CS50P - Lecture 6 - File I/O

Key Takeaways

foreign [Music] [Music] this is cs50's Introduction to programming with python my name is David Malin and this is our week on file IO input and output of files so up until now most every program we've written just stores all the information that it collects in memory that is in variables or inside of the program itself a downside of which is that as soon as the program exits anything you typed in anything that you did with that program is lost now with files of course on your Mac or PC you can hang on to information long term and file i o within the context of programming is all about writing code that can read from that is load information from or write to that is save information to files themselves so let's see if we can't transition then from only using memory and variables and the like to actually writing code that saves some files for us and therefore data persistently well to do this let me propose that we first consider a familiar data structure a familiar type of variable that we've seen before that of a list and using lists we've been able to store more than in one piece of information in the past using one variable we typically store one value but if that variable is a list we can store multiple values unfortunately lists are stored in the computer's memory and so once your program exits even the contents of those disappear but let's at least give ourselves a starting point so I'm over here in vs code and I'm going to go ahead and create a simple program using Code of names dot Pi a program that just collects people's names students names if you will and I'm going to do it super simply initially in a manner consistent with what we've done in the past to get user input and print it back out I'm going to say something like this name equals input quote unquote what's your name thereby storing in a variable called name the return value of input as always and as always I'm going to go ahead and very simply print out a nice F string that says hello comma and then in curly brace's name to print out hello David hello world however happens to be using the program let me go ahead and run this just to remind myself what I should expect and if I run python names.pi and hit enter type in my name like David of course I now see hello comma David so suppose though that we wanted to add support not just for one name but multiple names maybe three names for the sake of discussion so that we can begin to accumulate some amount of information in the program such that it's really going to be a downside if we keep throwing it away once the program exits well let me go back into names.pi up here atub let me proactively give myself a variable this time called names plural and set it equal to an empty list recall that the square bracket notation especially if nothing's inside of it just means give me an empty list that we can add things to over time well what do we want to add to it well let's add three names each from the user and let me say something like this for underscore in range of three let me go ahead and prompt the user with the input function and getting their name in this variable and then using list syntax I can say names dot append name to that list and now I have in that list that given name one two three of them other points to note is I could use a variable here like I which is conventional but if I'm not actually using I explicitly on any subsequent lines I might as well just use underscore which is a pythonic convention and actually if I want to clean this up a little bit right now notice that my name variable doesn't really need to exist because I'm assigning it a value and then immediately appending it well I could tighten this up further by just getting rid of that variable altogether and just appending immediately the return value of input I think we could go both ways in terms of design here on the one hand it's a pretty short line and it's readable on the other hand if I were to eventually change this phrase to be not what's your name but something longer we might want to break it out again into two lines but for now I think it's pretty readable now later in the program let's just go ahead and print out those same names but let's sort them alphabetically so that it makes sense to be gathering them all together then sorting them and printing them so how can I do that well in Python the simplest way to sort a list in a loop is probably to do something like this for name in names but wait let's sort the names first recall that there's a function called sorted which will return a sorted version of that list now let's go ahead and print out an F string that says again hello bracket name close quotes all right let me go ahead and run this so python of names dot pi and let me go ahead and type in a few names this time how about Hermione how about Harry how about Ron and notice that they're not quite in alphabetical order but when I hit enter and that Loop kicks in it's going to print out hello Harry hello Hermione hello Ron in sorted order but of course now if I run this program again all of the names are lost and if this is a bigger program than this that might actually be pretty painful to have to re-input the same information again and again and again wouldn't it be nice like most any program today on a phone or a laptop or desktop or Cloud to be able to save this information somehow instead and that's where file i o comes in and that's where Files come in they are a way of storing information persistently on your own phone or Mac or PC or some Cloud servers disk so that they're there when you come back and run the program again so how can we go about saving all three of these names on in a file as opposed to having to type them again and again let me go ahead and simplify this file and again give myself just a single variable called name and set the return value of input equal to that variable so what's your name as before quote unquote and now let me go ahead and let me do something more with this value instead of just adding it to a list or printing it immediately out let's save the value of the person's name that's just been typed in to a file well how do we go about doing that well