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[no audio]

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Friends here we are going to

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discuss about variable or variables, and we

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are going to see, what is a variable, how to declare a variable

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and use, and displaying a variable value using print, re-declare

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a variable, and how to delete a variable, and finally

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we are going to see rules to define a variable.

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Let me start with very first one, that is what is a variable.

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So before going to enter with your Python variable, let me

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explain your known thing, that is in your mathematics suppose

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if you write x=2,

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and then some y=3 x + 2, what you are doing?

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Any number 3, or 4, or 5, whatever it may be

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so if x=2, what is y value?

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Then you are saying, y=3 x 2=> 6=>6+4=>10, right

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Now here x is a variable, y is a variable, so you are assigning

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2 to x, and finally you are getting y value as 10, or simply

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10 assigned to Y.

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Now you can say finally after completion of this equation,

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you can say simply x value is 2, and y value is 10.

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Right. Now here

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we are saying that x and y are variables. Same meaning in

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your mathematics also.

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Right. But here in your mathematics simply you are assigning

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2 to x, and you are saying x=2 on paper, but here

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whenever if you define x=2 in your Python program

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or script what will happen means, your python will assign

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some memory location for your x,

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and in that memory location the value will be stored, and

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whenever if you want to access this value always you have to use x

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That's it.

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[no audio]

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So let me go with simple meaning for your variable.

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So a variable is nothing but a reserved memory location to

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store values or value.

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So whenever if you want to do some action, right, on your variable,

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yes, you can do it by taking that variable value. Right. How to

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take and how to use we will see. Then how to declare and use a variable?

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[no audio]

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No need to declare a variable before using them.

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That means, suppose guys

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I want to store x value as 2 in my Python program, or I want

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to store y value equals some 4.5. Now if you observe here

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x is an integer and y is a float point.

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So before going to assign 2 to x or 4.5 to y, you don't

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need to tell to your Python,

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I'm going to take x as an integer, 'int x', or y as integer, 'float y'.

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You don't need to tell this information to your Python. Python

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will automatically identify which type of data you are storing

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into your 'x',

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and that's why Python is called dynamic type language.

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Okay. Now let me show you this on your command line, or maybe through

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program, through script, or using your Python command line, right?

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So what you have to do is just open some editor, guys

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you can use either command line editors or if you have some GUI editors

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you can also use that, right. There write

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some line as 'a' equals to some 10, right. And if you want

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to print your 'a' value, you have to write 'print (a)', so that

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the value of 'a' will be printed, and already we have seen, right

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if you want to print a string using 'print' statement you are

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writing 'print', parentheses, inside of quotation some string,

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but whenever if you are printing a variable value don't use

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quotations. If you use quotations it will print as it

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is. I mean, let me do one thing.

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I am going to open my command line.

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[no audio]

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Okay.

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[no audio]

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Now I'm entering into my Python terminal.

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So guys whatever the line I am typing here the same lines

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you can write in your program as well.

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Let's say I am writing 'x' equals to some 4 number.

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Now, this is very first line in your program. Next line

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I am writing 'print(x)'. Now you can see the result.

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But if I write 'print("x")', it will print as it is,

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I mean 'x' will be printed as it is.

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So if 'x' is a variable you have to print without quotations

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for your variable. If it is not a variable and if you want

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to feel that as a string itself, right, you can print it in this way

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also. If you print in this way, you will get only 'x' not the 'x' value.

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[no audio]

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First of all, if you print in this way there is no relationship

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between your 'x' and this "x", because this is a

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string and this is our variable.

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Right. See if I print simply "Hello',

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[no audio]

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it is printing your string as it is because you are providing

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your message, or string, or text inside of quotations

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that's why your 'print' will print as it is.

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[no audio]

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Fine. Now, let me take 'y' equal to some number 5.6. Now I am

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going to print 'y' value.

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Yes, you are getting. And see that

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I want to know the type of your 'x' and 'y'. 'type(x)', integer

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'print', same lines

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we can write all lines in a program, and you can execute guys.

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'y', float.

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[no audio]

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Right. Let me open an editor.

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So I am going to use here Sublime Text.

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Of course, you can use 'vim' or 'PyCharm', right,

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whatever editor you prefer, right.

