WEBVTT

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in this video we're going to look at

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parts of generators and generator

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functions now because one of the

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advantages of generators is that they

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save memory and actually a lot of memory

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in many cases I'm going to start by

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showing how we check how much memory

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things use in Python so go ahead and

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create a new project and I'm going to

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call mine if I select the relevant

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things we're going to call it Jen

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example let's make sure in my case is in

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the right place and they're going to

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open a new file or rather create a new

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file and when I call this one sizes 5 ok

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so let's go ahead and type in some code

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we'll start by typing import this and a

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line three big underscore range is equal

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to range and a thousand in parentheses

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in line five with indoor print

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parentheses double quotes and Biggums

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called range is left and right curly

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braces space bytes in a dot format then

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within parentheses size dot gets sized

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off and then we type in big underscore

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range that's what we want to get the

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size from alright and that's the size

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before we've actually got some data in

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it now what we're going to do is on line

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seven we're going to create a list

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containing all the values so create a

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list containing all the values and we're

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going to put in ving underscore range so

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I'll start off by initializing a big end

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of score range equals in left or right

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square brackets and I'm going to type in

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for space val space in being on school

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range : and there's going to be big

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another school list sort of pins Val and

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actually that should have been big

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let's call list because we're using the

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range that we've defined on line three

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and we're creating this list which is

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containing all those values in that

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range like so and the last thing we'll

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do is let's just print out the size of

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the list so the little print parentheses

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double quote double quotes rather

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becomes called a list is a left-to-right

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curly braces then bytes then dot format

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and so I'm going to this dot get sizeof

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and I'm surprisingly big on the score

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list this time so there's our program so

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the part of the code itself creates two

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objects we've got two big underscore

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range that's a range that returns the

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numbers from 0 to 999 and it's not

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really that big a range but it'll do for

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our purposes here to demonstrate the

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memory usage you can make it bigger if

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you want but I'll wait until we run this

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program before using a larger number

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now a big underscore list on the other

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hand that object that is created by

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pending all the values from the bigger

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let's go range into a list and you can

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see the list being initialized on line 8

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then we then using to get sizeof

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function and eat in both cases from the

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sis module to see how much memory each

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object uses so I've actually run this

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you should find that they occupy 48

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bytes the range okay range itself as you

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can see occupies 48 bytes and the big

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underscore list though you can see

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that's taking up not nearly 9 kilobytes

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not a two-storey exactly the same values

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so I'll just move this down so I can get

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a better view down the bottom there if

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we go ahead and change and add an extra

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0 to the range up here on line 3 and

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then run it again you can see in this

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case the range itself is still only 48

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bytes but the list now is around 87 K

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hopefully there you can see why I

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suggested you wait before changing the

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actual numbers of values in the range if

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you make it too big you could eat up all

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your computer's memory now I should

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mention at this point that this isn't a

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very pythonic way of doing this and you

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probably use the list function itself to

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convert the range to a list but I wanted

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it to be obvious what we're doing which

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is why this code loops through the

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values in the range and appends each one

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to the list so what's actually going on

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here and how is it such a big difference

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in the memory that the two objects are

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using which actually is still exactly

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the same set of numbers well both big

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underscore range and Biggums call lists

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are actually iterators so we can use

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either in a for loop to iterate over all

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the values now big underscore range is a

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special case of an iterator called a

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generator and in fact we can run our own

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version of bigger than score range so

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I'm going to go ahead and do that

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and I'm going to call it my in the score

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range I'm going to go ahead up up here

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and start typing this code on line four

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so I'm going to define a function so def

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my own score range and in parentheses n

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colon and then int colon the end then we

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can have start equals zero while start

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is less than end colon then we're going

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to do yield type in the word yield space

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start and start space plus equals one

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and what we're going to do then is I'm

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going to comment out this next loan this

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bigger score range but was hard coded

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and we're going to change that to bigger

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than score range is equal to my

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underscore range and we're going to use

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the same number ten thousand so we're

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going to call the function that we've

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created so let's go ahead now and run

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the program again noting the previous

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values that we had down here and you can

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see in this case Big M is called ranges

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do eighty eight bytes and it may be 72

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by or something it's actually between 72

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to 80 eight bytes depending on the

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Python implementation point is that

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what's different 248 but it's still a

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vast improvement over 8k and that's

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because in this case of course we've

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written our own range instead of using

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the built-in one alright so I think I've

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made the point about the memory

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requirements so I'm going to reduce the

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range to just 5 otherwise I'm going to

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spend most of the time scroll

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and three thousands of lines of output

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that we're going to add so I'm going to

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go back here and just change the range

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to five Vanille and let's also now go

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ahead and actually print out the two

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objects at the end of the program so

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we've got the get sizeof function call

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which we're going to leave in there

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we're going to print this out as well as

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a little print be going to school range

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and print vegan school lists just to be

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consistent that's also changes commented

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out to range to five as well all right

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run it again so beginners called range

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in this case is eighty eight bytes

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bearing in mind that may be different on

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your operating system you might get a

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remember between 72 and 88 the point is

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it's the same as it was when I read last

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time and I'm surprisingly big on a

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school list is a lot smaller now because

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of storing fuel values in looking at the

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two lines of output that we've generated

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here down the bottom big underscore

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range is a generator object based on the

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Miami school range function that we

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wrote a big underscore list though as

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you can see is just a list and contains

