Python Jumpstart by Building 10 Apps Transcripts
Chapter: App 9: Real Estate Analysis App
Lecture: Python 3 AND Python 2 Compatible Code

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0:00 This code is working perfectly in Python 3, as we saw.
0:05 Let's see how it works in Python 2.
0:07 Now, there is a minor problem we are going to run into with our file IO
0:11 here so we'll solve that really quickly,
0:13 but let's go add it the configuration and say
0:15 hey, let's try to run this under Python 2.7.10.
0:18 No module named statistics.
0:23 Well, we need that, right?, we are using statistics.mean it turns out,
0:28 statistics was introduced later on in Python,
0:31 I think it was 3.4 but somewhere around that time frame, and it's definitely not in 2.7,
0:36 so what do we do, do we just say well,
0:38 we've got to take the mean so forget it, you can't write this code,
0:40 or another possibility would just be to say well,
0:43 we can't use the statistics module
0:45 we are going to have to just write our entire own statistics mechanism.
0:49 I suppose there is some answer out on PyPi but as a way of demonstration,
0:53 let me show you how we can fix this.
0:56 so that's the line where this is crashing,
0:58 and if you don't know for sure, you click here it says line 3 click
1:01 and it will navigate you right there, yeah, it looks like that's the line that's crashing,
1:04 and that's because there is no statistics module.
1:07 So what we can do, we are going to dig into this error handling later,
1:10 just roll with it here ok, what we are going to do is
1:15 we are going to say we are going to try to run this code,
1:17 and if it fails to import the statistics module,
1:20 we are going to fall back to something else, so let's say try, this is one of the,
1:25 this is basically the key concept of the next application,
1:28 so don't worry too much about the details,
1:30 what we'll say is we are going to try to run this code
1:33 and if there is some kind of error I'd rather run error code instead.
1:36 So either we are going to successfully import this
1:38 or we are going to have to do something else.
1:42 So let's add another file here, I'll call this statistics stand in for Python 2, ok,
1:49 name it whatever you want, it doesn't matter,
1:51 let me just capture the name really quick here,
1:53 we are going to come over here, we are going to say if there is an error,
1:56 we are going to instead import that now this is not going to work so well,
2:01 because we are going to still try to use this name below
2:03 and it's this is going to be here, right,
2:07 so what we can do is we can say as this,
2:09 so basically here is the real module name but forget it rename it to that
2:13 and as long as it has a mean method, that takes data,
2:19 and let's just say return 0 for a minute, things will run.
2:23 So remember, we were running on Python 2.7 first of all,
2:28 let's go run it back on Python 3, works, right,
2:34 here is our mean method, now this is going to return 0,
2:36 we are going to put real numbers there in a minute,
2:37 but let's just see that it doesn't crash in the same place,
2:40 there is going to be another crash we'll deal with hold on.
2:44 Look, no errors, there is this problem with encoding
2:46 and it turns out that this do like so, turns out that that didn't work in Python 2.7
2:53 exactly in that manner, so we'll drop it, but now watch this,
2:58 it used our method when it wasn't available from the system,
3:03 and that is in the case that it happens to be 2.7,
3:05 it could actually be Python 3 but Python 3.1
3:09 which preceded the sort of creation of that module.
3:13 So I guess it's up to us to go write some kind of a mean method,
3:17 so we'll have total=0 we'll have count=0, now we could do len of data
3:26 but if it's a generator it's better if we just do it this way,
3:29 for x in data:, count +=1, total+=x
3:37 and now we want to return total divided by count,
3:40 unless that's 0 then that's king of be a problem so then we could say
3:43 max of 1 and count so that way a count of 0 we won't get it
3:50 divided by 0 error but otherwise we'll use max.
3:53 Let's run it and see if we get some decent numbers here,
3:55 look at that, in Python 2 we have exactly the same answers, right,
4:05 that's Python 2, go back to Python 3,
4:08 notice 234, 165, 234, 165, same numbers,
4:15 so how do we do that, besides ignoring the encoding, we said look,
4:22 we'll try to use statistics if for whatever reason it's not available,
4:24 then we are going to fall back and use our own probably less performant,
4:28 maybe bug written implementation,
4:31 but at least we have something to fall back to and we named it this,
4:35 but we for the purposes of this file,
4:37 we sort of replaced the same name here as statistics.
4:41 There is a lot to know about writing code that runs on Python 3 and Python 2,
4:46 here is just one example hopefully you'll find this useful.