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Welcome back.

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Earlier in this specialization, we took
dive into aliasing, and the confusions

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that can occur when you mutate a list or
a dictionary that has many aliases.

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Many different variable names pointing
to the same list or dictionary.

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The same thing is true if you
have instead of variable names

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pointing to a single list,

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if you have a single list that is included
as an element in multiple other lists.

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If you mutate the inner list that
is part of several outer lists,

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all those outer lists will look
like the contents have changed.

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The best to keep track of these
confusing situations will be

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with the reference diagram, of course.

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So let's took at example one way that
you get a list included in multiple list

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by accident, and let's get this confusing
situation is when you copy a list.

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As in this codes slip it,
let's start by just running it.

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So you can see that on line1,
we create a list called original,

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it's value is a nested list.

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The outer list has two inner
lists as it's elements.

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Let's set ourselves up
with a reference diagram.

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We're going to say that, Original,

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Is the variable name, and
it's value is a list.

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That list has two items.

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The first item is itself a list,
and that list has dogs,

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As an element, and
it has puppies as an element.

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The second element of
original is another list,

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Which is as it values cats and kittens.

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So that's how it happens on line 1.

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On line 2, we make a copy of original.

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We use the slice operator,
colon inside of square brackets.

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You may recall that
always creates a slice.

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If I said, original[1:4],

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that would Make a slice beginning

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with the item at index one and going
up to but not including position four.

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Of course, I can't do that for
this list because it only has two items.

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But if we leave off
the value before the colon,

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we start at the beginning of the list.

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If we leave off the value after the colon
we go all the way to the end of the list.

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So this says take a slice beginning
at the beginning of original, and

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going to end of original.

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So that makes a new list, and
that new list has has its elements,

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the same elements that original had.

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The first element is dogs and puppies,
the second element is cats and kittens.

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And we assign that to a variable
called copied_version.

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That's what happens on line 2.

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And just to show you what
the output looks like.

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On line 3, we print out copied
_version and sure enough,

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it looks like it has exactly the same
stuff that original has in it.

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On line 4, we checked is copied_version
the same object as original.

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And the answer to that is false.

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Copied_version is pointing to one object.

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Original is pointing
to a different object.

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One line 5, we check whether
they have the same contents.

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The answer to that true.

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So original and
copied are not the same object,

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they're different objects,
but they're both list.

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They both have two elements, and
those elements are these inner list,

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dog, puppies, and cats, kittens.

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It's true that they are equal, equal.

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It's not true that they are is each other.

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So copied_version is not original but

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it is equal to the original,
it has the same contents.

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On line 6, we see where our
confusions are going to happen

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when we have multiple outer list
that point to the same inner list.

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So original square bracket 0 is the first
inner list, that's dog's puppies.

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And on line 6, we say, .append of canines.

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So that is going to append to
the end of this list, another item.

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But notice that this item is
canines in square brackets.

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So this item is actually another list.

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That list has only one item, canines.

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But it is a list,
it's not just this string canines.

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So now on line 7, if we print original,
We get this list here.

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Where the first element is dogs,
puppies like before, but

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we've appended to the end of that list,
the list canines.

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And then the second element
is still cats and kittens.

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On line 9, we print the copied version.

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Now on line 6,
we just said to change original,

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we didn't say anything about
changing copied_version.

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But on line 9, you can see

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that this square bracket canines has
magically appeared after dogs and puppies.

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And that's because copied_version,
even though we didn't say to change it,

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copied_version has as its first element
this list, and we mutated that list.

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That's shallow copies for you.

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See you next time.