in Python there's this function called open whose purpose in life is to do just that to open a file but to open it up programmatically so that you the programmer can actually read information from it or write information to it so open is like the programmer's equivalent of like double clicking on an icon on your Mac or PC but it's a programmer's technique because it's going to allow you to specify exactly what you want to read from or write to that file formally its documentation is here and you'll see that its usage is relatively straightforward it minimally just requires the name of the file that we want to open and optionally how we want to open it so let me go back to vs code here and let me propose now that I do this I'm going to go ahead and call this function called open passing in an argument for names.txt which is the name of the file I would like to store all of these names in I could call it anything I want but because it's going to be just text it's conventional to call it something.txt but I'm also going to tell the open function that I plan to write to this file so as a second argument to open I'm going to put literally quote unquote W for right and that's going to tell open to open the file in a way that's going to allow me to change the contents and better yet if it doesn't even exist yet it's going to create the file for me now open returns what's called a file handle a special value that allows me to access that file subsequently so I'm going to go ahead and sign it equal to a variable like file and now I'm going to go ahead and quite simply write this person's name to that file so I'm going to literally type file which is the variable a linking to that file dot write which is a function otherwise known as a method that comes with open files that allows me to write that name to the file and then lastly I'm going to quite simply going to go ahead and say file Dot close which will close and effectively save the file so these three lines of code here are essentially the programmers equivalent to like double clicking an icon on your Mac or PC making some changes in Microsoft Word or some other program and going to file save we're doing that all in code with just these three lines here well let's see now how this works let me go ahead now and run python of names.pi and enter let's type in a name I'll type in Hermione enter all right where did she end up well let me go ahead now and type code of names.txt which is a file that happens now to exist because I opened it in right mode and if I open this in a tab we'll see there's Hermione well let's go ahead and run names.pi once more I'm going to go ahead and run python of names.pi enter and this time I'll type in Harry let me go ahead and run it one more time and this time I'll type in Ron and now let me go up to names.text where hopefully I'll see all three of them here but no I've just actually seen Ron What might explain what happened to Hermione and Harry even though I'm pretty sure I ran the program three times and I definitely wrote the code that writes their name to that file what's going on here do you think I think because we're not appending them we should append the names since we are writing directly it is erasing the old content and it is replacing with the last uh set of characters that we mentioned exactly unfortunately quote unquote W is a little dangerous not only will it create the file for you it will also recreate the file for you every time you open the file in that mode so if you open the file once and write Hermione that worked just fine as we saw but if you do it again for Harry if you do it again for Ron the code is working but each time it's opening the file and recreating it with brand new contents so we had one version with Hermione one version with Harry and one final version with Ron but ideally I think we probably want to be appending as bashal says each of those names to the file not just clobbering that is overwriting the file each time so how can I do this it's actually a relatively easy fix let me go ahead and do this as follows I'm going to first remove the old version of names.text and now I'm going to change my code to do this I'm going to change the W quote unquote to just a quote unquote a for append which means to add to the bottom to the bottom to the bottom again and again now let me go ahead and rerun python of names.pi and enter I'll again start from scratch with Hermione because I'm creating the file new notice that if I now do code of names.txt enter we do see that Hermione is back so we've after removing the file it did get recreated even though I'm using append which is good but now let's see what happens when I go back to my terminal and this time I run python of names.pi again this time typing in Harry and let me run it one more time this time typing in Ron so hopefully this time in that second tab names.text I should now see all three of them but but but but this doesn't look ideal what have I clearly done wrong foreign something tells me even though all three names are there it's not going to be easy to read those back unless you know where each name ends and Begins the English form it's not correctly the English format is not correct like it's a correct it's concatenating them it is it's it can it's well it appears to be concatenating but technically speaking it's just appending to the file first Hermione then Harry then Ron it has the effect of combining them back to back but it's not concatenating per se it really is just a pending let's go to another hand here what really have I done wrong or equivalently how might I fix it would be nice if there were some kind of gaps between each of the names so we could read them more cleanly Hello uh we should add a new line before we write new name good we want to add a new line ourselves so whereas print by default recall always outputs automatically a line ending of backslash n unless we override it with the named parameter called end write does not do that right takes you literally and if you say right Hermione that's it you're getting the H through the E if you say right Harry you get the H through the uh y you don't get any extra new lines automatically so if you want to have a