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So now let me use Sublime Text,

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[no audio]

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sorry, yeah Sublime Text.

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[no audio]

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So I'm going to open Sublime Text. Now using that Sublime

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Text whatever the lines you executed on your Python terminal,

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Python Shell, Python command line, same lines

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I am going to execute.

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Right. Let me copy line-by-line, and let me paste it here somewhere.

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So I'm going to save this as,

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[no audio]

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let me save first my program.

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Okay. Let me go to Desktop, because there we have some directory

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called 'PythonScripts'.

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So I am going to write here

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'working_with_variables.py', any program name you have

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to take, right?

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So 'x=4', is the first line you've done on your terminal,

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then 'print(x)'.

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[no audio]

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That's it. Now save it and run it. To run your program from

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your script from Sublime Text

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you have to run 'Ctrl + B'.

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You can see the output. So you know the advantage by

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using GUI editors is you can see your program here itself

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and also output, right. Then

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let me take one more thing called 'print("x")'.

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So guys, you have to know the difference between command

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line and your Python program.

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So command line what you are doing?

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You are executing line by line by typing your required line.

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Program, you're going to execute all lines one by one

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[no audio]

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with your Python. Let's say 'y=5.6'.

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I am going to declare 'y' value,

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right? Don't provide any spaces before starting your line.

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If you provide, it will give an indentation. Now

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let me print your 'y', then let me print your 'type(x)', and

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let me print your 'type(y)' as well, right.

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Now I save and I am going to run. Guys whenever if you run

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your Python program it will run from top to down.

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It will take first line, 'x=4'.

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There is no print statement.

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You won't get any output. Simply 4 value is assigned to

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'x'. Now after first line in your remaining program 'x' value

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is 4. Now because of second line, it will print the 'x'. 'x'

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value not 'x'. 'x' value means 4. Third line

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it will print 'x' as it is because you are writing 'x' as a string.

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Even fourth line "hello" will be displayed.

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Because of Fifth Line 'y' value is 5.6. Now 'print(y)', 'y' value will

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be printed. 'type(x)', what type of data you're storing into

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'x', and what type of data you're storing into 'y' that type

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of data will be displayed because of last two 'print' statements.

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And if you observe the output, final output, you're getting

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'integer' and 'float', last two 'print' statements. Observe that because

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of this Python is called dynamic type language.

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Right. So guys if you want to define any variable you have to write

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simply 'x=4', or 'y' equals some number, right. Fine.

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Now next thing is, re-declare a variable. See that very first

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line 'x=4', right. Now

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what I am doing is in ninth line

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I am writing 'x' equals some 23, and if I 'print' in tenth Line

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'x' value, you modified your 'x' value from for 4 to 23.

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See the output, last 'print' statement, 23 you are getting. So even

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while re-declaring your variable you don't need to tell to your Python

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I'm going to change my 'x' value in so and so line. Whenever if

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you want to feel to change your variable value, you are allowed.

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Directly you can change your 'x' value,

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right, in this way. Suppose very first line I define 10, then

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after some lines I'm modifying 'x' value,

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yes, you are allowed, right. Then, delete a variable.

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See first of all why we need to delete a variable value or

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variable? Just assume that guys, you're having some 1000s

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of lines of program, right,

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so assume these are the 500 lines, and these are

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the 500 lines. Somewhere here

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you define some variable value called 'x equals some 10, and

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you know that after 500 lines,

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I mean from 501 to 1000 lines,

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so here you are, no where you're using your 'x' variable value,

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right? But whenever if you run your Python program, whatever

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the variables you are defining, and whatever the operations

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you are performing, all those things will be performed from RAM memory.

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Right. So if you are not going to use here your 'x' variable value

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why should we keep that in my program?

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Why should we keep that in RAM memory? Unnecessarily I am going

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to waste my RAM memory, right?

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You can delete, you can free up your RAM memory by deleting

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your variable value if you are not using that, right. Simply

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'del x'. See that after tenth line nowhere I am using variable

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called 'x'. Then in eleventh line what I will do? Simply I will

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delete 'x'. In case after deleting

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if you try to print your 'x' value, there is no more 'x', you

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will get an error. "name 'x' is not defined", because by this line

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your Python is removing from its history your 'x'. Now there

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is no 'x' in your python,

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right, that's why you are getting an error.

