Python 3, an illustrated tour Transcripts
Chapter: Language syntax
Lecture: Walk-through: Other Changes

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0:01 In this video we're going to look at other
0:03 Open it up in your editor, let's run it and make sure that it runs.
0:10 Okay, it looks like there's one failure,
0:13 that's because there's one test function, cool.
0:15 Let's go through it. Lazy range, get the 100th item
0:19 from the vals variable, store it in the variable named hun.
0:23 So we have a vals variable here
0:25 and it's range from 42 up to but not including two million
0:29 jumping by 32 and we want the 100th item here,
0:33 let's try and see if we can slice it off
0:36 hun is equal to vals 100, let's run that and see if it works.
0:42 Okay, it looks like that worked,
0:45 so the range function even though it's lazy in Python 3,
0:49 allows you to do some slicing off of it,
0:52 let's do a map, find the 100th item from mapping fn,
0:56 which is this function right up here, to vals using the map function
1:01 store the result in hun function.
1:03 So we say map we're going to map a function fn to vals.
1:09 And we want the hundredth guy from that.
1:12 So hun_fn is equal to that and let's see if we can slice off the hundredth guy.
1:18 We'll run it and we get an error, map is not subscriptable.
1:23 So even though range is lazy, map is also lazy,
1:28 but map doesn't support this index operation,
1:34 so what we would need to do to get the hundredth item
1:38 is do something like this,
1:43 seq is equal to the map of that,
1:47 and then for i in range 100 hun_fn equals next seq
2:01 let's see if that works
2:05 and I have a typo here let's change that and run it again.
2:09 Okay, and so it looks like that works
2:12 so this just shows you something that you may need to do
2:15 in this case I need to jump through a little hoop
2:17 and call next 100 times to get this item from my sequence here
2:23 because it's lazy.
2:25 Now there are pluses and minuses to this,
2:27 in Python 2 map materialized the list for you and you can slice a list
2:32 but if you have an arbitrary long list it might take a bunch of memory
2:34 so there are tradeoffs depending on what behavior
2:37 also I could course this into a list and do that
2:41 but again, we're going from 42 to 2 million
2:45 so this is pretty big I might not want to do that.
2:49 Sorting, sort the nums list as if they were integers
2:53 store the result in ordered, hint look at the sorted and the keys parameter
2:58 look at sorted and the keys parameter,
3:00 so there's a built in function called sorted and it has a keys parameter.
3:04 Okay, so in Python 2, Python 2 would allow us to sort lists with arbitrary types
3:11 and a Python 3 wants to be a little bit more explicit.
3:13 So if we just say ordered = sorted nums, let's run that and see what happens
3:25 I get a type error, less than operation is not supported
3:28 between instances of string and int
3:31 that's because I've got a string in here and I've also got integers in here.
3:34 So it wants me to sort these as if they were integers
3:38 so one thing we can do is we can cast them to integers
3:41 so we could make a little for loop or do a list comprehension and cast them to integers.
3:47 But they key parameter in the sorted function
3:50 will allow us to apply an arbitrary function to an item that needs to be sorted
3:58 and will sort based on that.
4:01 And this gives us the original ordered will now give us back the original list
4:05 but sorted as if they were integers.
4:08 So let's run and see if it works.
4:12 Okay, it looks like that worked.
4:15 So this is sorting that list as if they were integers.
4:18 Name leakage, sum the square of the numbers in nums,
4:23 store the result in square sum
4:26 so I'm going to put them right here in this space
4:29 and note that I've got some variables here that are just floating around
4:33 that maybe someone created or maybe I created if I'm typing code
4:38 and if I want to square a bunch of numbers and I've got them in sequence,
4:43 one way to do that is to use a list comprehension,
4:46 I can say nums is equal to, or I've already got nums,
4:52 so maybe sq is equal to num squared for num in nums.
5:01 And then I want to sum that,
5:04 and so I'm going to say sq.sum is equal to the sum of this whole guy here.
5:13 Let's run that see if it works.
5:17 Okay, I got an error here, unsupported operand for ** a string and an int,
5:25 again I've got this string in here, so maybe I want to corse these to ints.
5:30 Let's corse them to ints, and then do that see if that works.
5:37 Okay, it looks like it worked this might just seem like a silly thing
5:41 but you'll note that in this case I used num
5:46 as my variable in my list comprehension here
5:50 and in Python 2 if I used num here when I get out of this list comprehension
5:55 the value of no will be the last value of this list comprehension.
5:58 In Python 3 that behavior is changed and there is no "leakage"
6:02 of this variable into the surrounding scope here
6:06 so num stays at 42.
6:09 So I put an X in here and a num in here
6:12 in case you used those in a list comprehension.
6:14 Now if you did these in another way, if you made a for loop and used num in there
6:18 then you would overwrite this num guy
6:21 so it's just something to be aware of if you're not familiar with that
6:24 in Python 2 on list comprehensions,
6:27 note that generator expressions and set and dictionary comprehensions
6:32 behave as in Python 3, there's no leakage there
6:35 but in the list comprehension there is.
6:38 So this video showed some of the things that changed in Python 3,
6:43 we have laziness as a general theme
6:45 so range and map are both lazy
6:49 they support somewhat different interface so be aware of that.
6:53 Sorting, when you sort different types, you need to be specific
6:58 and make sure that they support sorting
7:00 and so we can use the key parameter of the sorted function to help enable that
7:05 and there's no name leakage in list comprehensions now.