new line at the end of each of these names we've got to do that manually so let me again close names.txt and let me remove the current file and let me go back up to my code here and I can fix this in any number of ways but I'm just going to go ahead and do this I'm going to write out an F string that contains name and backslash n at the end we could do this in different ways we could manually print just the new line or some other technique week but I'm going to go ahead and use my f strings as I'm in the habit of doing and just print to the name and the new line all at once I'm going to go ahead now and down to my terminal window run python of names dot Pi again enter we'll type in Hermione I'm going to run it again type in Harry I'm going to type it again and this time Ron now I'm going to run code of names.txt and open that file and now it looks like the file is a bit cleaner indeed I have each of the name on its own line as well as a line ending which ensures that we can separate one from the other now if I were you know writing code I bet I could parse that is read the previous file by looking at differences between lowercase and uppercase letters but that's going to get messy quickly generally speaking when storing data long term in a file you should probably do it somehow cleanly like doing one name at a time well let's now go back and I'll propose that this code is now working correctly but we can design it a little bit better it turns out that it's all too easy when writing code to sometimes forget to close files and sometimes this isn't necessarily a big deal but sometimes it can create problems files could get corrupted or accidentally deleted or the liked depending on what happens in your code so it turns out that you don't strictly need to call close on the file yourself if you take another approach instead more pythonic when manipulating files is to do this to introduce this other keyword called quite simply with that allows you to specify that in this context I want you to open and automatically close some file so how do we use with it simply looks like this let me go back to my code here I've gotten rid of the close line and I'm now just going to say this instead instead of saying file equals open I'm going to say with open then the same arguments as before and somewhat curiously I'm going to put the variable at the end of the line why that's just the way this is done you say with you call the function in question and then you say as and specify the name of the variable that should be assigned find the return value of open then I'm going to go ahead and indent the line underneath so that the line of code that's writing the name is now in the context of this with statement which just ensures that automatically if I had more code in this file down below no longer indented the file would be automatically closed as soon as line 4 is done executing so it doesn't change what has just happened but it does automate the process of at least closing things for us just to ensure I don't forget and so that something doesn't go wrong but suppose now that I wanted to read these names from the file all I've done thus far is write code that writes names to the file but let's assume now that we have all of these names in the file and heck let's go ahead and add one more let me go ahead and run this one more time python of names.pi and let's add in Draco to the mix so now that we have all four of these names here how might we want to read them back well let me propose that we go into names.pi now or we could create another program altogether but I'm going to keep reusing the same name just to keep us focused on this and now I'm going to write code that reads an existing file with Hermione Harry Ron and Draco together and how do I do this well it's similar in spirit I'm going to start this time with with open and then the first argument is going to be the name of the file that I want to open as before and I'm going to open it this time in read mode quote unquote R and to read a file just means to load it not to save it and I'm going to name the return value file and now I'm going to do this yes and there's a number of ways I can do this but one way to read all of the lines from the file at once would be this let me declare a variable called lines let me access that file and call a function or a method that comes with it called read lines so if you read the documentation on file i o and python you'll see that open files come with a special method whose purpose in life is to read all the lines from the file and return them to me as a list so what this line 2 is doing is it's reading all of the lines from that file storing them in a variable called lines now suppose I want to iterate over all of those lines and print out each of those names for line in lines this is just a standard for Loop in Python lines as a list line is the variable that will be automatically be set to each of those lines let me go ahead and print out something like oh hello uh comma and then I'll print out the line itself all right so let me go to my terminal window run python of names.pi now I have not deleted names.txt so it still contains all four of those names and hit enter and okay it's not bad but it's a little ugly here what's going on when I ran names.pi it's saying hello to Hermione to Harry to Ron to Draco but there's these gaps now between the lines what's explains that symptom if if nothing else it just looks ugly it happens because in the text file we have new line symbols uh in between those names and the print always adds another new line at the end so you you use the same symbol twice perfect and here's a good example of a bug a mistake in a program but if you just think about those first principles like how do each of the lines of code work that I'm using you should be able to reason exactly as Rafal did there to say that all right well one of those new lines is coming from the file after each name and then of course print all of these Weeks Later is still giving us for free that extra new line so there's a couple of possible solutions I could certainly do this which we've done in the past and pass in a named argument to print like end equals quote unquote