601 lines
19 KiB
Plaintext
601 lines
19 KiB
Plaintext
PEP: 435
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Title: Adding an Enum type to the Python standard library
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Version: $Revision$
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Last-Modified: $Date$
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Author: Barry Warsaw <barry@python.org>,
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Eli Bendersky <eliben@gmail.com>
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Status: Draft
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 2013-02-23
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Python-Version: 3.4
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Post-History: 2013-02-23
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Abstract
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========
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This PEP proposes adding an enumeration type to the Python standard library.
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Specifically, it proposes moving the existing ``flufl.enum`` package by Barry
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Warsaw into the standard library. Much of this PEP is based on the "using"
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[1]_ document from the documentation of ``flufl.enum``.
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An enumeration is a set of symbolic names bound to unique, constant integer
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values. Within an enumeration, the values can be compared by identity, and the
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enumeration itself can be iterated over. Enumeration items can be converted to
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and from their integer equivalents, supporting use cases such as storing
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enumeration values in a database.
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Status of discussions
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=====================
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The idea of adding an enum type to Python is not new - PEP 354 [2]_ is a
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previous attempt that was rejected in 2005. Recently a new set of discussions
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was initiated [3]_ on the ``python-ideas`` mailing list. Many new ideas were
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proposed in several threads; after a lengthy discussion Guido proposed adding
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``flufl.enum`` to the standard library [4]_. During the PyCon 2013 language
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summit the issue was discussed further. It became clear that many developers
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want to see an enum that subclasses ``int``, which can allow us to replace
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many integer constants in the standard library by enums with friendly string
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representations, without ceding backwards compatibility. An additional
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discussion among several interested core developers led to the proposal of
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having ``IntEnum`` as a special case of ``Enum``.
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The key dividing issue between ``Enum`` and ``IntEnum`` is whether comparing
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to integers is semantically meaningful. For most uses of enumerations, it's
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a **feature** to reject comparison to integers; enums that compare to integers
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lead, through transitivity, to comparisons between enums of unrelated types,
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which isn't desirable in most cases. For some uses, however, greater
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interoperatiliby with integers is desired. For instance, this is the case for
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replacing existing standard library constants (such as ``socket.AF_INET``)
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with enumerations.
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This PEP is an attempt to formalize this decision as well as discuss a number
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of variations that were discussed and can be considered for inclusion.
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Motivation
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==========
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*[Based partly on the Motivation stated in PEP 354]*
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The properties of an enumeration are useful for defining an immutable, related
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set of constant values that have a defined sequence but no inherent semantic
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meaning. Classic examples are days of the week (Sunday through Saturday) and
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school assessment grades ('A' through 'D', and 'F'). Other examples include
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error status values and states within a defined process.
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It is possible to simply define a sequence of values of some other basic type,
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such as ``int`` or ``str``, to represent discrete arbitrary values. However,
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an enumeration ensures that such values are distinct from any others including,
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importantly, values within other enumerations, and that operations without
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meaning ("Wednesday times two") are not defined for these values. It also
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provides a convenient printable representation of enum values without requiring
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tedious repetition while defining them (i.e. no ``GREEN = 'green'``).
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Module and type name
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====================
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We propose to add a module named ``enum`` to the standard library. The main
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type exposed by this module is ``Enum``. Hence, to import the ``Enum`` type
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user code will run::
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>>> from enum import Enum
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Proposed semantics for the new enumeration type
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===============================================
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Creating an Enum
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----------------
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Enumerations are created using the class syntax, which makes them easy to read
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and write. Every enumeration value must have a unique integer value and the
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only restriction on their names is that they must be valid Python identifiers.
