Collecting Things • Python's Collections (Data Structure Categories #5)

Collections • Part 5 of the Data Structure Categories Series

Picture a collection of artwork in a museum. The museum collection contains paintings. It also has a size—the number of items in the museum. And when you visit the museum, you can plan a route to take you from one painting to the other, going past all the items one at a time.

Did you spot three definitions you encountered in previous articles in this Data Structure Categories series?

The collection contains items, has a size, and you can go through each element one by one. In Python-speak, it's a container, a sized object, and an iterable.

The Data Structure Categories Series

We're on the fifth of seven articles in this series. You can read the previous ones by following the links in this overview:

• Iterable

• Sequence

• Mapping

• Container

• Collection (this article)

• Iterator

• Generator

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What's a Collection?

It's easy to get collection and container confused. And if you also throw sequence and iterable in the mix, things can get murkier. There's lots of overlap between the different terms. But they're all different.

This is a good time to visualise the hierarchy of these categories again—you've seen this earlier in this series:

Iterables, containers, and sized objects sit at the top. Each of these categories defines a specific property. Containers contain objects. Iterables are objects that can return their elements one at a time. And sized objects have a length.

A collection is an object that has all three of these properties. A collection is a sized, iterable container.

Let's look at some data types to determine whether they're collections. Let's start with a list:

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All three special methods __contains__, __len__, and __iter__ exist for a list object. This makes it a collection.

Let's look at another data type, the zip object:

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You create a zip object called team_points. It has an __iter__ special method, which means it's iterable. But the zip object doesn't have __contains__ or __len__. The error messages confirm this. Therefore, a zip object is not a collection.

Using the Abstract Base Classes

Let's look at another way to determine whether an object is a collection. In fact, this will allow you to check for any of the categories we've been discussing in this series.

You'll use the Abstract Base Classes, or ABCs for short, that you can find in the collections module. Note that you can find ABCs for all data type categories in the collections module, not just for those that meet the definition of a collection:

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You use the built-in function isinstance() to check whether the objects team_members and team_points are collections. You check whether they're instances of the abstract base class collections.abc.Collection.

The results confirm the earlier observation. Lists are collections, but zip objects aren't.

Let's explore a few more data types:

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You can also explore other categories of data types. Let's look at a few examples:

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You can explore other data types. There are abstract base classes for all the categories of data types we've discussed in this series.

The Next Rung Down

A collection is an object which is a sized, iterable container. However, most collections have additional properties. You already read about sequences and mappings earlier in this series. All sequences and mappings are collections. A set is another Python basic data type that's a collection.

Let's explore this hierarchy using a list. You can start by considering the "bottom" level of this hierarchy, which you can see in the image shown earlier in this article: a list is a sequence. You can use isinstance() and the abstract base class Sequence to confirm this:

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All sequences are collections:

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And all collections are sized, iterable containers:

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Etymology Corner

The term “collection” comes from the Latin colligere, which means “gather together” The Latin prefix con- means “together”, and the verb legere is “to gather”.

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Next in the series: iterator

Code in this article uses Python 3.11

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Stop Stack

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