Python Jumpstart by Building 10 Apps Transcripts
Chapter: App 9: Real Estate Analysis App
Lecture: CSV Processing From Scratch

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0:01 So now it's time to load the data,
0:03 we've got everything just lined up for us to grab this file and run with it.
0:05 Let's do a quick little context manager
0:07 we'll do a with open file name as we'll do read only text,
0:12 and let's go and set the encoding, to utf 8,
0:17 remember, this is the file we are going after and it's right at the top
0:22 you can see we have the street then the city,
0:25 then the zip code, then the state, beds, bath and so on,
0:27 this will give us information about all the real estate purchases
0:29 that have happened over here.
0:31 When we are processing the csv, there is actually a couple of options,
0:35 one option would be to just parse this directly as strings,
0:41 now three is another option we'll use in just a minute
0:44 but let me just kind of sketch that out for you
0:47 because understanding how that might go,
0:50 would help you approach file formats that there is not a built in reader for.
0:53 The first thing we need to do is capture the header,
0:55 and that's not the thing of the top that we are printing out but you know,
0:58 the sort of column names at the top of this,
1:00 so we can say fin.readline singular
1:03 and we could do something like print found header,
1:06 to be and then we'll just print that out.
1:08 And then the next thing we need to do is we need to go through
1:12 line by line and turn this into some kind of data structure,
1:15 we'll start simple and we'll get something more interesting in a minute,
1:18 so we can say for line in fin, now we could have just started out this way
1:23 but remember the first line is the header
1:26 and then every subsequent line is actually the data,
1:28 so we do one read line and then we stream over it,
1:31 and we need to turn the line into the individual columns of data
1:36 and with the csv comma separated value one way let's call this line data,
1:42 one way to do this would be to say line.split and we can just split on the comma,
1:47 now this is not perfect because you'll see that there are ways
1:50 to like escape commas and so on
1:52 and the edge cases can be a little bit tricky,
1:55 so we are not going to end up using this,
1:57 but let me just print out well, let's build up all this we'll say lines,
2:00 and we'll gather this here, and we'll say lines.append line data,
2:06 and then let's just print out let's say the first bunch here so we'll say print lines,
2:11 we don't want all like thousands of lines, so let's just say we'll take the first line,
2:15 remember we can slice from 0 to 5 and just get the first lines like so.
2:19 So let's run this to see what happens.
2:22 Fantastic, here is our header, that looks good
2:24 and look here is a string for High Street,
2:27 the string for Sacramento, now there is two lists here,
2:30 and we have basically a list of lists,
2:34 that was the first row, that's the second row,
2:35 now notice the very last bit has a backslash n \n
2:39 and that's because it's coming form when we read a line out of a file,
2:43 a text file in Python it keeps that on the end there,
2:46 so let's do a strip and we can strip the bits off the front and the back
2:51 and I suppose we could go and do that with the header as well,
2:54 now we try that again, perfect, our latitude is looking solid.
2:59 Ok, so now if we wanted to go work with the data, remember,
3:05 this is not where we are going to stop, this is where we are starting,
3:07 if we wanted to work with like the number of beds
3:10 if I want to just print out those,
3:12 I could somehow go over to this bit here and for each line, and the line data,
3:16 I could come over and put bed was I think the fifth one, it didn't really matter,
3:21 let's say it's a fifth one, then I would say 4 and I could say bed count equals that,
3:25 ok, now there is a couple of challenges,
3:27 because this actually comes back as a string so I need to parse it and so on,
3:31 but this is sort of the basic way of working with this data.
3:34 So that is kind of how we would attack csv files from scratch,
3:39 but Python has built in support for csvs and we can do better,
3:43 so let's use the csv module and then build on that.