PEP 802 – Display Syntax for the Empty Set

Posted by kirara0048@reddit | Python | View on Reddit | 262 comments

PEP 802 – Display Syntax for the Empty Set
https://peps.python.org/pep-0802/

Abstract

We propose a new notation, {/}, to construct and represent the empty set. This is modelled after the corresponding mathematical symbol ‘∅’.

This complements the existing notation for empty tuples, lists, and dictionaries, which use (), [], and {} respectively.

>>> type({/})
<class 'set'>
>>> {/} == set()
True

Motivation

Sets are currently the only built-in collection type that have a display syntax, but no notation to express an empty collection. The Python Language Reference notes this, stating:

An empty set cannot be constructed with {}; this literal constructs an empty dictionary.

This can be confusing for beginners, especially those coming to the language from a scientific or mathematical background, where sets may be in more common use than dictionaries or maps.

A syntax notation for the empty set has the important benefit of not requiring a name lookup (unlike set()). {/} will always have a consistent meaning, improving teachability of core concepts to beginners. For example, users must be careful not to use set as a local variable name, as doing so prevents constructing new sets. This can be frustrating as beginners may not know how to recover the set type if they have overriden the name. Techniques to do so (e.g. type({1})) are not immediately obvious, especially to those learning the language, who may not yet be familiar with the type function.

Finally, this may be helpful for users who do not speak English, as it provides a culture-free notation for a common data structure that is built into the language.