and that's fine I would argue a little better than that might actually be to do this to strip off of the end of the line the actual new line itself so that print is handling the printing of everything the person's name game as well as the new line but you're just stripping off what is really just an implementation detail in the file we chose to use new lines in my text file to separate one name from another so arguably it should be a little cleaner in terms of design to strip that off and then let print print out what is really just now a name but that's ultimately a design decision the effect is going to be exactly the same well if I'm going to open this file and read all the lines and then iterate over all of those lines and print them each out I could actually combine this into one thing because right now I'm doing twice as much work I'm reading all of the lines then I'm iterating over all of the lines just to print out each of them well in Python with files you can actually do this I'm going to erase almost all of these lines now keeping only the with statement at top and inside of this with statement I'm going to say this for line in file go ahead and print out quote unquote hello comma and then line Dot R strip so I'm going to take the approach of stripping off the end of the line but notice how elegant this is so to speak I've opened the file in line one and if I want to iterate over every line in the file I don't have to very explicitly load all read all the lines then iterate over all of the lines I can combine this into one thought it in Python you can simply say for line and file and that's going to have the effect of giving you a for Loop that iterates over every line in the file one at a time and on each iteration updating the value of this variable line to be Hermione then Harry then Ron then Draco so this again is one of the appealing aspects of python is that it reads rather like English for line and file print this it's a little more compact when written this way well what if though I don't want quite this Behavior because notice now if I run python of names.pi it's correct I'm seeing each of the names and each of the hellos and there's no Extra Spaces in between but just to be difficult I'd really like us to be sorting these hellos really I'd like to see Draco first then Harry then Hermione then Ron no matter what order they appear in the file so I could go in of course to the file and manually change the file but if that file is changing over time based on who is typing their name into the program that's not really a good solution in code I should be able to load the file no matter what it looks like and just sort it all at once now here is a reason to not do what I've just done I can't iterate over each line in the file and print it out but sort everything in advance right logically if I'm looking at each line one at a time and printing it out it's too late to sort I really need to read all of the lines first without printing them sort them then print them so we have to take a step back in order to add now now this new feature so how can I do this well let me combine some ideas from before let me go ahead and start fresh with this let me give myself a list called names and assign it an empty list just so I have a variable in which to accumulate all of these lines and now let me open the file with open quote unquote names.txt and it turns out I can tighten this up a little bit it turns out if you're opening a file to read it you don't need to specify quote unquote r that is the implicit default so you can tighten things up by just saying open names.text and you'll be able to read the file but not write it I'm going to give myself a variable called file as before I am going to iterate over the file in the same way for line in file but instead of printing each line I'm going to do this I'm going to take my names list and append to it and this is appending to a list in memory not appending to the file itself I'm going to go ahead and append the current line but I'm going to strip off the new line at the end so that all I'm adding to this list is each of the students names now I can use that familiar technique from before let me go outside of this with statement because now I've read the entire file presumably so by the time I'm done with lines four and five again and again and again for each line in the file I'm done with the file it can close I now have all of the students names in this list variable let me do this for name in not just names but the sorted names using our python function sorted which does just that and do print quote unquote with an F string hello comma and now I'll plug in bracket name so now what have I done I'm creating a list at the beginning just so I have a place to gather my data I then on lines three through five iterate over the file from top to bottom reading in each line one at a time stripping off the new line and adding just the student's name to this list and the reason I'm doing that is so that on line seven I can sort all of those names now that they're all in memory and print them in order I need to load them all into memory before I can sort them otherwise I'd be printing them out prematurely and Draco would end up last instead of first so let me go ahead in my terminal window and run python of names.pi now and hit enter and there we go the same list of four hellos but now they're sorted and this is a very common technique when dealing with files and information more generally if you want to change that data in some way like sorting it creating some kind of variable at the top of your program like a list adding or appending information to it just to collect it in one place and then do something interesting with that collection that list is exactly what I've done here now I should note that if we just want to sort the file we can actually do this even more simply in Python particularly by not bothering with this names list nor the second for Loop and let me go ahead and instead just do more simply this let me go ahead and tell python that we want the file itself to be sorted using that same sorted function but this time on the file itself and then inside of that for Loop let's just go ahead