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alright so let's have a look in more

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detail now the stack trace has been

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printed out here now because of the way

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that the stack trace is generated it may

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be interspersed with the program's

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output if i run the program a few times

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you can probably see or you will see we

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don't always get the exact same contents

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well technically the content will be

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identical each time but the stack trace

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and the output can get mixed up and when

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it is mixed up we can't rely on what's

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been printed first we can say every time

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is that 4 ducks fly before we get an

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error to show you what I mean lets run

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this a few times so you can see that

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the stack trace is different there

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we've got dispersed in a different

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position than it was previously run it

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again we three at the top

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three lines of output and again we're

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getting different results pretty much

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every time we run it so we can't rely on

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what's been printed first all we can really

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say every time is that the four ducks

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fly before we get an error and that's

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true even if the trace starts to print

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before we see the fourth duck take to

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the air so reading from the bottom first

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and ignoring any text that doesn't

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appear mostly in red

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we've got an attribute error because the

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penguin has no attribute called fly

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right here down the bottom up until that

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the error as you can see here occurred on

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line 55 the migrate method and that's

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when it tried to called duck.fly so

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that's useful because it tells us where

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the error happened

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however we know there's not a problem

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with that particular line of code

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because we've seen 4 ducks already

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start flying

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so this is where the stack trace is

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useful now we've got code that works as

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long as its operating on the correct type

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of object to find out what's really

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caused the error we have to keep reading

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up the call stack so next we can see the

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migrate method was called on line 22

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duck.migrate as you can see here

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line 22 and in module and that means the

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main program when it called the flock .

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migrate method now sometimes that's all

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you need

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so if migrate accepted arguments that

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it would probably indicate a problem

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with one of the parameters

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but here though it's not quite

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that simple because we set up the flock

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earlier in the code but there are now

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enough clues to let us know that

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somehow the flocks got a penguin it now

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the important thing is that the cause

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of the error in this case isn't in ducks

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.py it's in a migration .py

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module now we've only got two modules here

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but we could have a coordinator program

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that calls another program to select

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different species of ducks that in turn

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calls our migrate module that uses

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the classes from ducks.py and

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things can get quite complex but with a

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stack trace like this it's much easier

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to find the root cause of errors and

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crashes alright so let's go see what

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we can do about this exception now in

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fact we've already done the first thing

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we could do we have done nothing so we

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didn't handle the exception in the

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migrate method so the exception

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propagated up the call stack to wherever

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called migrate and that was our main

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program and because that didn't handle

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the exception

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it got propagated up the python run time

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the Python runtime prints out this useful

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stacked trace and then terminates the

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program so if an exception isn't handled

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it will get passed up the call stack

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until either something handles it or it

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reaches the python runtime now having

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a program crashed like this isn't good

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but that doesn't mean that migrate

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method should handle the error it's so

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often better to leave errors for

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something further up the call chain to

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deal with so that's one option now is

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there anything useful that the migrate

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method could do in this case if

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there is we should actually do it but if

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there isn't then we should let the

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exception propagate to allow code

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that's in a better position to deal with

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the situation handle it now I don't know

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much about ducks and i'm pretty sure

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that once you've caused the flock of

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ducks to take flight

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there's no way to get them back on the

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ground now humane way that is but we can

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make sure that all the ducks that are

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capable of flying managed to migrate so

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to do that I'm going to start by

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catching the exception in the migrate

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method let's have a look at doing that

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i'm looking at our migrate method here

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so after the four we're going to....

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...

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...

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so this time you can see that all seven

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ducks manage to take off poor Percy is left

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on the ground but at least the rest of

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the flock are away but unfortunately the

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main programs now and none the wiser as

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far as our migration program is

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concerned we've got eight birds in the

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air so we didn't take any action in the

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exception handler so the exception was

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effectively just ignored so what we can

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do now that all the ducks safely flying

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is to raise the error that we've got

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when Percy attempted to fly now if we

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attempt to do that within the for loop the

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loop will terminate when we raise an

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exception so raising it in the exception

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handler will leave us no better off than

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before and to show you what i mean i'm

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going to change this Pass will print a

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message here one duck down they're and

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put raise run that again we're back to

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the same scenario where we were before

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or be that we've got an extra message

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here informing us that the percy

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didn't make it but importantly with now

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unfortunately only got four ducks in

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flight now you can log the values of

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local variables for example before

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raising the exception again some of the

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words are here and let the caller

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deal with it so to raise the exception

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that you just caught you just used the

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raise statement that has been typed in and

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actually preserves the stack trace so

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that all the useful information is still

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available for debugging and again if you

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run that again you can see that's the

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case so we still got our information

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and pir stack trace which we can see

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propagating up the call stack much as

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the case was before

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so basically all the useful information

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still available for debugging npw as O have

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said many times before everything in

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python is an object exceptions are no

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exception

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so we can obtain a reference to the

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exception that's just been raised and we

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do that with a slightly different form

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of the except Clause sp come back to

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our ducks.py to our migrated method

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and we do instead of typing accept

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attribute error at the end of that we do

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....so when used like this a

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reference to the exception is stored in

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a variable for us to do something with

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now one useful thing we could do is

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store the exception then raise it once

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all the Ducks are off the ground so to

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show you what I mean will delete the

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raise line which always commented out and

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we'll put..

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...

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...

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you can see we've got problem here local variable problem

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might be referenced before assignment so

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to fix that we should really assign

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value to this outside of the for loop so

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problem equals and we'll start with none

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that then remove that error

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00:08:05,340 --> 00:08:07,530
so let's now run this to see what

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00:08:07,530 --> 00:08:08,460
happens

192
00:08:08,460 --> 00:08:12,410
you can see this time we've actually got

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all seven ducks in the air one duck down

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for penguins didn't work and then we've

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got the error at the end so in other

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words that the seven ducks that can fly

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do so and our migration programs

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notified of the problem once they are all

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of the ground now you probably won't do

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that very often but it can be useful and

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it also shows another way to use raise

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00:08:33,659 --> 00:08:36,299
which is to specify the exception that

203
00:08:36,299 --> 00:08:39,600
you want to raise so looking at the stack

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trace now we still got all the useful

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information that we had before the

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original problem was on line 57 ducks.py

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i can click that link to have a look

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at it

209
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that's the duck .fly called their we got

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another entry this time and clicking

211
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that takes us to the line where the

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exception was raised

213
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this one here in line 63 and just move

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00:09:00,240 --> 00:09:03,390
this down a little bit line 63 is we've

215
00:09:03,390 --> 00:09:06,150
raised the exception clicking on the

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00:09:06,150 --> 00:09:08,990
first link takes us to well the actual

217
00:09:08,990 --> 00:09:11,550
line where we call the migrate method

218
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so the stack trace is slightly different

219
00:09:15,090 --> 00:09:16,890
but it still contains all the

220
00:09:16,890 --> 00:09:20,340
information you had before so

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let's end the video here in the next

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video we're going to talk about creating

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your own exceptions see you in the next

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video