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To define an enumeration, derive from the ``Enum`` class and add attributes
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with assignment to their integer values::
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>>> from enum import Enum
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>>> class Colors(Enum):
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... red = 1
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... green = 2
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... blue = 3
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Enumeration values are compared by identity::
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>>> Colors.red is Colors.red
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True
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>>> Colors.blue is Colors.blue
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True
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>>> Colors.red is not Colors.blue
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True
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>>> Colors.blue is Colors.red
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False
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Enumeration values have nice, human readable string representations::
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>>> print(Colors.red)
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Colors.red
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...while their repr has more information::
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>>> print(repr(Colors.red))
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<EnumValue: Colors.red [int=1]>
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The enumeration value names are available through the class members::
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>>> for member in Colors.__members__:
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... print(member)
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red
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green
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blue
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Let's say you wanted to encode an enumeration value in a database. You might
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want to get the enumeration class object from an enumeration value::
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>>> cls = Colors.red.enum
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>>> print(cls.__name__)
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Colors
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Enums also have a property that contains just their item name::
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>>> print(Colors.red.name)
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red
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>>> print(Colors.green.name)
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green
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>>> print(Colors.blue.name)
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blue
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The str and repr of the enumeration class also provides useful information::
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>>> print(Colors)
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<Colors {red: 1, green: 2, blue: 3}>
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>>> print(repr(Colors))
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<Colors {red: 1, green: 2, blue: 3}>
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You can extend previously defined Enums by subclassing::
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>>> class MoreColors(Colors):
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... pink = 4
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... cyan = 5
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When extended in this way, the base enumeration's values are identical to the
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same named values in the derived class::
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>>> Colors.red is MoreColors.red
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True
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>>> Colors.blue is MoreColors.blue
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True
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However, these are not doing comparisons against the integer equivalent values,
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because if you define an enumeration with similar item names and integer values,
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they will not be identical::
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>>> class OtherColors(Enum):
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... red = 1
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... blue = 2
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... yellow = 3
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>>> Colors.red is OtherColors.red
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False
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>>> Colors.blue is not OtherColors.blue
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True
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These enumeration values are not equal, nor do they hash equally::
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>>> Colors.red == OtherColors.red
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False
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>>> len(set((Colors.red, OtherColors.red)))
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2
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Ordered comparisons between enumeration values are *not* supported. Enums are
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not integers (but see ``IntEnum`` below)::
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>>> Colors.red < Colors.blue
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Traceback (most recent call last):
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...
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NotImplementedError
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>>> Colors.red <= Colors.blue
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Traceback (most recent call last):
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...
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NotImplementedError
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>>> Colors.blue > Colors.green
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Traceback (most recent call last):
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...
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NotImplementedError
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>>> Colors.blue >= Colors.green
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Traceback (most recent call last):
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...
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NotImplementedError
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Equality comparisons are defined though::
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>>> Colors.blue == Colors.blue
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True
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>>> Colors.green != Colors.blue
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True
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Enumeration values do not support ordered comparisons::
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>>> Colors.red < Colors.blue
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Traceback (most recent call last):
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...
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NotImplementedError
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>>> Colors.red < 3
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Traceback (most recent call last):
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...
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NotImplementedError
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>>> Colors.red <= 3
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Traceback (most recent call last):
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...
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NotImplementedError
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>>> Colors.blue > 2
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Traceback (most recent call last):
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...
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NotImplementedError
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>>> Colors.blue >= 2
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Traceback (most recent call last):
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...
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NotImplementedError
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While equality comparisons are allowed, comparisons against non-enumeration
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values will always compare not equal::
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>>> Colors.green == 2
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False
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>>> Colors.blue == 3
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False
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>>> Colors.green != 3
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True
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>>> Colors.green == 'green'
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False
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If you really want the integer equivalent values, you can convert enumeration
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values explicitly using the ``int()`` built-in. This is quite convenient for
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storing enums in a database, as well as for interoperability with C extensions
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that expect integers::
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>>> int(Colors.red)
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1
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>>> int(Colors.green)
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2
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>>> int(Colors.blue)
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3
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You can also convert back to the enumeration value by calling the Enum
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subclass, passing in the integer value for the item you want::
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>>> Colors(1)
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<EnumValue: Colors.red [int=1]>
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>>> Colors(2)
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<EnumValue: Colors.green [int=2]>
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>>> Colors(3)
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<EnumValue: Colors.blue [int=3]>
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>>> Colors(1) is Colors.red
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True
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The Enum subclass also accepts the string name of the enumeration value::
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>>> Colors('red')
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<EnumValue: Colors.red [int=1]>
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>>> Colors('blue') is Colors.blue
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True
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You get exceptions though, if you try to use invalid arguments::
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>>> Colors('magenta')
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Traceback (most recent call last):
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...
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ValueError: magenta
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>>> Colors(99)
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Traceback (most recent call last):
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...
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ValueError: 99
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The Enum base class also supports getitem syntax, exactly equivalent to the
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class's call semantics::
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>>> Colors[1]
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<EnumValue: Colors.red [int=1]>
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>>> Colors[2]
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<EnumValue: Colors.green [int=2]>
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>>> Colors[3]
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<EnumValue: Colors.blue [int=3]>
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>>> Colors[1] is Colors.red
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True
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>>> Colors['red']
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<EnumValue: Colors.red [int=1]>
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>>> Colors['blue'] is Colors.blue
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True
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>>> Colors['magenta']
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Traceback (most recent call last):
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...