and print right away our hello comma followed by the line itself but still stripping off of the end of it any white space therein if we go ahead and run this same program now with pythonupnames.pi and hit enter we get the same result but of course it's a lot more compact but for the sake of discussion let's assume that we do actually want to potentially make some changes to the data as we iterate over it so let me undo those changes leave things as is where by now we'll continue to accumulate all of the names first into a list maybe do something to them maybe forcing them to uppercase or lowercase or the like and then sort and print out each item let me pause and see if there's any questions now on file IO reading or writing or now accumulating all of these values in some list hi is there a way to sort the files but instead if you want it from alphabetically from A to Z is there a way to reverse it from zet to a is there a like a little extension that you can add to the end to do that or would you have to create a new function if you wanted to reverse the contents of the file yeah so if you instead of sorting them from a to z in ascending order if you're one of them in descending order is there an extension for the there is indeed and as always the documentation is your friend so if the goal is to sort them not in alphabetical order which is the default but maybe reverse alphabetical order you can take a look for instance at the formal python documentation there and what you'll see is this summary you'll see that the sorted function takes a first argument generally known as an iterable and something that's iterable means that you can iterate over it that is you can Loop over it one thing at a time what the rest of this line here means is that you can specify a key like how you want to sort it but more on that later but this last named parameter here is reverse and by default per the documentation it's false it will not be reversed by default but if we change that to true I bet we can do that so let me go back to vs code here and do just that let me go ahead and pass in a second argument to sorted in addition to this iterable which is my names list iterable again in the sense that it can be looped over and let me pass in Reverse equals true thereby overriding the default of false let me now run python of names Dot pie and now Ron's at the top and draco's at the bottom so there too whenever you have a question like that moving forward consider what does the documentation say and see if there's a germ of an idea there because odds are if you have some problem odds are some programmer before you have had the same question other thoughts um and the second question can we find a specific name really good question can we limit the number of the names in the file and can we find a specific one we absolutely could if we were to write code we could for instance open the file first count how many lines are already there and then if there's too many already we could just exit with sys.exit or some other message to indicate to the user that sorry the class is full as for finding someone specifically absolutely you could imagine opening the file iterating over it with a for loop again and again and then adding a conditional like if the current line equals equals Harry then we found the chosen run and you can print something like that so you can absolutely combine these ideas with previous ideas like conditionals to ask those same questions how about one other question on file IO uh so I just thought about this function like uh read all lines and it looks like it's uh like separate all the lines by this special character backslash n but it looks like we don't need it a character and we we always trip it and it looks like some bad design of function why why wouldn't we just strip it inside this function a really good question so we are in my examples thus far using our strip to reverse uh to strip from the end of the line all of this white space you might not want to do that in this case I am stripping it away because I know that each of those lines isn't some generic line of text each line really represents a name that I have put there myself I'm using the new line just to separate one value from another in other scenarios you might very well want to keep that line ending because it's a very long series of text or a parallel graph or something like that where you want to keep it distinct from the others but it's just a convention we have to use something presumably to separate one chunk of text from another there are other functions in Python that will in fact handle the removal of that white space for you read lines though does literally that though it reads all of the lines as is well allow me to turn our attention back to where we left off here which is just names to propose that with names.text we have an ability it seems to store each of these names pretty straightforwardly but what if we wanted to keep track of other information as well suppose that we wanted to store information including a student's uh name and their house at Hogwarts be it Gryffindor or Slytherin or something else well where do we go about putting that you know Hermione lives in Gryffindor so we could do something like this in our text file Harry lives in Gryffindor so we could do that Ron lives in Gryffindor so we could do that and Draco lives in Slytherin so we could do that but I worry here but I worry now that we're mixing apples and oranges so to speak like some lines or names some lines are houses so this probably isn't the best design if only because it's confusing or it's ambiguous so maybe what we could do is Adopt A convention and indeed this is in fact what a lot of programmers do they change this file not to be names.text but instead let me create a new file called names.csv CSV stands for comma separated values and it's a very common convention to store multiple pieces of information that are related in the same file and so to do this I'm going to separate each of these types of data not with another new line but simply with a comma I'm going to keep each student on their own line but I'm going to separate the information about each student using a comma instead