Python Jumpstart by Building 10 Apps Transcripts
Chapter: App 8: File Searcher App
Lecture: Recursion applied

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0:01 Ok, let's see how recursion will us search
0:03 not just the current directory that we give it
0:05 but the entire directory tree starting at the directory we provided as the root.
0:11 Here we have search folders, this is going to be our recursive function,
0:14 and we are going to come here, we are going to list all the items here,
0:17 we want to for each item build up the full path to that item.
0:20 Remember these little bits are just relative,
0:23 they are sort of the name but not including the path.
0:26 Before we said we don't know what to do with directories
0:28 let's just get out of here and then we said if it is a file
0:31 we will go get all the matching lines and information
0:34 about that in extend to all matches,
0:36 well now we can use recursion to solve this problem
0:39 so let's try this like else really quick,
0:41 so we sort of put these in these two cases
0:44 so if it's a directory we do one thing otherwise,
0:46 if it's a file we go search it. so what are we going to do,
0:50 we are just going to say if it's a directory,
0:56 let's also search that directory and what method do we have that can search folders,
0:58 well, the one that we are in.
1:00 But we are going to search a different folder,
1:02 a subfolder, and the name of the subfolder is full item
1:04 and we still want to search for the same text,
1:06 now this will come back with matches as well,
1:08 different sets of matches probably more, you never know, right,
1:11 but it's all the stuff within that subfolder not just an individual file,
1:15 and then we'll say all matches.extent those matches as well.
1:19 Look how perfectly recursion solves this problem
1:22 like it was really one or two lines of code,
1:26 we removed the continue we just said search but a smaller subset of this tree
1:32 we want to go search and we are just going to recursively do that
1:34 until there is no more directories we are just down to the end,
1:37 the end of that hierarchy if you will.
1:39 So let's run this and see if it's working.
1:41 So again, we are going to search our books,
1:43 now remember there is that classics folder that has Sherlock Holmes in it
1:45 previously we did not have results for Sherlock Holmes
1:49 so hopefully we are going to search the books folder and within there
1:52 we are also going to search the classics subfolder, let's give it a shot.
1:56 Again, we are going to search for Holmes, and here we go,
1:59 wow, now we have a lot more results and we've got the Ulysses book
2:03 but we also have classics, The Adventures of Sherlock Holmes
2:07 you can see on line 11587 the table had not been cleared yet, Sherlock Holmes had been...
2:13 beautiful, and you saw that recursion was the key
2:17 to making this work in the most dramatically simple way possible.
2:21 So in some sense our app is done,
2:24 if what we want to give our app is 10 books that are just all text
2:30 and maybe that's a couple of megabytes
2:32 you can see that we can actually search that quite quickly and there is no problem,
2:36 but if the number of files and the quantity of files gets to be gigabytes of text
2:41 you'll see there is a severe performance problem especially around memory
2:45 and we can do much better using some very cool features
2:49 called generator methods that's what we are going to do next.