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Now that we've seen lists as well as dictionaries

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I want to talk about a concept that you often see in both of these collection

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types and that's something called nesting. Now,

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if we imagine a list or a dictionary

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being something like a folder where lots of things can be stored inside it,

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then nesting lists and dictionaries is just a matter of putting one inside the

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other. For example,

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here;s a dictionary that is very simple,

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it's only got one key-value pair.

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Now we know that we can add multiple key-value pairs into the same dictionary by

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just adding a bunch of commas to separate them. Now, what if

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instead of having a simple value like a string or a number,

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I could also put a list as a value as well.

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Similarly,

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I could also use a dictionary as a value. In this case

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we've got a list as the value for key, this first one,

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and we've got a dictionary as the value for key2. Notice how this is a list and

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a dictionary nested inside another dictionary.

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The structure gets a little bit more complex,

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but it gives us more flexibility when we're trying to store more complex pieces

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of data. Coming back to our code,

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let's go ahead and create some sample dictionaries to see what this actually

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looks like in real life. For example,

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this could be a dictionary that contains the country as the key and the city,

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that is the capital, as the value. So for France,

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that's Paris, for Germany that's Berlin.

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This is a very simple dictionary that you've seen already. Now,

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if I wanted to nest a list in a dictionary,

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then I would be able to represent even more complex data. For example,

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if I had a travel log where I was going to collect a dictionary of all the

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cities I had been for each of the countries I've traveled to. So for example,

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if I had traveled to France and I wanted to say that I've been to multiple

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cities, I can't simply just say Paris,

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and then Lille, Dijon, that doesn't really work because each key can only have one

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value. The only way that we can make these three pieces of data

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one value is by turning it into a list, like so.

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So now in our travel log dictionary

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as represented by the curly braces,

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we have one key value pair and it just so happens that

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the value in this case is a list.

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And of course you can go on and add as many entries as you would like,

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and still preserving this kind of structure of key being a string and the value

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being a list. Now,

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this idea of nesting isn't limited dictionaries by the way.

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You could also just nest a list in a list.

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So you could have a list which is A

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and then B, and then the third item just happens to be another list which has C and D.

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This is perfectly valid Python code

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but it's not quite as useful as nesting a list in a dictionary or a dictionary

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in a dictionary, because the way that data is structured. Now,

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what if you wanted to nest a dictionary in a dictionary?

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Let's say that we wanted to expand our travel log.

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And instead of just storing the cities that I visited in each country,

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what if I wanted to also keep track of how many visits I've made to that

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country?

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Or what if I wanted to actually label what this piece of data is?

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Because at the moment it's kind of like here's

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a bunch of cities that's associated with the country,

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but it doesn't really describe this list.

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And I'd like to give this to you as a challenge,

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have a go at changing the France entry so that it contains another dictionary.

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For the nested dictionary use the key cities_visited,

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and for the value you can keep the list of Paris, Lille and Dijon.

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Pause the video now and see if you can get that to work.

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Yeah.

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All right, so here's the solution. By nesting a dictionary

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we can label the Paris, Lille and Dijon lists as cities visited.

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All I need to do is create the string,

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"cities_visited" and add a colon after it. But there's one more step.

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We have to add a pair of curly braces so that France is associated with a single

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value, namely our nested dictionary.

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Now that we have a dictionary of the travel log,

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all the countries I've been to, and each country has a value that is a dictionary

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in itself and it can store multiple pieces of data

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including the cities I've visited in that country

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as well as things such as the total number of visits,

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which in this case would have a string as the key and a

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number of as the value. Now

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have a go at creating your own travel log or modifying the second part of the

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travel log so that you also have a dictionary nested in a dictionary and you're

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able to represent something like this,

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the number of cities you visited in a list and another additional piece of data,

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if you wanted to.

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Right,

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And hopefully would have done something similar to this.

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And so now our data structure is such that we've got a list nested inside a

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dictionary, which is in itself nested in another dictionary.

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Now that we've seen nesting lists inside dictionaries, dictionaries inside

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dictionaries,

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the last thing I want to show you is nesting a dictionary inside a list.

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So we could basically have multiple dictionaries inside a single list.

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So remember that lists are ordered and they're accessed by the positions inside the

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list. So this dictionary would be the item at index zero,

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this one at index one and so on and so forth.

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Whereas inside a dictionary,

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the items are accessed by their keys like this France or Germany.

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Now I want to do the last type which is nesting a dictionary

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in a list.

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And I'm going to be working with the same travel log that we had previously,

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but I want to change it so that instead of having this being one giant

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dictionary with key-value pairs,

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I want to change it so that each of these entries are a dictionary in itself.

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Instead of having France be the key of this dictionary here,

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I want the turn it into its own key-value pair. Going inside this dictionary,

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I'm going to create a new key

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which is called country and then put France as the value for that key.

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So now I have an entire dictionary that has three pieces of data.

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The country visited, the cities visited,

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and the total number of visits. Let's do the same for Germany as well.

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So now you can see we've got two dictionaries and instead of holding these two

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dictionaries inside a dictionary,

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I'm going to change it so that it's now a list. I'm going to turn these curly

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braces into square brackets

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and this way we can add as many of these dictionaries inside the list and

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iterate through them if we need to.

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When we've been creating dictionaries that only have one or two key-value pairs,

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we've kind of just kept it all on one line.

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But once you've got more and more key-value pairs in the dictionary,

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it's usually a good idea to go ahead and separate out each of the entries,

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Right?

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Like what I'm doing here.

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Now it's a little bit easier to see that we have a list which contains two

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items, and each item is a dictionary.

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Each dictionary has three key-value pairs and they all contain different types

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of data. The first one has a value which is a string,

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the second one holds a list and the third one holds a number.

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The data types inside a dictionary can be completely mixed up if you want it to.

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But what can't change is you still need a key and a value separated by a colon.

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Now the only downside to this is you want to make sure that you're actually

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aware that later on when you try to pull out the number of visits,

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it's actually a number that you're working with rather than a string or when

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you're trying to get hold of the cities visited

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it's a list rather than a dictionary, for example.

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So now that we've the various different types of nesting,

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it's time for you to actually really understand this and get to grips with this

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concept by completing a challenge yourself. In the next lesson,

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I've got a coding challenge for you based on this travel log and you'll need to

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use what you've learned so far to be able to complete it.

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So for all of that and more, I'll see you on the next lesson.