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The transposition cipher uses an integer for a key which is stored in your Michy Istar variable, and

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the program accepts the more ETR variable which will store either in or decrypt to tell it to encrypt

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or decrypt the file input namaskar fine.

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But before we can read from the input finally Mystere, we need to check that it exist using the OS

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dot but not exist.

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So coming to checking whether a fire is existing, we are reading files is always harmless, but you

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need to be careful when writing to the files.

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So calling the open function in the right mode on a file name that already exist overrides the original

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content.

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So using the OS dot, dot, dot exist function in your program can check whether or not that file already

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exist, not understanding the overstored part, the DOT exist function.

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It takes a single string argument for a file name or a part or to a file and returns.

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True, if the file already exist and falls.

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If it doesn't, the always path don't exist function exist inside the part module which exists inside

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the OS module.

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So when we import the module, the path module is important also.

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So for example, if we just go to our interactive shell and here if we try to enter the same import

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OS first, then we say always stored but not exist.

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And here we give say ABC Dot, the 60 in the bracket.

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We get the value through.

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If we try to say overstored but not exist in bracket, we give your face equal numbers, then Windows

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CE System 32, then calc dot.

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Except for example, we get it through.

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Suppose you are not on Windows your own Mac OS, then you will have to write down something like No,

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I will execute this, but just showing you how the command would look like it will be slash, say USCIRF,

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then say local, then say Binn and then say, oh, fine name, which is there for Mac OS.

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And if you're on Linux then you will have to write down, say, user slash bin and slash the fine.

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That's how you can check the path existing on all the given operating systems now.

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In the particular example that we have checked for the function, it confirms that the TalkTalk file

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is there in Windows.

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Of course, you will only get the results if you are running Python on Windows.

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Now, remember to escape the backslash in the Windows file part by typing another backslash before it.

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If you're using Mac OS, only the Mac OS example will return to no.

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Coming back, we will check whether the input file exist with the oyster or part does not exist function.

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So when we are using this in our Demo 17 file to check whether the filename in the input file name is

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still exist, otherwise we have no file to encrypt or to decrypt it.

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So if we say if not always dot, dot, dot, exist the input, find the Mystere, then we are printing

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a message that is the file.

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The name does not exist, giving quiting and parsing that the input filename.

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So if the file doesn't exist, we display a message to the user and then with the particular program

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next, we are using the string methods to make user input more flexible.

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Now next, the program checks here whether a file with the same name as the output filename exist,

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and if so, it asks the user to type see if they want to continue running the program or to quit the

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program now because the user might type various responses, like a small C, a capital C or even the

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word continue with C capital.

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We want to make sure that the program will accept all of these versions.

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To do this, we are basically using more string methods here.

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That is upper lower title string methods.

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So that is over here that we're checking out.

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So the upper and lower string methods will return the string.

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They are called on in an upper case or lower case letters respectively, and enter the following into

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the interactive shell to see how the methods basically work on the same string.

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So, for example, if we go back to the python shell here and we try to enter, say, Python, Doc Apple,

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we get that all in the uppercase or we try to enter Python doc level, we get all in the lowercase.

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Now, just as the lower and the upper string methods return a string and lowercase or uppercase, the

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title string method returns string in the title keys title cases where the first character of every

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word is uppercase and the rest of the characters are in the lowercase.

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For example, if we enter something like Python Dot type it, we get it.

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First character in upo dressed all in small.

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Similarly, if you enter everything in some capital and still you title, it will be converted in the

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same manner.

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Or if you have, say, python hacking goes beginner level everything you'll want to convert to type

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them.

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So every word, the first character will be in capital.

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So we are using title a little later in the session to format messages we output for the user.

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Now on we would be coming with first as the using this starts with and the ends with string methods.

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So this start with method returns.

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True, if it's string argument is found at the beginning of the string, for example, if we see Python

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then thought starts with and here we give capital P for example, we get the values through, or in

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that case we see Python starts with and we give here.

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I get the values Foote's for this case.

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You can also take a variable, for example, C John is the value in the variable and we use this variable

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that starts with and we give your up.

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Still, the value would be true.

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So this starts with method is case sensitive and can also be used on strings with multiple characters.

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Similarly, we have the ends with string method, which is used to check whether a string value ends

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with another specified string value.

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For example, if we say here, python hacking dot C ends with in brackets, we give hacking.

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It is true.

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Or if we try to say again in single code five ten hacking ends would give your ex capital list everything.

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Same at this for.

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So the string values must match perfectly.

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Notice that the lack of exclamation mark or your capital letters will also cause an injury to return

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value as Foote's now will come to using these string methods in our program.

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So now let us go back to the more 17.

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As noted, we want the program to accept any response that starts with a capital C, regardless of capitalization.

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So this means that we want the file to be overridden.

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Whether the user types moisy continue in small C capital or another string that begins with C Soulsville

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use the string methods lower, starts to make the program more flexible when taking the user input.

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So over here, when we are checking over here, if the Oyster Bar don't exist, the output filename

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is still reprint.

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This will be overridden.

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Type C or Q4 quick and then reticular.

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The response saying response SDR is equal to input and use that the more than sine, if not response

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total overall starts with C, then you exit.

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So over here we are taking the first letter of the string and check whether at this C using this starts

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with method, starts with method that is used in case sensitive and checks for a Lawsie.

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So we have used a lower method to modify the response of the string capitalization to always be in the

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lowercase.

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So if the user didn't enter a response starting with C, then this starts with would return for it's

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not otherwise which will make the if statement to evaluate to true, because if not in the if statement,

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this is what it is called to end the program.

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So technically the user doesn't have to enter.

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Q Two quick.

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Any string that doesn't begin with C causes the system exists function to be called and with the program.

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That's it from the session.

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Thank you very much.