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- [Narrator] In this video

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I'd like to talk more

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about passing objects to functions.

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Now you may recall back
in the preceding lesson

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that we talked about the fact

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that every single object in Python

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is passed to a function by reference.

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And the reason for that is,

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in Python everything is an object,

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and the way you manipulate
objects is through references

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to those objects,

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not by having direct access

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to the objects themselves.

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So when I pass something
like a list to a function,

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because a list is modifiable

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and because the function is receiving

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what is in fact the location
of that list in memory,

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the function can talk directly

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to the list object

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through that reference

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and modify the contents of the list.

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So I'm going to paste a snippet

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into this session here

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for a function called Modify Elements

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which is going to multiply

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all of the elements in Items,

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its parameter by two,

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and we want to use this

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to demonstrate the fact that

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when pass a list into a function,

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the function can modify the list.

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So we're going to iterate

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through every index representing

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the element positions

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within the list called Items

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and for each of those items,

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we're going to multiple its value by two

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and store the result back into the element

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within the original list,

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wherever that thing
happens to be in memory.

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So let's go ahead define that function

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and, of course, we need
some values to work with

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so let's create a numbers list

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and we'll give it some values.

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Let's do like 5 values here

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and let's evaluate numbers

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just so we can see it's contents.

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And, of course, if we go
and call Modify Elements

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and we hand it numbers as an argument,

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now numbers hopefully is modified

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and we can confirm that

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with the evaluation of numbers once again.

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And you can see that indeed,

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each of the values was doubled

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from its original value in memory.

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So clearly, we define
numbers outside the body

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of the function Modify Elements

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when we passed it into Modify Elements.

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We saw that the result was

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to modify the original list.

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Now there's nothing in this
original function definition

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that says Items must be a list.

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If we study the code,

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we can see that whatever we're working on

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has to be something that's modifiable,

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and, of course, the only thing we know

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with this syntax for
accessing its elements

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right now that's modifiable
is a list element

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so we know Items must be a list.

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But I can pass whatever I
want into Modify Elements.

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If Python doesn't like what I pass it,

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it will tell me by issuing
an exception at runtime.

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So let's go and create another collection.

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Let's create a Numbers Tuple.

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And let's assign it a tuple

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of 10, 20 and 30.

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And, of course, we can evaluate
that to see its contents,

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and now let's try to call
Modify Elements again.

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This time we'll call
it with Numbers Tuple.

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Now when I do that, you can see here

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that I'm getting an
exception at execution time.

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There's two pieces to
the exception message

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indicating line numbers
where problems occurred,

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so I do want to talk
about that a little bit.

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And we can see at the
bottom of the error message,

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a tuple object does not
support item assignment.

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So we're getting a type error

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because tuple's can't do
what we're trying to do

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in the body of our
Modify Elements function.

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Now if you look at the trace back,

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it tells you information
about what happened

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in this particular case.

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So first of all,

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it's telling me

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that I called Modify Elements
in snippet number eight,

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which if I look up above is right here

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in snippet number eight
and that caused a problem.

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And then separately it's
saying in sippet number one,

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which if I scroll back up

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is the definition of Modify Elements

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in snippet number one.

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In line four of that snippet
is where that problem occurred

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and the problem at that point

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was the fact that tuples
don't support Item assignment.

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Remember tuples are
immutable, nonmodifiable.

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So what you're seeing here
are actual line numbers

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in snippet number one.

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So let's go look back at
line four of snippet one.

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So one, two, three, four.

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This is line number
four of Modify Elements,

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and that's where the problem occurred.

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Now all of that information

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in the trace back can be used

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to help you debug your code

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when you run into these types
of problems at execution time.