PEP: 218 Title: Adding a Built-In Set Object Type Version: $Revision$ Author: gvwilson@nevex.com (Greg Wilson) Status: Draft Type: Standards Track Python-Version: 2.1 Created: 31-Jul-2000 Post-History: Introduction This PEP proposes adding sets as a built-in type in Python. Rationale Sets are a fundamental mathematical structure, and are commonly used to specify algorithms. They are much less frequently used in implementations, even when they are the "right" structure. Programmers frequently use lists instead, even when the ordering information in lists is irrelevant, and by-value lookups are frequent. (Most medium-sized C programs contain a depressing number of start-to-end searches through malloc'd vectors to determine whether particular items are present or not...) Programmers are often told that they can implement sets as dictionaries with "don't care" values. Items can be added to these "sets" by assigning the "don't care" value to them; membership can be tested using "dict.has_key"; and items can be deleted using "del". However, the three main binary operations on sets --- union, intersection, and difference --- are not directly supported by this representation, since their meaning is ambiguous for dictionaries containing key/value pairs. Proposal We propose adding a new built-in type to Python to represent sets. This type will be an unordered collection of unique values, just as a dictionary is an unordered collection of key/value pairs. Constant sets will be represented using the usual mathematical notation, so that "{1, 2, 3}" will be a set of three integers. In order to avoid ambiguity, the empty set will be written "{,}", rather than "{}" (which is already used to represent empty dictionaries). We feel that this notation is as reasonable as the use of "(3,)" to represent single-element tuples; a more radical alternative is discussed in the "Alternatives" section. Iteration and comprehension will be implemented in the obvious ways, so that: for x in S: will step through the elements of S in arbitrary order, while: {x**2 for x in S} will produce a set containing the squares of all elements in S, Membership will be tested using "in" and "not in". The binary operators '|', '&', '-', and "^" will implement set union, intersection, difference, and symmetric difference. Their in-place equivalents will have the obvious semantics. The method "add" will add an element to a set. This is different from set union, as the following example shows: >>> {1, 2, 3} | {4, 5, 6} {1, 2, 3, 4, 5, 6} >>> {1, 2, 3}.add({4, 5, 6}) {1, 2, 3, {4, 5, 6}} Note that we expect that items can also be added to sets using in-place union of temporaries, i.e. "S |= {x}" instead of "S.add(x)". Elements will be deleted from sets using a "remove" method, or using "del": >>> S = {1, 2, 3} >>> del S[1] >>> S {2, 3} >>> S.remove(3) {2} The "KeyError" exception will be raised if an attempt is made to remove an element which is not in a set. A new method "dict.keyset" will return the keys of a dictionary as a set. A corresponding method "dict.valueset" will return the dictionary's values as a set. A built-in converter "set()" will convert any sequence type to a set; converters such as "list()" and "tuple()" will be extended to handle sets as input. Alternatives A radical alternative to the (admittedly clumsy) notation "{,}" is to re-define "{}" to be the empty collection, rather than the empty dictionary. Operations which made this object non-empty would silently convert it to either a dictionary or a set; it would then retain that type for the rest of its existence. This idea was rejected because of its potential impact on existing Python programs. A similar proposal to modify "dict.keys" and "dict.values" to return sets, rather than lists, was rejected for the same reasons. Copyright This document has been placed in the Public Domain. Local Variables: mode: indented-text indent-tabs-mode: nil End: