Python Jumpstart by Building 10 Apps Transcripts
Chapter: App 9: Real Estate Analysis App
Lecture: Parsing CSV data into Classes
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0:00
Ok, we saw that the csv dict reader was really the way to go from the text file into memory for comma separated data.
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Now, there are still some things that are not quite perfect, like imagine I was given this data, I would somehow want to go through it,
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if you asked me what is the average number of beds, well, I still have to know every single time to say quote beds,
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and if I say bed instead of beds, crash, that's not great. It's not discoverable what all the rows are,
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and the rows all come back as text even though beds represent a number,
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it's just a string that came out of the text file so if I look at the type here, I'm going to say type of that, it should come back as str,
0:52
guess what, type is str because it's a string, we need to convert it to its either an integer or for things like longitude, latitude to floats,
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so let's bundle up that sort of implicit knowledge of the format here, and we can put that into a separate class,
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so I'll create a file and I'll call this just data types, probably a better descriptive name would be great there
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but let's define a class and we'll call this, let's just call it a purchase, right, I was going to call it real estate purchase
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but within the context of our real estate app hopefully the real estate component is clear.
1:29
Now, I am going to show you something that you've not seen in this class about classes, what we have done so far is we have defined a __init__()
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and we are going to do that again here, let me just really quickly sketch out the pieces that are going to go in here,
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you don't need to see me type this, ok, so now I've sketched out a fairly complicated initialization method but you know,
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it's just a matter of what pieces of data you want to store in here and where do we go. Now, we could go back to program and make it this method,
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this load files job to allocate a bunch of those purchase objects do the conversion type here and stash the data in here
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by passing them to the initializer. But, this class already knows the structure and it knows the pieces that it expects,
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so let's make this class responsible for creating other ones, ok. So we can come over here and define the method,
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let's just call this create_from_dict(), something like this, and we'll call this a lookup, or something like that, ok,
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we will come down here we will say something this, return purchase and we are going to allocate one of these things,
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the first thing that goes in here is going to be the street so we would say lookup of street like so,
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again just let me type this out and I'll skip ahead, ok, so I wrote this method, now there is a few more things to do here,
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we want to convert the number of beds into an integer, so we need to parse this, similarly we want to convert the number of baths the square footage,
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I believe that is an integer and a price, let's make that a float, just to be safe. I suppose we could do that here as well.
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Ok, so this will create these purchase objects and from then on, I can say things like . (dot) and here is all my list of items,
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and the price is always going to be a float, and that will be a strong structure that we can work with in our app.
3:34
But, let's try to use this method this is not going to be so cool, so come over here to our program, let's go ahead and make a purchases list here,
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so we are just going to store all the purchases basically one for each row, let's say purchases.append() purchase,
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let's just say p, keep it short to fit on the screen, so we'll say p=purchase, we need to import this, if I try this, it's- let's give it the row here,
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row is a dictionary, if we try to run this, it's not going to go so well, it says missing one position lookup,
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the other thing that it wanted here is self, we have to order these this way, I have to do this weird thing,
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I have to say p=purchase and I got it created and then I have to say p. well, that almost works, notice this line little error went away,
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the line above now says oh, you are missing these 12 parameters, yayks. So, let's go back through this.
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This method has nothing to do with the individual objects, it deals with the class, right, this just is associated with this class
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but it doesn't have anything to do with the self parameter, it could be a static method, it could be one that we can just call right on the type
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and it maybe will allocate instances, of the purchase. Now PyCharm is actually telling us to head as well like see the little squiggles there
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it says you know what, you can make this method static and the way we do that is we add what is called a decorator,
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a decorator starts with the @ symbol, and they have a cool color in PyCharm, so we can say @staticmethod there is other things you can say,
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you can say @property, and define the method that would be a property instead of a method and so on, what we are going to use is static method
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and that lets us go back here and instead of requiring an instance we say like this, purchase.create_from_lookup and it only takes the dictionary here,
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so let's go and append these, and let's top the print out here, I'll just delete those,
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when we are done, I'll just print let's just print the first entry, our first class, all right, run this- oops,
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I mean purchases, so we go this first one now that's not super helpful,
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let's use this internal dictionary just to see what the data is like really quick,
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here we go, sale date, such and such, state number of baths, perfect,
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and notice, the type, residential has a quote but the number of baths is an actual number, it's the 1, not quote '1',
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which means we can do math on it similarly for things like the longitude, latitude, number of bedrooms and so on.
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So this is great, now we are just going to return our purchases,
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what will be in this list, this will be our specifically typed purchase class that has to do, that knows specifically about the data containing here
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and we can leverage that to help us do math like ask what is the average number of bathrooms and so on.