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Welcome to lesson 31 which roughly covers pages 181 to 185 of the automate the boring stuff with Python

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textbook now that we know about file names and file paths we can start writing strings to files that

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we create.

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This is a great way to save information to the hard drive.

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You can then later read these files or any file on the hard drive back into your program.

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This is just like saving and opening documents in an app with type of file that will be reading and

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writing are called text or plain text files plain text files only contain basic text characters and

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they don't include any information about font or size or color text files usually have the text file

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extension.

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Python scripts are an example of plain text files except they also have the dot why file extension plain

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text files can be opened with programs like Windows Notepad or tense text edit application.

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For example I'll just open up one of my plain text python files with Notepad

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and these files just have the plain text information.

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They don't have anything else like a word processor would have with fonts or centering text or anything

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else like that.

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It's just text just like a python string is just text.

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Now the other type of files besides plain text files are called binary files and these are every other

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type of file.

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Word processing documents PTF images spreadsheets executable programs.

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If you open a binary file in Notepad like I'll open up windows calculator program calc.

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You see you're going to find that it's going to be impossible to understand.

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Since every different type of binary file has to be handled in its own way.

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This book really won't go into reading and writing raw binary files directly.

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Most of the time there are modules to handle these binary file formats for you anyway and will be going

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into them in future lessons in this course.

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There are three steps to reading or writing files in Python.

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First you're going to call the open function.

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You'll pass it a file name that you'd like to open.

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So I would like to open

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users slash now.

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Hello 0.60 fact let's go ahead and use Notepad to create this file.

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So an O is 10 you can use text at it or some other plain text file editor program.

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You can create this file right here and then save it to your user accounts home folder.

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On Windows this is going to be something like C slash users slash Al and I'll just save this as whole

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0.60.

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If you're on OS 10 your users folder is going to be slash users slash whatever your account name is

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and then you can save haloed at 60 there.

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So this function call will open the file in plain text read mode.

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We'll just call this read mode for short when the file is open read mode.

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Python lets you only read data from the file.

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You can't write or modify it in any way.

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Reimold is the default mode for files you open in Python.

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With this open function we call the Open returns a file object.

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So we are going to save that too.

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Hello file.

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So the file data type has several methods including one is the read method and this will return a string

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of the entire plain text files contents plus just copy and paste this and run it in the interactive

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shell

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file to read and that returns the contents of that file that we just typed.

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You notice that the new line character is represented as slash in and after we're done reading the file

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contents will call the close method to close that file.

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Now we can only read through the file contents once if we want to read it again.

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We'll have to call open again so we should probably save this to a variable save content.

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Alternatively there is also a read lines method for file objects.

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This will return all of the lines as strings inside if a list is just a different way of getting the

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file contents in a structure that you want realigns will return a list of strings whereas read will

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simply just return a single string

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.

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Python allows you to write content to files in a similar way that how the print function writes strings

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to the screen.

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You can't write to a file that you've opened in read mode though instead you'll have to open it in write

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mode or append mode so right mood will overwrite an existing file and start from scratch with a blank

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text file just like when you overwrite variables value with a new value to open in write mode.

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Pass the string W to the open function as the second argument append mode.

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On the other hand will append the text that you're writing to the end of an existing file so you can

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think of this is like a pending to a list in a variable rather than overwriting the variable altogether

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to open a pen mode you'll just pass a lower case a as the second argument to the open function.

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Now in both cases if this file doesn't already exist Python will just create a new blank text file for

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you to write to.

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I'm just going to create a file hello to and open it and write mode.

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And in order to write strings to this file or call the right method and pass it a string that I want

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to write to it

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and write will return the integer of how many bytes of how many characters it wrote to it.

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And generally you'll just ignore this information so I can call this multiple times and finally close

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that file.

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So I'll use Notepad to go ahead and open this hello to that text file.

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And you'll notice that the right method doesn't automatically add a newline character to the end of

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each string that we passed it the way the print function does when it prints things to string prints

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things to the screen.

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So if we want new lines we're going to have to add them ourselves to the end of the strings that we

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passed like this slash in for the new line character.

