Containers

Table of Contents

Introduction

Python provides several A container or collection types–these are data structures, values that can hold other values. Individual values in a collection are referred to as elements or items.

Capabilities

The behavior and features of any particular collection will depend on its type. Here are some of the key aspects.

State	Mutable Collections that can be modified after they are created.	Immutable Collections that cannot be modified once they are created.
Position	Ordered Elements retain their position in the collection.	Unordered Elements are not stored in any particular order.
Composition	Homogeneous All elements in the collection are of the same type.	Heterogeneous Elements in the collection may be of any type.
Diversity	Unique Only unique elements are added to the collection.	Repeatable Duplicate elements are allowed in collection.
Access	Subscriptable Elements can be accessed via bracket notation.	Not subscriptable Bracket notation is not supported.
Value	Hashable Collection is immutable and can produce a hash value, therefore it can be a member of a set or used as a dict key.	Not hashable Collection is mutable or cannot produce a hash value, therefore cannot be a member of a set or used as a dict key.

Sequences

An ordered collection accessed via index numbers.

State

Mutable

Position

Ordered

Composition

varies

Diversity

Repeatable

Access

Subscriptable

Value

Hashable

Sequences are organized in terms of the position of each element in the collection.

Each element is assigned a successive index number, starting at 0. You can imagine each element grabbing a ticket from one of those “take a number” dispensers.

The below diagram shows a visualization of a hypothetical sequence with three items.

Sequence types

Type	Description
list	mutable collection of arbitrary objects
tuple	immutable collection of arbitrary objects
set	unordered collection with no duplicate elements.
range	collection containing a arithmetic progressions of numbers
str	sequence of unicode characters
bytes	immutable collection of integers between 0 and 256
bytearray	mutable collection of integers between 0 and 256
memoryview	provide access to internal memory of bytes or bytesarray object

See also

See also

• python.org > Sequence Types

• python.org > Comparing Sequences and Other Types

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Lists

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Sets

An unordered collection with no duplicate elements.

State

Mutable

Position

Unordered

Composition

Heterogeneous

Diversity

Unique

Access

Not subscriptable

Value

Not hashable

Set types

Type	Description
set	mutable collection of unique immutable objects
frozenset	immutable collection of unique immutable objects

Mappings

A collection of key: value pairs.

State

Mutable

Position

Ordered

Composition

Heterogeneous

Diversity

Repeatable

Access

Subscriptable

Value

Not hashable

Mapping types

Type	Description
dict	mutable collection of arbitrary objects indexed by nearly arbitrary values

Reference

Collection capabilities table

Type	Base	Syntax	Subscriptable	Mutable	Ordered	Unique
list	Sequence	[item,...]	Yes	Yes	Yes	No
tuple	Sequence	(item,...)	Yes	No	Yes	No
str	Sequence	"text"	Yes	No	Yes	No
bytes	Sequence	b"text"	Yes	No	Yes	No
set	Set	{item,...}	No	Yes	No	Yes
dict	Mapping	{key: value,...}	Yes	Yes	Yes	No
range	Sequence		Yes	No	Yes	Yes
bytearray	Sequence		Yes	Yes	Yes	No
memoryview	Sequence		Yes	Yes	Yes	No
frozenset	Set		No	No	No	Yes

Glossary

Data Types

bracket notation

subscript

subscription

Accessing an element from a collection object using [ ] after the object which enclose a selector expression. The expression may be an index number, key, or slice depending on its type.
The syntax is: COLLECTION[SELECTOR].

data structure

A particular way to store, organize, and access multiple values. A data type that can store other values.

container

collection

A value that can hold other values, for example list objects.

element

item

An individual value in a collection.

index

index number

An integer representing an elements position in a sequence.

slice

Selecting a range of elements from a collection. In Python, this is done either using subscription or the slice type.

sequence

A category of data types which are charactarized as ordered collection accessed via index numbers, such as list and tuple.

See also

See also

• python.org > Abstract Base Classes for Containers

• Python’s built-in container data types: categorisation and iteration

• Data Structures

• Subscription