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ValueError: magenta
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>>> Colors[99]
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Traceback (most recent call last):
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...
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ValueError: 99
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The integer equivalent values serve another purpose. You may not define two
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enumeration values with the same integer value::
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>>> class Bad(Enum):
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... cartman = 1
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... stan = 2
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... kyle = 3
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... kenny = 3 # Oops!
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... butters = 4
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Traceback (most recent call last):
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...
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TypeError: Multiple enum values: 3
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You also may not duplicate values in derived enumerations::
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>>> class BadColors(Colors):
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... yellow = 4
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... chartreuse = 2 # Oops!
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Traceback (most recent call last):
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...
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TypeError: Multiple enum values: 2
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The Enum class support iteration. Enumeration values are returned in the
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sorted order of their integer equivalent values::
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>>> [v.name for v in MoreColors]
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['red', 'green', 'blue', 'pink', 'cyan']
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>>> [int(v) for v in MoreColors]
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[1, 2, 3, 4, 5]
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Enumeration values are hashable, so they can be used in dictionaries and sets::
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>>> apples = {}
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>>> apples[Colors.red] = 'red delicious'
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>>> apples[Colors.green] = 'granny smith'
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>>> for color in sorted(apples, key=int):
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... print(color.name, '->', apples[color])
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red -> red delicious
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green -> granny smith
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IntEnum
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-------
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A variation of ``Enum`` is proposed that also subclasses ``int`` - ``IntEnum``.
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Such enumerations behave much more similarly to integers. In particular, they
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can be compared to integers; by extensions, enumerations of different types can
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also be compared to each other::
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>>> from enum import IntEnum
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>>> class Shape(IntEnum):
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... circle = 1
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... square = 2
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...
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>>> class Request(IntEnum):
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... post = 1
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... get = 2
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...
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>>> Shape == 1
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False
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>>> Shape.circle == 1
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True
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>>> Shape.circle == Request.post
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True
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However they still can't be compared to ``Enum``::
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>>> from enum import Enum, IntEnum
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>>> class Shape(IntEnum):
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... circle = 1
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... square = 2
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...
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>>> class Colors(Enum):
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... red = 1
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... green = 2
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...
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>>> Shape.circle == Colors.red
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False
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For the vast majority of code, ``Enum`` is recommended. Only if a greater
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degree of interoperatility with integers is required and ``Enum`` does not
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fit the bill, ``IntEnum`` should be used.
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Pickling
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--------
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Enumerations created with the class syntax can also be pickled and unpickled::
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>>> from enum.tests.fruit import Fruit
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>>> from pickle import dumps, loads
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>>> Fruit.tomato is loads(dumps(Fruit.tomato))
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True
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Convenience API
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---------------
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You can also create enumerations using the convenience function ``make()``,
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which takes an iterable object or dictionary to provide the item names and
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values. ``make()`` is a module-level function.
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The first argument to ``make()`` is the name of the enumeration, and it returns
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the so-named `Enum` subclass. The second argument is a *source* which can be
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either an iterable or a dictionary. In the most basic usage, *source* returns
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a sequence of strings which name the enumeration items. In this case, the
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values are automatically assigned starting from 1::
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>>> import enum
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>>> enum.make('Animals', ('ant', 'bee', 'cat', 'dog'))
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<Animals {ant: 1, bee: 2, cat: 3, dog: 4}>
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The items in source can also be 2-tuples, where the first item is the
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enumeration value name and the second is the integer value to assign to the
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value. If 2-tuples are used, all items must be 2-tuples::
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>>> def enumiter():
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... start = 1
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... while True:
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... yield start
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... start <<= 1
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>>> enum.make('Flags', zip(list('abcdefg'), enumiter()))
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<Flags {a: 1, b: 2, c: 4, d: 8, e: 16, f: 32, g: 64}>
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Proposed variations
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===================
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Some variations were proposed during the discussions in the mailing list.
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Here's some of the more popular ones.
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Not having to specify values for enums
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--------------------------------------
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Michael Foord proposed (and Tim Delaney provided a proof-of-concept
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implementation) to use metaclass magic that makes this possible::
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class Color(Enum):
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red, green, blue
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The values get actually assigned only when first looked up.
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Pros: cleaner syntax that requires less typing for a very common task (just
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listing enumeration names without caring about the values).