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the values would expect from a range of

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five so what's the Miami school range

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function doing and what's this yield

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statement that we worked on on line

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seven what's that actually doing what's

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that all about

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well when you write a normal function

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the function finishes after returning

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the result you can see that if we change

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this yield and change that keyword to

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return and they get this warning here

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that the code is unreachable

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unsurprisingly because return is

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actually returning it the execution back

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to the calling code now if we go ahead

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and actually run that because that was

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just a warning we end up with an error

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the type error down the bottom there in

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object is not iterable that's because we

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can iterate over an int the point is

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that the - called range now returns a

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single value then stops as opposed to

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what it was returning before the other

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thing to note though down here you can

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see in the output the Bignell is called

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range there was only 24 bytes noting

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that the last few times I ran it it was

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88 and might have been a number between

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70 to 90 and in your operating

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the point is it's a different value to

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what it was when it was previously read

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so once that change in behavior to just

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return one value than stop there's

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something going on with the structure of

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the - called range object as well now

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when it's a generator and uses yield it

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actually takes up a bit more memory so

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we change line 7 again back to what it

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was to yield of course that warning goes

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that way the difference is that when we

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use yield the function actually returns

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the yielded value then goes into a sort

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of suspended state so it doesn't just

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return the value it keeps track of all

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its variables and worry about in its

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code it is so the next time it's called

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it wakes up and continues from where it

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left off so looking at this code on line

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12 this the function call

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you might be thinking it only gets

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called once but in fact it's called each

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time it's iterated over in the loop on

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line 18 down here and we can see that

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better by adding some diagnostic print

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statements so let's go ahead and do that

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so come up here to the - called range

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function let's add some code to print

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out so print parentheses double quote

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why underscore range starts and we'll

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also put one in the wild statement after

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the wild start to less than end

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statement or line

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then we'll do print parentheses double

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quotes my underscore range is returning

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and left the right curly braces dot the

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format's then start and if we actually

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run this and have a look at the output

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you can actually see that in that case

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my underscore range starts then we're

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getting a value 0 1 2 3 4 so katie has

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been called five times now it doesn't

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quite show the full story though now I

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said you might think that my underscore

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range is only called once or what was

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line 12 which is down line 14 over here

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but it's not actually called on that

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line so to see what's really going on a

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bit better we can pause the program by

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getting it to wait for input at various

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points so let's go ahead and do that and

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that should

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feels a little bit clearer so I'm going

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to go back up here onto line 12 and I'm

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going to put underscore equals input and

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parenthesis and the double quotes line

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12 they're going to come down to line 15

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let's take a copy of that line 12 paste

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it in for line 15 and then we'll change

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the reference to line 15 and make sure

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that we've deleted the extra line there

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let's also add a third one we'll come

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down here after the big underscore list

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let's go ahead and do that to line 22 so

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line 22 then lastly within the for loop

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itself let's actually put one in there

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so that's going to be line 24 so I'm

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going to make that line 20 for it just

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to be clear

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we'll add some extra text inside loop

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okay so we're using an underscore by the

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way aligns 12 15 22 and 24 that's of

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using that as a variable name in here

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because that's a Python convention and

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indicates that we're not interested in

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the value of the variable what we're

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doing is we're just using those input

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statements to get the program to wait

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until we press ENTER now if you're also

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programming in Java don't use a single

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underscore as a variable name in Java

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and stop doing it if you currently are

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now it's valid in Java right but it's

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being removed in Java and on though it's

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an accepted convention in Python it

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doesn't translate well to rid

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practically all other languages alright

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so now that we've added those input

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lines let's actually run the program to

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see how it behaves now okay so we're

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down here we've got tired waiting on

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line 12 and you can see there's no

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output at this point just waiting until

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the end of is pressed so when I press

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ENTER it's come back and said it's

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stoppable it's actually stopped again on

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line 15 so the big range equals my

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underscore range here in line 14 well

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that's already been executed but notice

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that nothing else is printed at this

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stage just the input prompt so in other

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words - call range the function hasn't

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been executed at this point so you

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wouldn't normally include this line in

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code you just use my underscore range

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five in the four loop in other words

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this is sort of an extra line there you

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just go straight down to here and you

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put my underscore range there on line 23

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direct in the four loop itself we've got

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it here so you can check the size of the

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object but it's really not necessary and

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in fact it can even be a bad idea and

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I'm going to show you why shortly now if

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I just continue on though back in the

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run window if I press ENTER again you

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can see it stopped on line 22 just

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before the for loop starts but now it

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gets interesting we press ENTER again

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and we've entered the for loop now you

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can see that this was the first time

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looking at the output that my own score

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range gets called when the first value

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is actually needed so we get the message

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my underscore range starts which of

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course we put up a start on the function

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and the first value in this case is zero

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is return so pressing enter again and

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the next fill is returned and notice

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that my own score range doesn't start

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again at the beginning it's continuing

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from line eight so you can see light on

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the screen there so we don't get the

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start message printed again now each

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time I press ENTER the next value is

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returned until the generator runs out of

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valleys to return and the loop axis and

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I'll just go ahead and do that and you

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can see our program is now all closed so

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the loop went around five times so the

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line 24 inside loop prompt was displayed

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five times all right so let's end the

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video here in the next video we'll go

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ahead and start talking more about this

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generator and add some extra code to

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this method to clear things up so I'll

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see you in the next video.