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Chapter 16. Sets

Many algorithms need to work with simple containers of data values, irrespective of order or any key. This is a simple set of objects, which is supported by the Python set container. We'll look at Sets from a number of viewpoints: semantics, literal values, operations, comparison operators, statements, built-in functions and methods.

Set Semantics

A set is, perhaps the simplest possible container, since it contains objects in no particular order with no particular identification. Objects stand for themselves. With a sequence, objects are identified by position. With a mapping, objects are identified by some key. With a set, objects stand for themselves.

Since each object stands for itself, elements of a set cannot be duplicated. A list or tuple, for example, can have any number of duplicate objects. For example, the tuple ( 1, 1, 2, 3 ) has four elements, which includes two copies of the integer 1; if we create a set from this tuple, the set will only have three elements.

A set has large number of operations for unions, intersections, and differences. A common need is to examine a set to see if a particular object is a member of that set, or if one set is contained within another set.

A set is mutable, which means that it cannot be used as a key for a dict (see Chapter 15, Mappings and Dictionaries for more information.) In order to use a set as a dict key, we can create a frozenset, which is an immutable copy of the original set. This allows us to accumulate a set of values to create a dict key.


 
 
  Published under the terms of the Open Publication License Design by Interspire