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So if you are not using then only you have to delete it, right,

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so simply remember how to delete a variable value. Simply 'del x'.

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[no audio]

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Fine. Now next thing is, rules to define variable names.

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So as of now guys, we have taken variable names as like 'x'

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and 'y', but in real time you may take, suppose you are going

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to find Java version then instead of 'x' and 'y',

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I will take a variable called 'java_version=', something.

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I will write some logic to find my Java version.

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Right. So likewise, you can take some variable names. No need to

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take like a 'x' and 'y'. Suppose I am going to install some Java,

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right? So 'install_pkg', I will take simply "java",

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so this is my variable name.

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'java_version' is my variable name.

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[no audio]

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Right. So now while selecting your variable names you have to follow

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some rules, right.

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Let me go with that rules one by one.

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It contains letters, numbers, and underscore.

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So whatever the variable name you are selecting,

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it may be simply 'xyz' or 'pq', right. But also you can take

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some lengthy variable names. While taking lengthy variable

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names, your variable names should include only this group

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of, not include, it may contain either letters,

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or numbers, and underscore.

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Don't take anything

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other than from these three groups.

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Suppose '$' is not there in our three, either in this, or

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in this, or in this, so don't take '$' in your variable

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name. Suppose

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I want to define 'xy', 'xy' equals some number 10. It's fine.

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But 'x$' or '$x',

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don't take in that way.

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'$x' or 'x$',

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they are invalid variable names.

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Don't take them.

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Right. Then second one,

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it should not be a keyword.

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See actually, if we take simply 'if', right, 'if' is a valid variable

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name only based on your three groups because it is containing only letters.

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Right. But the thing is 'if' is already predefined word for

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your Python to do some particular action,

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that's why don't take predefined words as your variable names

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like 'print'. You know, what is the purpose of 'print'. 'print' is

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used to print something. And don't take 'print' as suppose some

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variable name.

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Let me take 'print' equals some number 40. Don't take 'print'

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as your variable name because 'print' is already predefined

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word for your python. Right.

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So guys, you know need to mug up about your predefined words

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or keywords, while practicing you will come to know, right.

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So after completion of your complete Python, you will come

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to know which are the keywords.

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[no audio]

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Right. Then, can't contain spaces.

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You should not include a space in your variable name.

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Let's say, let me take command line.

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Anything is fine, no.

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I am taking 'my name' equal to suppose "narendra".

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So 'my name', my variable name contains a space.

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So it is invalid.

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[no audio]

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Right. So don't take any spaces in your variable names. Then,

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it should not start with a number.

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Yeah, one more thing guys, don't take,

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don't take space in your variable name. Whenever if you want

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to feel to take a space, instead of space just club those

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two in this way, 'my_name'. Now it is like two words, but we

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are clubbing these two words with underscore.

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Right. So underscore is allowed in our variable name, right. From

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here underscore is allowed,

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so that's why I can take simply 'my_name',

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or 'my_value', 'my_number', anything you can. No problem.

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Okay. So whenever if you want to take a space inside of your

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variable just replace that space with underscore. Then fine.

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Next one is, it should not start with a number. Suppose guys

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You can write 'x1' equal to some number, and 'x2' equal to some,

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one more number, right?

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And if you print simply 'x1', and if you print simply 'x2', it's

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working perfectly.

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But if we take '1x', that is invalid variable name.

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So don't start your variable name with number.

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Right. And it is case sensitive.

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Case sensitive means small 'x=4' and 'X' is

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some different variable.

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Let me print both 'x' and 'X'. Guys they are

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different. Case sensitive means 'x' and 'X', they are different.

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You can use 'x' as one variable and 'X' as one more variable.

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Okay, and the last one is guys,

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[no audio]

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You should not start with the number, right but in your valid,

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for your valid variable names you have these three groups -

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letters, numbers, and underscore.

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You can also start your variable with underscore.

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Let's say '_x' equals some number.

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It is valid but don't start with a number.

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[no audio]

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Okay. So guys,

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these are the important points to remember while defining

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variable name, or while selecting a variable name whenever

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if you are writing your Python scripts.

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[no audio]

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Right. Okay guys,

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thank you for watching this video.

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[no audio]