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Let's just have another example.

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Let's say they can file.

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And let's just say and at TXI he will open this in write mode we'll just write the string.

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Bacon is not a vegetable.

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And then we'll close that.

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Now this is a relative file path so you might be wondering where on my hard drive is this actually Wolf

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you remember from last lesson.

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You can import the OS module and the OS module has a function called Get current working directory.

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So Python is just going to use this file path relative to the current working directory.

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So here is where we're going to find that vacante XTi file.

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Print this out so it's easier to copy to the clipboard control see the copy file open and paste it here

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and see here in this folder is where that big and text file is.

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And just as an example I'll open this file now in append mode so that it doesn't overwrite the contents

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that are already there.

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And I'll just write some more text I'll add a new line maybe a couple new lines and I'll just add some

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text.

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Bacon is delicious

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.

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I'll just have to re-open this file and you see there were two new line characters added along with

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bacon is delicious and then append mode the original contents of the file weren't erased.

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Writing and reading text files is a good way to store a single long string but if you want to store

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variables that have lists and dictionaries and other complex data structures you can see variables in

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your Python programs to binary shell files using the shell module.

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So I'll give you an example let's just import the shell module and then we'll call shelved up open and

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we'll just say this file is going to be called my data and this will return a shell file object to store

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that in a variable called Shell file.

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You can make changes to the shell value as if it were a dictionary and when you're done just cold call

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close on the shelf value.

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So for example let's say shelf.

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I'll have a key called cats and in this I'll just store a list of my cat names.

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Sophie

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Simon fantail Cleo.

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I actually don't have this many cats and they are called File clothes and then later when I run this

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program my program again in the future I could just have a code that reopens that shelf file and I can

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grab the value just like a dictionary.

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In fact if you think about it variables inside of Python programs are kind of like key value pairs in

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a dictionary.

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I had a variable called cats and I saved this string to it.

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Consider this cat's variable to sort of be the key in this list to be the value.

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So that's why it has a dictionary like structure for this.

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And see here we just have a keys.

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The string cats and then the value of whatever we want to store the benefit of using the shell module

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is that you can store things like list and dictionaries and non-text data to a file and then reopen

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them in the future with your python program.

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Now you might be wondering what does this actually look like on your hard drive.

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So let's go back to the current working directory folder where I originally had this.

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My data.

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I'll just open up the file explorer.

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Here you see on Windows This actually creates three new files where all this data is stored my data

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that are my data that dat my data up back on always 10 Linux.

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I believe that there is only one file that's created my data.

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D-B but these are the files where this information for the shelf is all stored.

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And these are binary files so if you open this in Notepad

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there would also have this weird sort of binary file format.

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You can kind of see the data here and really you could just open this file as a plain text file to read

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this as a string but it would be hard to figure out.

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Oh I guess this means the beginning of the list value and this means like that comma insert.

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It's better just to let the shelf marshal handle all these details for you.

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So Shell file objects are very similar to dictionaries.

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In fact they even have PE keys and values method that will return a list like values for all the keys

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and values inside them so let's just convert this to an actual list.

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So here are all the keys inside that shelf module are inside that shell file

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.

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We can also call its values method as well.

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So if you just need to store plain text you can open up a program using the open function and then just

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use write or read or read lines to write and read the text to these files as strings.

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But if you want to store complex data like lists and dictionaries or if you just want to store strings

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but use this sort of dictionary format you can use a shell file in the shell module.

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To recap.

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Open will return a file object which has reading and writing related methods.

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You can pass the string are or more commonly nothing as the second argument to open to open the file

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in read mode or pass the string W for right mode or the string a for append mode opening a non-existent

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file in right where we're pen mode will create that file.

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It'll just create a blank file that you'll start writing to.

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You can call the read or write methods on a file object to read the contents of the file or write a

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string to that file.

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Or you could call the read lines method to return a list of strings of the files content then call close

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after you're done with the file.

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Meanwhile there is also a shell module that Python can use to store Python data structures like lists

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and dictionaries into a binary file and you can get a shelf a file object by calling shell open to return

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it.

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This dictionary like shelf value