1 00:00:05,200 --> 00:00:10,459 The strip, lstrip, and rstrip methods are commonly used when processing text files. 2 00:00:10,560 --> 00:00:15,404 We've already seen how to strip whitespace, including the newline, from the end of a line. 3 00:00:15,520 --> 00:00:20,246 I'm including this video, because the exact behaviour of these methods isn't obvious. 4 00:00:20,320 --> 00:00:25,273 Many students get confused, when they try to strip something and get strange results. 5 00:00:25,360 --> 00:00:28,880 Create a new Python file called stripping.py for this video. 6 00:00:28,880 --> 00:00:32,179 I'll start by reading the first line of the Jabberwocky file: 7 00:01:01,920 --> 00:01:05,528 Run the program, to check the string that we'll be working with. 8 00:01:05,760 --> 00:01:10,560 Note that you don't have to read every line from a file. We saw that in the last video, when we 9 00:01:10,560 --> 00:01:15,680 broke out of the loop, and we're only reading one line here. After the first line has been read, 10 00:01:15,680 --> 00:01:21,738 the with block exits and the file gets closed. Ok, let's strip the apostrophe from the string. 11 00:01:21,840 --> 00:01:26,688 I'll use strip for these examples, but the behaviour is the same for lstrip and rstrip. 12 00:01:26,688 --> 00:01:30,312 The only difference is that strip will strip from both ends of the string. 13 00:01:49,840 --> 00:01:54,794 Run the program, to confirm that no_apostrophe doesn't start with an apostrophe. 14 00:01:55,200 --> 00:01:57,920 That's probably the behaviour you expected, but it can lead 15 00:01:57,920 --> 00:02:01,979 you to believe that the string we stored in chars is removed from either end of first. 16 00:02:02,160 --> 00:02:05,320 That's not what's happening here. To demonstrate that, 17 00:02:05,320 --> 00:02:10,899 I'll attempt to strip the apostrophe and the word "Twas" from the string. Change line 7: 18 00:02:17,120 --> 00:02:21,840 That looks like it should work. Let's run it and see: 19 00:02:22,880 --> 00:02:28,138 At first glance, it seems to have worked. But what's happened to the end of the string? 20 00:02:28,320 --> 00:02:34,045 The "s" of "toves" has been removed, as well! This is the behaviour that can be confusing. 21 00:02:34,240 --> 00:02:37,644 These strip methods don't remove the string that you pass as an argument. 22 00:02:37,840 --> 00:02:41,440 They take each of the characters in the string, and keep removing them, 23 00:02:41,440 --> 00:02:45,625 until a non-matching character is found. The only character that matched, 24 00:02:45,625 --> 00:02:51,200 at the end of the string, was the s. When strip got to the e in "tove", that doesn't match any of the 25 00:02:51,200 --> 00:02:56,560 characters in our chars variable. So strip stopped removing characters from the end of the string. 26 00:02:56,560 --> 00:03:01,191 I'll add some more characters to chars, so we can get a clearer idea of the behaviour: 27 00:03:07,280 --> 00:03:11,040 Our chars string now contains the characters "e", "b" and "v", 28 00:03:11,040 --> 00:03:14,960 in addition to the ones we had before. That means any of the 8 characters 29 00:03:14,960 --> 00:03:19,419 in chars will be removed, until strip reaches a character that isn't in chars. 30 00:03:19,600 --> 00:03:25,072 The "b" of "brillig", and the "v" and "e" from the end of "tove", should also be removed. 31 00:03:25,200 --> 00:03:29,712 Run the program, to check if that's what happens. And it's not! 32 00:03:29,840 --> 00:03:34,560 The "e" and the "v" were removed from the end, but we've still got the "b" of "brillig". 33 00:03:34,560 --> 00:03:40,739 Pause the video, and see if you can work out why. Start the video again, to see if you were right. 34 00:03:42,320 --> 00:03:46,915 Ok, did you work it out? strip checks each character at the ends of the string, 35 00:03:46,915 --> 00:03:50,880 in turn. If a character is in the characters we told it to remove, 36 00:03:50,880 --> 00:03:54,880 it removes it and moves on to the next. It stops when it finds a character 37 00:03:54,880 --> 00:03:59,440 that's not one that we told it to remove. In this case, when examining the left hand 38 00:03:59,440 --> 00:04:04,632 side of the string, it removed the apostrophe and "Twas", then it examined the space. 39 00:04:04,800 --> 00:04:08,080 We didn't include a space in chars, so strip stopped processing 40 00:04:08,080 --> 00:04:12,080 the left hand side of the string. When processing the right hand side, 41 00:04:12,080 --> 00:04:17,539 it removed the "s", the "e" and the "v". It then got to the "o" and stopped. 42 00:04:17,600 --> 00:04:22,240 We don't have "o" in our chars string. Add a space to chars, to confirm that 43 00:04:22,240 --> 00:04:27,000 strip will remove any character it finds, until it finds something we didn't include: 44 00:04:33,280 --> 00:04:39,165 I've included a space, between the apostrophe and the letter "T". Run the program again 45 00:04:40,880 --> 00:04:45,593 and the space, and the letter "b", are now removed from the start of the string. 46 00:04:45,840 --> 00:04:49,885 This is one of those things that, once you understand it, it's obvious. 47 00:04:50,000 --> 00:04:53,378 But until you do understand it, it can be really confusing. 48 00:04:53,440 --> 00:04:58,160 I'm sure some students are still a bit confused, so I'll include another explanation. 49 00:04:58,160 --> 00:05:01,600 We'll write some code, to do the same job as strip. 50 00:05:01,600 --> 00:05:05,782 Comment out lines 8 and 9, and we'll use a couple of for loops. 51 00:05:06,960 --> 00:05:11,911 We'll use one for loop to process the start of the string, and another one to process the end. 52 00:05:35,760 --> 00:05:41,232 Ok, that will tell us which characters will be removed from the start of the string. Run it 53 00:05:42,720 --> 00:05:48,432 and we can see the behaviour of strip. It removes each of the characters, if they're in chars. 54 00:05:48,560 --> 00:05:52,337 As soon as it finds a character that isn't in chars, it stops. 55 00:05:52,640 --> 00:05:57,897 Processing the end of the string is very similar. I'll print some asterisks to separate the output: 56 00:06:17,440 --> 00:06:22,068 We iterate backwards, from the end of the string. I've used a slice here, 57 00:06:22,068 --> 00:06:26,400 but you could also use the reversed function. The body of this for loop will be the same as the 58 00:06:26,400 --> 00:06:30,659 previous one, so I'll copy and paste the code. 59 00:06:34,836 --> 00:06:37,250 Run the program again 60 00:06:38,480 --> 00:06:42,059 and we can see the characters that strip removed from the end of the string. 61 00:06:42,160 --> 00:06:46,539 That might not be the behaviour you expected, but that's how the strip methods work. 62 00:06:46,640 --> 00:06:49,680 You may have expected the entire string to be stripped from the start, 63 00:06:49,680 --> 00:06:54,160 or end, of the string in first. That's not what the strip methods do, 64 00:06:54,160 --> 00:06:58,600 but we'll have a look at how we can achieve that. See you in the next video.