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in the previous video we looked at how

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to catch exceptions and handle them

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using the try and accept statements and

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we looked at how to use finally to make

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sure that we can run clean up code for

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example even if an exception is raised

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and then we also saw yet another use of

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the else keyword this time to execute

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code if an exception isn't raised now

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handling exceptions is important to make

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sure your code doesn't crash so if you

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can take some useful action when an

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exception is raised then you should

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certainly do so but what happens if you

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code can't handle the exception now

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if your module relies on a file name that has

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been provided by the user for example

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they may have entered an invalid volume or

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drive letter on Windows now your module

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may be called by some other code that

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requested the file name so you can't

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just go picking a random location on

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the computer to write your data to so

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in that case there's not a lot that your code

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can do about the exception

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now you may be tempted to ask the user

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for another file name but if you're

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code is running on a remote server or as a

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windows service

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no one will ever see your prompt and

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your program will sit there waiting for

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input that will never come so in that

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case theirs 2 actions you can take one

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action really the first thing you can do

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is do nothing and we'll see in a minute

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that doing nothing can actually be the

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best approach and they can be nothing

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wrong with ignoring exceptions that you

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can't deal with but the other option is

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to raise an exception to let the calling

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code know that something's gone wrong so

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we're going to use our duck class to

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demonstrate this so got the game project

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open and you can see I've got the duck

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stop py file opened so if you haven't

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got this file or you've been experimenting

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or modifying it then you will find the

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original ducks .py file in the

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resources section for this video so

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we're going to use our duck class in a demo

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program to model durk migration now

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it's all a bit simplified but if I

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waited until we really needed to use

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exceptions in our database code you'd

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have to learn about them while also

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having a lot of new database concepts

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to understand so I put this

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example together to explain raising

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exceptions before we need to do things

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like this in real code now our duck

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class maybe we'll be very simple but

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what we're about to do is exactly what you

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would do in real modules to make them

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more robust and to make debugging

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easier

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ok so should either have the game project

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loaded which I've got on screen there or

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you should have created a new python

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project with a Python file called ducks

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and then paste in the text from ducks.py

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from the resources section of this

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video so what I'm going to do is

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start by adding a new class to duck.py

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to represent a flock of ducks i'm

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going to call this class flock and add it

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after the penguin class down here at

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the bottom going to clean up some of

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this code that's been commented out so

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I'm actually going to add this

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class here on line 45 actually what I

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might do just make it a space down here so

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it's a little bit easier for us to

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see going to start with class...

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....

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....

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ok so there's our flock class so it's a

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fairly basic classes you can see when we

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create a new flock the unique method in

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line 48 creates an empty list called

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flock and that's with a lowercase as you

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can see there now we got an add

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duck method to add new ducks to the

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flock and also this migrate method that

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causes every duck in the flock to fly by

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calling their fly method so next what

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we're going to do is use our duck

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objects in another program so we're

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going to create a new python file

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called migration migration so we need to

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import ducks so that we can use the

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object from ducks.py and then i'm going

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to create a new flock and add some ducks

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to that so import...

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....

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....

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...okay so

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obviously I didn't try to think up other

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names so for each duck but if you're in

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a habit of naming duck then

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you've either got a good scientific

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reason for doing so or a very strange

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hobby

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alright so we've now got a new flock

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object

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we've also got the seven ducks that

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we've defined herein line 4 through

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10

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so let's now go ahead and add the Ducks

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to the flock....

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...

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....

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....

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....so

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it now contains seven ducks and we

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should be able to make the migrate so

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we do that by typing...

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so try running that to confirm that it

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works you can see that time we've got

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seven messages their so each duck is

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successfully flying so far that's

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working fine but the problem comes if i

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try to add percy the penguin to the

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flock let me show you what i mean so i'm

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going to do....

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...

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...

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....

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....you see that we've

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actually got an error so as you can see

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here we've only got four of our ducks

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that have managed to fly the other three

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grounded and failed to take off and again

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percy he had a problem trying to fly as

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well and i put this into

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floating mode right now to make it easier

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and probably see those links a little

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easier so you probably recall from we're

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setting up a penguin that this was going

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to happen and that's because penguins

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can't fly so the migrate method crashes

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when it tries to call Percy's fly method

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so we're going to use this simple

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example to look at how we can handle

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problems like this and the first thing we

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should do is to provide the users of our

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modules with a hint as to what they

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should be passing to our methods now

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python 3.5 added support for type hints

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in function and method declarations but

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function annotations have been around

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since python 3.0 you can use an

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annotation in exactly the same format as

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as a type in what we're about to do will

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work with all versions of python 3 in

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fact type hints it's just a formalization of

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using function annotations for static type

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analysis and it was mainly done so that

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type checking tools have a standard to

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work with now the lint checker in

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IntelliJ that gives us those warnings

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and errors in the right margin

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is one such checker alright so let's add

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some hints to our add duck method so that

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at least anyone using attacks module has

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an indication of what they should be

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providing to the method so we're going

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to actually just come back here close down

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run window and i'm going to add

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the code back in ducks.py and

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specifically what we're going to be

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looking at is the flock object and we're

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going to add hint to the add duck method

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so parameters are annotated by using a

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colon followed by the type of the

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parameter and return values are

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indicated using the literal a dash and a

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greater than symbol

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let's go ahead and do that so with duck

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we're going to put...

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...

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...

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...so again parameters

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are annotated by using a colon as you

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can see here and then now followed by

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the type of the perimeter and the return

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value can see I use a dash and a

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greater than sign and then the type of

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the parameter in this case we're using

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none when I switch back now that I've

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done that to migration py so you can

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see that intelliJ lint checker has

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picked up the annotation and we've

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actually got this percy showing a

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warning if I can hover over that now

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expected type duck got penguin instead if

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you click on one of the add duck method calls

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doesn't matter which one then use

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control q or ctrl J on a Mac there's now

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indication that we should be adding

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objects of type duck as you can see there

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now note that self has been annotated

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automatically self flock up here there's

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no need to annotate the self parameter

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in method calls so function annotations

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or type hints as they're called they

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do use exactly the same mechanism can be

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very useful now they're very simple to

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use so there's only one other thing i

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want to say about them now if you use a

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hint

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for one of the parameters then add a

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hint for all of them including the

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return value you can find more

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information of this in pet 484 lets

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just to open a browser I will quickly show

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that

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[Music]

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if you scroll down here

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here under the non goals section it

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should be emphasized that python will

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remain a dynamically typed language and

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the authors have no desire to ever make

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type hints mandatory even by convention

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and the second parameter under here the

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next section the meaning of annotations

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that recommends that you add hints for

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all arguments and the return type if

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you're going to add hints for any of them

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and I think that's probably good

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practice so do that

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ok so we've given the users of our

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module an indication of what they should

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pass to the add_duck_

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method as if the name was an

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indication enough in this case but that

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still doesn't prevent it from crashing

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if they ignore the hint so if we go ahead

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and run this migration .py we still

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get crashing

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so let's stop the video here and in the

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next video we're going to have a look at

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this stack trace and talk about this a

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little bit further so we'll see you in

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the next video