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So what is a dictionary?
well a dictionary is a collection of

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values that are stored using a key.
As an example, employees in a company

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might be given an employee id. That
would form their key, and the value

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would be their personal records.
When a card triggers a speed camera, the

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car's registration number,
or license number,

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is recorded. That becomes the key that's
used to find the owner of the vehicle,

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so that a speeding fine can be issued.

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If the camera only recorded that it was
a blue car, the system wouldn't function

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very well. The key's needed
to find the records for

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that particular blue car, in
the collection of all cars in the

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country. So that's basically
what a dictionary is.

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Every value that's stored in the
dictionary has a key.

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We use the key to get the
values from the dictionary.

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You'll find dictionaries described
as storing key value pairs.

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Each value has a unique key,
that's used to refer to it.

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Alright, so let's look at some examples
that are going to demonstrate this.

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So we need to - in our project
we created in the last video -

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create a new python file, and we'll call
this one dict_intro.

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So we're going to start with a dictionary
to store the values for different vehicles;

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vehicles equals and a left curly brace,

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and you can see that the right curly
brace was added by IntelliJ.

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So to create a dictionary literal,
we use curly braces.

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Within the curly braces,
we have keys and values,

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separated by a colon. Now, I gave the
example of vehicle records in the last video.

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The key, in that case, would be the
vehicle's registration number.

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But things are complicated enough,
without having to deal with

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real registration numbers
such as HHU494V, for example,

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so I'm going to use friendlier names for
the keys. Now Honda called their 250 T motorbike,

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the dream, for example. That's the sort
of names I'm going to use for the keys.

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So I'm going to paste in some data,

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over the top of what I typed.
Now pause the video while you catch up

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with the typing.
As you can see, a dictionary looks a bit

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like a list. We use curly braces instead
of square brackets -

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you can see them on lines 1 and 10. The
other difference is that each entry is

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in two parts.
We have a key - things like dream, virago

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and tenere.  After the key, and separated
from it with a colon, comes the value.

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The values in this dictionary are the
string containing the manufacturer's

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name and model. That's why we talk about

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dictionaries storing key value pairs.
Each entry on lines 2 to 9,

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consists of a key
and a value. Okay, so we've got a

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dictionary containing details on eight
vehicles. Let's see some things we can do

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with it.
We can get values from the dictionary

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by providing the key.
That works in a similar way to indexing

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into a list, but we can use the
key instead of the index position.

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Let's type some code in.
I'm going start on line 12;

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my_car is equal to vehicles
and square brackets,

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single quote and fiesta.
Then on the next line,

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print parentheses my_car.
So if you run the program now,

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and my_car is a Ford Fiesta Ghia 1.4.
So you can see there that we provided

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the key inside the square brackets,
and we get the value returned.

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So it looks a bit like indexing a sequence,
such as a string or a list. In fact, it is

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similar to indexing,
but when working with dictionaries,

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we use the key as the index, rather
than the position. So note that we use

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curly braces to create a dictionary -
that's on lines one and ten.

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I've already mentioned that.
I'm repeating it because we use square

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brackets when we want
to retrieve a value.

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We can see the square
brackets on line 12.

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Alright, so let's see another example of
retrieving a value, before I move on;

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commuter is equal to vehicles
square bracket single quotes virago,

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and you can see there that
Python actually recognizes

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the dictionary, and it's returning the
dictionary key virago. You can see on

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the right-hand side, on the
on the pop-up there, it says dict key.

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So virago, and we'll print that
out as well - print commuter.

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J-P used a little virago to get to work
because it did over 86 miles to the gallon,

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So I've called the variable
commuter, there.

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Now that gets bound to the
dictionary value for the key virago.

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If we run the program,
check the right value -

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or the correct value - you can see
that it shows the yamaha XV250.

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Alright, so that's one
way to retrieve values

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from a dictionary. You index a dictionary
using the key for the value you want.

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Dictionaries also have a get method.
I'll demonstrate how to use get,

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then we'll see why you might use it,
rather than indexing.

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Alright, so we'll start on
line 18 by typing learner

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is equal to vehicles.get
parentheses, and in double quotes er5

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double quote right parenthesis, as you can
see there. On the next line,

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we're going to print parentheses learner.

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The Kawasaki ER5 was popular with
riding schools, which is why I've called

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the variable learner.
So on line 18, we call the dictionary's

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get method.
We provide the key for the item we want,

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and the method returns the value from
the dictionary.

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When I run the program now,

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you can see that Kawasaki ER5
is printed out.

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Alright, so we've now got two ways to
retrieve values from the dictionary.

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You can use the key as an index,
which I'm doing on lines 12,

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and also on lines 15, or we can
pass the key to the get method,

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as I'm doing on line 18. I'm going to
finish this video by discussing

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when you might use each method,
and what their advantages are.

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Before that,  I want to point out
something that might not be obvious.

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The keys that we use, when either
indexing or calling dot get,

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must match exactly. Our keys
are strings in this dictionary,

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and strings are case sensitive.
That means we can't use er5 in capitals,

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for example.
Let's see what happens if we do that.

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Let's capitalize ER there,

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and if we run the program again now,
you can see down the

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output at the bottom left-hand corner, we
get None as the last line of the output.

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The string ER5 in uppercase, doesn't
match any of the keys

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in the dictionary, and you
can see the dictionary there.

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If the key doesn't exist
when we use it for indexing,

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we instead, get an error. I'm going
to demonstrate that by changing

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fiesta to have a capital F.

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If we run that again now,

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the program's crashed, as you can see,
and we've got a key error showing there,

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Fiesta.
An error helpfully tells us which key

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doesn't exist -
Fiesta with a capital F, in this case.

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So that's one reason why you might
use dot get, rather than indexing.

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So if the key doesn't exist,
dot get will return none

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whereas indexing will crash
with a key error, as you saw.

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The get method is useful when you're
not sure if the key exists or not.

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Now you may be wondering why not just
use the get method all the time. Why

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bother with indexing, if it can crash?
One answer to that is indexing is faster.

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If you know that the key will be present,
then use indexing with square brackets;

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if there's a possibility that
the key won't be present,

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then call the get method. We'll see how
we can be sure that the key is present

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in the next few videos.

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Another reason is that sometimes
you want to get an error.

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We'll talk more about that when
we look at handling exceptions.

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But sometimes it's better
for your code to crash,

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rather than for it to
process invalid data.

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So experiment by printing out different
values from the vehicle's dictionary,

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using both the dot get method
and indexing, and I'll see you in the

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next video.