and so now we sort of have a two-dimensional file if you will row by row we have our students but if you think of these commas as representing a column even though it's not perfectly straight because of the lengths of these names it's a little it's a little Jagged you can think of these commas as representing a column and it turns out these CSV files are very commonly used when you use something like Microsoft Excel Apple numbers or Google spreadsheets and you want to export the data to share with someone else as a CSV file or conversely if you want to import a CSV file into your preferred spreadsheet software like Excel or numbers or Google spreadsheets you can do that as well so CSV is a very common very simple text format that just separates values with commas and different types of values ultimately with new lines as well let me go ahead and run code of students.csv to create a brand new file that's initially empty and we'll add to it those same names but also some other information as well so if I now have this new file students.csv inside of which is one column of name so to speak and one column of houses how do I go about changing my code to read not just those names but also those names and houses so that they're not all on one line we somehow have access to both type of value separately play well let me go ahead and create a new program here called students.pi and in this program let's go about reading not a text file per se but a specific type of text file a CSV a comma separated values file and to do this I'm going to use similar code as before I'm going to say with open quote unquote students.csv I'm not going to bother specifying quote unquote R because again that's the default but I'm going to give myself a variable name of file and then in this file I'm going to go ahead and do this for line and file as before and now I have to be a bit clever here let me go back to students.csv looking at this file and it seems that on my loop on each iteration I'm going to get access to the whole line of text I'm not going to automatically get access to just Hermione or just Gryffindor recall that the loop is going to give me each full line of text so logically what would you propose that we do inside of a for Loop that's reading a whole line of text at once but we now want to get access to the individual values like Hermione and Gryffindor Harry and Gryffindor how do we go about taking one line of text and gaining access to those individual values do you think just instinctively even if you're not sure what the name of the functions would be you can access access it as you would and if you were using a dictionary like using a key and value so ideally we would access it using it a key in value but at this point in the story all we have is this Loop and this Loop is giving me one line of text that is the time I'm the programmer now I have to solve this there is no dictionary yet in question about another suggestion here um so you can somehow split the two words based on the comma yeah even if you're not quite sure what function is going to do this intuitively you want to take this whole line of text Hermione comma Gryffindor Harry comma Gryffindor and so forth and split that line into two pieces if you will and it turns out wonderfully the function we'll use is actually called split that can split on any characters but you can tell it what character to use so I'm going to go back into students.pi and inside of this Loop I'm going to go ahead and do this I'm going to take the current line I'm going to remove the white space at the end as always using R strip here and then whatever the result of that is I'm going to now call split and quote unquote comma so the split function or method comes with strings stirs in Python any stir has this method built in and if you pass in an argument like a comma what this strip split function will do is split that current string into one two three maybe more pieces by looking for that character again and again ultimately strip uh ultimately split is going to return to us a list of all of the individual parts to the left and to the right of those commas so I can give myself a variable called row here and this is a common Paradigm when you know you're iterating over a file specifically a CSV it's common to think of each line of it as being a row and each of the values they're in separated by commas as columns so to speak so I'm going to deliberately name my variable row just to be consistent with that convention and now what do I want to print well I'm going to go ahead and say this print how about the following in F string that starts with curly braces well how do I get access to the first thing in that row well the row is going to have how many parts two because if I'm splitting on commas and there's one comma per line that's going to give me a left part and a right part like Hermione and Gryffindor Harry and Gryffindor when I have a list like row how do I get access to individual values well I can do this I can say Row Bracket zero and that's going to go to the first element of the list which should hopefully be the student's name then after that I'm going to say is in and I'm going to have another curly brace here for Row Bracket one and then I'm going to close my whole quote so it looks a little cryptic at first glance but most of this is just F string syntax with curly braces to plug in values and what values am I plugging in well rho again is a list and it has two elements presumably Hermione in one and Gryffindor and the other and so forth so bracket zero is the first element because remember we start indexing at zero in Python and one is going to be the second second element so let me go ahead and run this now and see what happens python of uh students.pi enter and we see Hermione's in Gryffindor Harry's and Gryffindor Ron is in Gryffindor and Draco is in Slytherin so we have now implemented our own code from scratch that actually parses that is reads and interprets a CSV file ultimately here now let me pause to see if there's any questions but we'll make this even easier to read in just a moment any questions on what we've just done here by splitting by comma so my question is uh can we edit any