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