Writing to Files

[MUSIC] 0:00 Hi, I'm Kenneth, the Python teacher here at Treehouse. 0:04 Quite often, when we're writing code, 0:08 we get the idea that we should put handy information into files. 0:09 Or just write things out to a file so we'll have it later. 0:12 I don't mean our scripts, of course. 0:15 Those are saved into files. 0:17 I mean things like, I'm going to ask the user how they're feeling today, 0:19 and then save it into a file. 0:22 Thankfully, Python makes this easy. 0:24 We only have to know one function to open the file, and 0:26 a couple of flags to control how it's opened. 0:28 Let's get over to workspaces to see how to do this. 0:30 Opening a file in Python is surprisingly easy. 0:34 But before we can read a file, we need to write a file, all right? 0:37 I want to write a script that will let me jot down reminders and notes and such. 0:42 Let's call it remember.py. 0:47 So let's make a new file, remember.py. 0:49 Alright, cool, and let's add in a def rememberer, and a thing to remember. 0:54 And we'll say open file, write thing to file, close file. 1:03 And our function has to do something so we'll just say pass. 1:09 And then let's say if name is equal to main 1:13 then run rememberer, and remember should get some input. 1:18 What should I remember? 1:24 So cool, there's that let's go ahead and run it. 1:28 And what should I remember? 1:33 I don't know remember the milk. 1:34 All right, so that's cool. 1:36 But we actually need to do the commented things, right? 1:38 So luckily Python gives us this function that's called open. 1:41 And so we can do like file equals open 1:46 database.txt and let's just try that, all right? 1:49 So we get the file open and 1:54 we've put it into this pointer, this variable, called file. 1:56 All right, so let's try running that. 2:01 And you should remember the milk. 2:03 That's a big exception. 2:06 What this exception tells me, is that I don't have a file named database.txt. 2:08 That's why we get the error. 2:14 We get an error because the file doesn't exist. 2:15 This a good time to talk about flags. 2:18 Pythons open function uses flags to tell it how to open the file. 2:20 Like we want to open it for reading, we want to open it for writing, 2:25 we want to open it for appending, and other ways. 2:28 By default, if we don't give it an argument, which is what we've done here. 2:33 Then, it opens it for reading and reading, kind of requires the file to exist, right? 2:36 Currently, there isn't a database.txt so Python can't open it. 2:43 So, let's add in the "w" flag. 2:48 W, for write. 2:52 Right? 2:54 So, Cool. 2:55 Let's run this again. 2:56 And I want you to remember the milk. 2:58 No errors. 3:00 Great. 3:01 All right, so now, let's see, let's refresh this over here. 3:01 And when you see that we have a database.txt, there's nothing in it. 3:06 That's fine, we didn't actually write anything to the file. 3:09 Let's do that now. 3:13 All right so, writing the files is really simple. 3:14 We do whatever variable points to the file and then we do a dot and 3:17 then we do write and then we do whatever it is we want to write that file. 3:23 So I'm going to do thing and then I'm gonna add in a new line character. 3:29 That way each thing gets to be put on to it's own line. 3:35 All right, so let's run that. 3:37 And again, let's say remember the milk. 3:41 And let's go look at database.txt. 3:43 And milk is in there and there's a new line. 3:46 So, cool, that's pretty neat. 3:49 That's the thing we wanted to be remembered, and it's in there. 3:51 It's important that we can remember more than one thing, right. 3:54 So, let's do that. 3:58 And I've already put in milk, so the other thing I need to remember is, 3:59 I need to remember to get orange juice. 4:03 So, I'm gonna put in orange juice. 4:06 And let's go check and make sure both those things are in our file. 4:08 Only have orange juice. 4:12 Our old record's gone and it's been replaced with a new one. 4:13 So, I wonder why that is? 4:18 Let's check the documentation and let's find out. 4:20 Let's go over here and look for open and there's open. 4:25 And let's look at these, these are the modes that I was telling you about. 4:33 So there's R for reading which is the default. 4:37 There's W for writing which truncates the file first. 4:39 Aha! See it truncates. 4:45 Truncates means it deletes everything out of the file. 4:47 It shortens the file down to 0 bytes. 4:49 There's nothing in the file, and then it writes. 4:53 Okay, that's not what we want. 4:56 This is why you read the documentation. 4:57 So A opens it, appends to be in the file if it exists. 5:00 Okay, that's that probably what we want. 5:05 We probably want A. 5:06 And again, this is why it pays to read the docs. 5:08 Alright, let's fix our script. 5:10 So come over here, we don't wanna open it with W, we wanna open it with A. 5:12 All right, so right now my database 5:15 file has orange juice in it, so I'm gonna add milk cuz I still need to get milk. 5:21 All right, so milk and let's come over here and look at database. 5:24 Sweet, there's orange juice and there's milk. 5:30 Everything is in there, and they each have their own line. 5:32 They're all in the file. 5:35 We need to do one more thing, we need to close the file. 5:37 Now if you're really building this, and 5:41 you're really building it the way that I'm building it, when this function ends, 5:43 when rememberer hits the end of it and it's done. 5:46 Then everything inside it gets garbage collected. 5:50 Python takes all that stuff and goes, you don't need this anymore, and 5:53 throws it away. 5:56 Which means that the file being open, the file would be closed. 5:57 So it would be fine, it would be garbage collected and then it would be closed. 6:01 But, I like being really explicit about this. 6:06 I don't want to know that I'm closing the file. 6:08 I want to make sure that file is closed. 6:10 Some I'm gonna do it on purpose. 6:12 So I'm gonna do file.close, not clone, 6:14 close and I know that the file has been closed. 6:17 So if I run this again, and 6:22 I need to pick up a sweater, I know the file's been closed. 6:23 Nothing changed outwardly, but Python opened the file, Python wrote to the file, 6:29 and then Python closed the file. 6:33 This pattern is really, really common, and it's actually so 6:36 common that there's a built in easier way to do it. 6:39 We can use the with keyword to make what's known as a context manager. 6:43 We're defining a block like a if or a while and inside of that block, 6:48 the variables only exist for a certain time. 6:54 If I come over here, and I say with, and 6:58 I take out the file equals, open that as file. 7:03 Now we have, inside here, we have a block. 7:07 Inside this block, the file exists and the file points to that opened file. 7:09 But once this block ends, file goes away, which is really awesome and handy. 7:14 So we're actually just going to indent both of those and since file 7:20 goes away once that with is done, I don't need those two lines anymore. 7:25 So all I need is, 7:31 open up the file In a context manager, write the thing into the file. 7:33 So, cool we have two lines that are replaced our three lines. 7:38 Let's run it one more time to make sure that it works. 7:41 I remember to get a sword and 7:44 if come look at my database, I have orange juice, milk, sweater and a sword. 7:47 And it's kinda funny that orange shows up like that. 7:52 Anyway, so great. 7:55 Now you know how to write to files. 7:56