line of code anytime we want or uh the only option that we have is to append uh the lines or let's say if we want to let's say change headies uh house to let's say Slytherin or some other house yeah a really good question what if you want to in Python change a line in the file and not just a pen to the end you would have to implement that logic yourself so for instance you could imagine now opening the file and reading all of the contents in then maybe iterating over each of those lines and as soon as you see that the current name equals equals Harry you could maybe change his house to Slytherin and then it would be up to you though to write all of those changes back to the file so in that case you might want to in simplest form read the file once and let it close then open it again but open for writing and change the whole file it's not really possible or easy to go in and change just part of the file though you can do it it's easier to actually read the whole file make your changes in memory then write the whole file out but for larger files where that might be quite slow you can be more clever than that well let me propose now that we clean this up a little bit because I actually think this is a little cryptic to read Row Bracket zero Row Bracket one it's it's not that well written at the moment I would say but it turns out that when you have a variable that's a list like row you don't have to throw all of those variables into a list you can actually unpack that whole sequence at once that is to say if you know that a function like split returns a list but you know in advance that it's going to return two values in a list the first and the second you don't have to throw them all into a variable that itself is a list you can actually unpack them simultaneously into two variables doing name comma house so this is a nice python technique to not only create but assign automatically in parallel two variables at once rather than just one so this will have the effect of putting the name in the left Hermione and it will have the effect of putting Gryffindor the house in the right variable and we now no longer have a row we can now make our code a little more readable by now literally just saying name down here and for instance house down here so just a little more readable even though functionally the code now is exactly the same all right so this now works and I'll confirm as much by just running it once more python of students.pi enter and we see that the text is as intended but suppose for the sake of discussion that I'd like to sort this list of output I'd like to say hello again to Draco first then hello to Harry then Hermione then Ron how can I go about doing this well let's take some inspiration from the previous example where we're only dealing with names and instead do it with these full phrases so and so is in-house well let me go ahead and do this I'm going to go ahead and start scratch and give myself a list called students equal to an empty list initially and then with open students.csv as file I'm going to go ahead and say this for line in file and then below this I'm going to do exactly as before name comma house equals the current line stripping off the white space at the end splitting it on a comma so that's exact same as before but this time before I go about uh printing the sentence I'm going to store it temporarily in a list so that I can accumulate all of these sentences and then sort them later so let me go ahead and do this students which is my list dot append let me append the actual sentence I want to show on the screen so another F string so name is in house just as before but notice I'm not printing that sentence I'm appending it to my list not a file but to my list why am I doing this well just because as before I want to do this for student in the sorted students I want to go ahead and print out students like this well let me go ahead and run python of students.pi and hit enter now and I think we'll see indeed Draco is now first Harry a second Hermione is third and Ron is fourth but this is arguably a little sloppy right it seems a little hackish that I'm constructing these sentences and even though I'm technically want to sort by name I'm technically sorting by these whole English sentences so it's not wrong it's achieving the intended result but it's not really well designed because I'm just kind of getting lucky that English is reading from left to right and therefore when I print this out it's sorting properly it would be better really to come up with a technique for sorting by the students names not by some English sentence that I've constructed here on line six so to achieve this I'm going to need to make my life more complicated for a moment and I'm going to need to collect information about each student before I bother assembling that sentence so let me propose that we do this let me go ahead and undo these last few lines of code so that we currently have two variables name and house Each of which has name in the student's house respectively and we still have our Global variable students but let me do this recall that python supports dictionaries and dictionaries are just collections of keys and values so you can associate something with something else like a name with Hermione like a house with Gryffindor that really is a dictionary so let me do this let me temporarily create a dictionary that stores this Association of name with house let me go ahead and do this let me say that the students here is going to be represented initially by an empty dictionary and just like you could create an empty list with square brackets you can create an empty dictionary with curly braces so give me an empty diction missionary that will soon have two keys name and house how do I do that well I could do it this way student Open Bracket name equals the student's name that we got from the line student bracket house equals the house that we got from the line and now I'm going to append to the students list plural that particular student now why have I done this I've admittedly made my code more complicated it's more lines of code but I've now collected all of the information I have about students while still keeping track what's