PEP: 354 Title: Enumerations in Python Version: $Revision$
Last-Modified: $Date$ Author: Ben Finney <ben+python@benfinney.id.au>
Status: Superseded Type: Standards Track Content-Type: text/x-rst
Created: 20-Dec-2005 Python-Version: 2.6 Post-History: 20-Dec-2005
Superseded-By: 435

Rejection Notice

This PEP has been rejected. This doesn't slot nicely into any of the
existing modules (like collections), and the Python standard library
eschews having lots of individual data structures in their own modules.
Also, the PEP has generated no widespread interest. For those who need
enumerations, there are cookbook recipes and PyPI packages that meet
these needs.

Note: this PEP was superseded by PEP 435, which has been accepted in May
2013.

Abstract

This PEP specifies an enumeration data type for Python.

An enumeration is an exclusive set of symbolic names bound to arbitrary
unique values. Values within an enumeration can be iterated and
compared, but the values have no inherent relationship to values outside
the enumeration.

Motivation

The properties of an enumeration are useful for defining an immutable,
related set of constant values that have a defined sequence but no
inherent semantic meaning. Classic examples are days of the week (Sunday
through Saturday) and school assessment grades ('A' through 'D', and
'F'). Other examples include error status values and states within a
defined process.

It is possible to simply define a sequence of values of some other basic
type, such as int or str, to represent discrete arbitrary values.
However, an enumeration ensures that such values are distinct from any
others, and that operations without meaning ("Wednesday times two") are
not defined for these values.

Specification

An enumerated type is created from a sequence of arguments to the type's
constructor:

    >>> Weekdays = enum('sun', 'mon', 'tue', 'wed', 'thu', 'fri', 'sat')
    >>> Grades = enum('A', 'B', 'C', 'D', 'F')

Enumerations with no values are meaningless. The exception
EnumEmptyError is raised if the constructor is called with no value
arguments.

The values are bound to attributes of the new enumeration object:

    >>> today = Weekdays.mon

The values can be compared:

    >>> if today == Weekdays.fri:
    ...     print "Get ready for the weekend"

Values within an enumeration cannot be meaningfully compared except with
values from the same enumeration. The comparison operation functions
return NotImplemented[1] when a value from an enumeration is compared
against any value not from the same enumeration or of a different type:

    >>> gym_night = Weekdays.wed
    >>> gym_night.__cmp__(Weekdays.mon)
    1
    >>> gym_night.__cmp__(Weekdays.wed)
    0
    >>> gym_night.__cmp__(Weekdays.fri)
    -1
    >>> gym_night.__cmp__(23)
    NotImplemented
    >>> gym_night.__cmp__("wed")
    NotImplemented
    >>> gym_night.__cmp__(Grades.B)
    NotImplemented

This allows the operation to succeed, evaluating to a boolean value:

    >>> gym_night = Weekdays.wed
    >>> gym_night < Weekdays.mon
    False
    >>> gym_night < Weekdays.wed
    False
    >>> gym_night < Weekdays.fri
    True
    >>> gym_night < 23
    False
    >>> gym_night > 23
    True
    >>> gym_night > "wed"
    True
    >>> gym_night > Grades.B
    True

Coercing a value from an enumeration to a str results in the string that
was specified for that value when constructing the enumeration:

    >>> gym_night = Weekdays.wed
    >>> str(gym_night)
    'wed'

The sequence index of each value from an enumeration is exported as an
integer via that value's index attribute:

    >>> gym_night = Weekdays.wed
    >>> gym_night.index
    3

An enumeration can be iterated, returning its values in the sequence
they were specified when the enumeration was created:

    >>> print [str(day) for day in Weekdays]
    ['sun', 'mon', 'tue', 'wed', 'thu', 'fri', 'sat']

Values from an enumeration are hashable, and can be used as dict keys:

    >>> plans = {}
    >>> plans[Weekdays.sat] = "Feed the horse"

The normal usage of enumerations is to provide a set of possible values
for a data type, which can then be used to map to other information
about the values:

    >>> for report_grade in Grades:
    ...     report_students[report_grade] = \
    ...         [s for s in students if students.grade == report_grade]

Rationale -- Other designs considered

All in one class

Some implementations have the enumeration and its values all as
attributes of a single object or class.

This PEP specifies a design where the enumeration is a container, and
the values are simple comparables. It was felt that attempting to place
all the properties of enumeration within a single class complicates the
design without apparent benefit.

Metaclass for creating enumeration classes

The enumerations specified in this PEP are instances of an enum type.
Some alternative designs implement each enumeration as its own class,
and a metaclass to define common properties of all enumerations.

One motivation for having a class (rather than an instance) for each
enumeration is to allow subclasses of enumerations, extending and
altering an existing enumeration. A class, though, implies that
instances of that class will be created; it is difficult to imagine what
it means to have separate instances of a "days of the week" class, where
each instance contains all days. This usually leads to having each class
follow the Singleton pattern, further complicating the design.

In contrast, this PEP specifies enumerations that are not expected to be
extended or modified. It is, of course, possible to create a new
enumeration from the string values of an existing one, or even subclass
the enum type if desired.

Values related to other types

Some designs express a strong relationship to some other value, such as
a particular integer or string, for each enumerated value.

This results in using such values in contexts where the enumeration has
no meaning, and unnecessarily complicates the design. The enumerated
values specified in this PEP export the values used to create them, and
can be compared for equality with any other value, but sequence
comparison with values outside the enumeration is explicitly not
implemented.

Hiding attributes of enumerated values

A previous design had the enumerated values hiding as much as possible
about their implementation, to the point of not exporting the string key
and sequence index.

The design in this PEP acknowledges that programs will often find it
convenient to know the enumerated value's enumeration type, sequence
index, and string key specified for the value. These are exported by the
enumerated value as attributes.

Implementation

This design is based partly on a recipe[2] from the Python Cookbook.

The PyPI package enum[3] provides a Python implementation of the data
types described in this PEP.

References and Footnotes

Copyright

This document has been placed in the public domain.



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[1] The NotImplemented return value from comparison operations signals
the Python interpreter to attempt alternative comparisons or other
fallbacks.
<http://docs.python.org/reference/datamodel.html#the-standard-type-hierarchy>

[2] "First Class Enums in Python", Zoran Isailovski, Python Cookbook
recipe 413486
<http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/413486>

[3] Python Package Index, package enum
<http://cheeseshop.python.org/pypi/enum/>