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Cons: involves much magic in the implementation, which makes even the
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definition of such enums baffling when first seen. Besides, explicit is
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better than implicit.
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Using special names or forms to auto-assign enum values
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-------------------------------------------------------
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A different approach to avoid specifying enum values is to use a special name
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or form to auto assign them. For example::
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class Color(Enum):
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red = None # auto-assigned to 0
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green = None # auto-assigned to 1
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blue = None # auto-assigned to 2
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More flexibly::
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class Color(Enum):
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red = 7
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green = None # auto-assigned to 8
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blue = 19
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purple = None # auto-assigned to 20
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Some variations on this theme:
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#. A special name ``auto`` imported from the enum package.
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#. Georg Brandl proposed ellipsis (``...``) instead of ``None`` to achieve the
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same effect.
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Pros: no need to manually enter values. Makes it easier to change the enum and
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extend it, especially for large enumerations.
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Cons: actually longer to type in many simple cases. The argument of explicit
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vs. implicit applies here as well.
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Use-cases in the standard library
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=================================
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The Python standard library has many places where the usage of enums would be
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beneficial to replace other idioms currently used to represent them. Such
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usages can be divided to two categories: user-code facing constants, and
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internal constants.
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User-code facing constants like ``os.SEEK_*``, ``socket`` module constants,
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decimal rounding modes, HTML error codes could benefit from being enums had
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they been implemented this way from the beginning. At this point, however, at
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the risk of breaking user code (that relies on the constants' actual values
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rather than their meaning) such a change cannot be made. This does not mean
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that future uses in the stdlib can't use an enum for defining new user-code
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facing constants.
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Internal constants are not seen by user code but are employed internally by
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stdlib modules. It appears that nothing should stand in the way of
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implementing such constants with enums. Some examples uncovered by a very
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partial skim through the stdlib: ``binhex``, ``imaplib``, ``http/client``,
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``urllib/robotparser``, ``idlelib``, ``concurrent.futures``, ``turtledemo``.
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In addition, looking at the code of the Twisted library, there are many use
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cases for replacing internal state constants with enums. The same can be said
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about a lot of networking code (especially implementation of protocols) and
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can be seen in test protocols written with the Tulip library as well.
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Differences from PEP 354
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========================
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Unlike PEP 354, enumeration values are not defined as a sequence of strings,
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but as attributes of a class. This design was chosen because it was felt that
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class syntax is more readable.
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Unlike PEP 354, enumeration values require an explicit integer value. This
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difference recognizes that enumerations often represent real-world values, or
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must interoperate with external real-world systems. For example, to store an
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enumeration in a database, it is better to convert it to an integer on the way
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in and back to an enumeration on the way out. Providing an integer value also
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provides an explicit ordering. However, there is no automatic conversion to
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and from the integer values, because explicit is better than implicit.
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Unlike PEP 354, this implementation does use a metaclass to define the
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enumeration's syntax, and allows for extended base-enumerations so that the
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common values in derived classes are identical (a singleton model). While PEP
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354 dismisses this approach for its complexity, in practice any perceived
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complexity, though minimal, is hidden from users of the enumeration.
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Unlike PEP 354, enumeration values can only be tested by identity comparison.
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This is to emphasize the fact that enumeration values are singletons, much
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like ``None``.
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Acknowledgments
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===============
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This PEP describes the ``flufl.enum`` package by Barry Warsaw. ``flufl.enum``
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is based on an example by Jeremy Hylton. It has been modified and extended
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by Barry Warsaw for use in the GNU Mailman [5]_ project. Ben Finney is the
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author of the earlier enumeration PEP 354.
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References
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==========
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.. [1] http://pythonhosted.org/flufl.enum/docs/using.html
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.. [2] http://www.python.org/dev/peps/pep-0354/
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.. [3] http://mail.python.org/pipermail/python-ideas/2013-January/019003.html
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.. [4] http://mail.python.org/pipermail/python-ideas/2013-February/019373.html
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.. [5] http://www.list.org
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Copyright
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=========
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This document has been placed in the public domain.
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Todo
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====
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* Mark PEP 354 "superseded by" this one, if accepted
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* New package name within stdlib - enum? (top-level)
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* For make, can we add an API like namedtuple's?
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make('Animals, 'ant bee cat dog')
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I.e. when make sees a string argument it splits it, making it similar to a
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tuple but with far less manual quote typing. OTOH, it just saves a ".split"
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so may not be worth the effort ?
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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coding: utf-8
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End:
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