a name what's a house the list meanwhile has all of the students names and houses together now why have I done this well let me for the moment just do something simple let me do for students in students and let me very Simply Now say print the following F string the current student with this name uh is in this current student's house and now notice one detail inside of this F string I'm using my curly braces as always I'm using inside of those curly braces the name of a variable as always but then I'm using not bracket zero or one because these are dictionaries now not list but why am I using single quotes to surround house and to surround name why single quotes inside of this F string to access those Keys yes um because you have double quotes in that in that line 12 and so you have to tell python to differentiate exactly because I'm already using double quotes outside of the F string if I want to put quotes around any strings on the inside which I do need to do for dictionaries because recall when you index into a dictionary you don't use numbers like lists 0 1 2 onward you instead use strings which need to be quoted but if you're already using double quotes it's easiest to then use single quotes on the inside so python doesn't get confused about what lines up with what so at the moment when I run this program it's going to print out those hellos but they're not yet sorted in fact what I now have is a list of dictionaries and nothing is yet sorted but let me tighten up the code too to point out that it doesn't need to be quite as verbose if you're in the habit of creating an empty dictionary like this on line six and then immediately putting in two keys name and house each with two values name and house respectively you can actually do this all at once so let me show you a slightly different different syntax I can do this give me a variable called student and let me use curly braces on the right hand side here but instead of leaving them empty let's just Define those keys and those values now quote unquote name will be name and quote unquote house will be house this achieves the exact same effect in one line instead of three it creates a new non-empty dictionary containing a name key the value of which is the student's name and a house key the value of which is the student's house nothing else needs to change that will still just work so that if I again run python if students.pi I'm still seeing those greetings but they're still not quite actually sorted well what might I go about doing here in order to what could I do to improve upon this further well we need some mechanism now of sorting those students but unfortunately you can't do this we can't sort all of the students now because those students are not names like they were before they aren't sentences like they were before each of the students is a dictionary and it's not obvious how you would sort a dictionary inside of a list so ideally what do we want to do if at the moment we hit line 9 we have a list of all of these students and inside of that list is one dictionary per student and each of those dictionaries has two keys name and house wouldn't it be nice if there were ran code to tell python sort this list by looking at this key in each dictionary because that would give this the ability to sort either by name or even by house or even by any other field that we add to that file so it turns out we can do this we can tell the sorted function not just to reverse things or not it takes another position National it takes another named parameter called key where you can specify what key should be used in order to sort some list of dictionaries and I'm going to propose that we do this I'm going to first Define a function temporarily for now called get name and this functions purpose in life given a student is to quite simply return the student's name from that particular dictionary so if student is a dictionary this is going to return literally the student's name and that's it that's the sole purpose of this function in life what do I now want to do well now that I have a function that given a student will return to me the student's name I can do this I can change sorted to say use a key that's equal to whatever the return value of get name is and this now is a feature of python python allows you to pass functions as arguments in two other functions so get name is a function sorted is a function and I'm passing in get name to sort it as the value of that key parameter now why am I doing that well if you think of the get name function as just a bunch a block of code that will get the name of a student that's handy because that's the capability that sorted needs when given a list of students Each of which is a dictionary sorted needs to know how do I get the name of the student in order to do alphabetical sorting for you the authors of python didn't know that we were going to be creating students here in this class so they couldn't have anticipated writing code in advance that specifically sorts on a field called student let alone called name let alone house so what did they do they instead built into the sorted function this named parameter key that allows us all these years later to tell their function sorted how to sort this list of dictionaries so now watch what happens if I run python of students.pi and hit enter I now have a sorted list of output why because now that list of dictionaries has all been sorted by the student's name I can further do this if as before we want to reverse the whole thing by saying reverse equals true we can do that too let me rerun python of students.pi and hit enter now it's reverse now it's Ron then Hermione Harry and Draco but we can do something different as well what if I want to sort for instance by house name reversed I could do this I could change this function from get name to get house I could change the implementation up here to be get house and I can return not the student's name but the student's house and so now notice if I run python of students.pi enter notice now it is sorted by house in reverse order Slytherin is first and then Gryffindor if I get rid of the reverse but keep the get house and rerun this program now it's sorted by hou