which means python interpreter will release the memory that allocated for these variables so these are the local variables in contrast to local variables we have global variables so if we move this message variable outside of great function now it's a global variable which means it's accessible anywhere in this file so the scope of this variable is this file we can use it anywhere in this file in any functions or outside of a function now for this reason global variables stay in memory for a longer period of time until they are garbage collected and you should not use them that often in fact global variables are really evil so as a best practice create functions with parameters and local variables now what if here in the greet function we set message to let's say B now let's delete this second function we don't really need it for now so we call the greet function in this function we assign a new value to the message variable now let's print this message and see what we get what do you think we're gonna get well let's take a look we get a but do we change the value of message variable to be not really because by default python interpreter treats this message variable as a local variable in the greet function even though it has the same name as the global variable and we have on line one so these two variables are separate now I've seen some tutorials or even books teaching you bad practices for example they show you how to modify the value of this global message variable inside of a function let me show you how to do that but remember this is a bad practice and you should avoid it at all times I will explain the reason in a second so these tutorials or books teach you to use the global keyword here and reference the message variable when python interpreter sees this line it will realize that in this function we want to use the global message variable so it will not define a local variable in this function now with this change if we run our program you can see we get beyond the terminal because in this function we are modifying the value of the global message variable why is this bad because it is possible that you might have multiple functions that rely on the of this global variable if you're accidentally or deliberately change the value of this global variable in one function this might have a side effect in other functions those functions may not behave properly and this way we can create a bug in our program so global variables have always been bad this is not a Python thing this is a discussion that's been going on for decades avoid them as much as you can or if there are situations that you really need to define a variable globally in a file that's okay but do not modify it in a function as you see here this is a really bad practice so that's all about local and global variables one of the questions that often comes in programming interviews is the fizzbuzz algorithm you might be surprised that there are a lot of developers out there with even years of experience but they don't know how to solve this simple programming problem that's why I decided to include this in my course so let's see how this algorithm works and then you can spend 10 to 15 minutes on this exercise on your own so here we have a function phase buzz that takes on input and depending on the input we give it it returns different results here are the rules if the input that we give it is divisible by 3 it will return the string fist let me show you so we run this program we get Fitz if the input is divisible by 5 it will return Buzz take a look here's buzz now the third rule if the input is divisible by both 3 & 5 it will return fizzbuzz so let's pass 15 here we'll get fizzbuzz for any other numbers it will return the same input so if I pass 7 it's not divisible by 3 or 5 it will return 7 itself so spend 10 to 15 minutes on this exercise you will see my solution next all right let's see how we can solve this problem so we get the input and check to see if input is divisible by 3 then we can set a variable like result to phase now let me temporarily simplify these rules because I want to show you a really cool technique so let's imagine if the input is divisible by 3 we'll return fizz otherwise we'll return bus so LS : we set the result to buzz and finally we'll return this resolved variable this is one way to implement these rules but it's not the best way there is a better and cleaner way let me show you so instead of defining this result variable and then returning it we can simply return this string here and also here and then we can get rid of line six so we remove one unnecessary line of code this was one technique the other technique I want to show you is when you have an if statement and in your a flock you're returning a value you don't really need an else clause because if this condition is evaluated to false the control moves to the else clause and all we're doing here is returning a different value so this is the only possible outcome so here we can remove this else clause and simply return bus so with this technique I remove an unnecessary indentation as well I've made our code more linear okay now let's go back to our original algorithm so if the input is divisible by three we'll return fizz now we check to see if the input is divisible by five we'll return buzz so here we don't really need an Elif because if this condition is false we'll get here anyway so we can check if input is divisible by five we will return buzz once again if this condition is false the control moves here now we check to see if the input is divisible by three and it is also divisible by five then we will return fizz buzz now this expression is a little bit long I would prefer to make it more readable by putting parentheses around the first and second parts like this that is cleaner now if this condition is also false the control moves here that means our number is not divisible by three or five so we should simply return the input once again we don't need an else or another if we simply return the input now let's run this program and see if it's working properly or not so we call our function with three and we get fizz beautiful let's call it with five we get buzz perfect what if we call it with 15 we should get fizz bus however we get fizz what is going on here the reason this happened was because when we pass 15 here line two is executed obviously 15 is divisible by three so that is why we immediately return fizz to solve this problem we need to move this condition which is more specific to the top so I've selected these two lines using alt + up I move these lines up beautiful alright now let's run the program one more time so we get fizzbuzz once again let's test it with three now we get fares let's test it with five when writing functions don't assume that your function is working make sure to test it with all different kinds of values let's test it but five we get Buzz beautiful what if you pass seven which is not divisible by three or five we get seven beautiful hi guys thank you for watching this tutorial my name is Mohammad Ani and I have tons of tutorials like this for you on my channel so be sure to subscribe and also please like and share this video if you want to learn Python properly from scratch with depth I have a comprehensive python tutorial for you the link is below this video so click the link to get started thank you and have a fantastic day

Channel: Programming with Mosh

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Keywords: python functions, functions in python, python function, python kwargs, python def, python args, python *args, python **kwargs, python language, python for beginners, python for absolute beginners, python basics, getting started with python, python tutorial, python 3, python, args, kwargs, **kwargs, def, *args, absolute beginners, tutorial, beginners

Id: u-OmVr_fT4s

Length: 30min 33sec (1833 seconds)

Published: Tue Nov 06 2018