============================ Using the flufl.enum library ============================ The ``flufl.enum`` package provides yet another enumeration data type for Python. While this is similar intent to the reject `PEP 354`_, this package defines an alternative syntax and semantics. 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. Motivation ========== [Lifted from 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, and that operations without meaning ("Wednesday times two") are not defined for these values. 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 flufl.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. >>> print Colors.__members__ ['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.enumclass >>> print cls.__name__ Colors Enums also have a property that contains just their item name. >>> print Colors.red.enumname red >>> print Colors.green.enumname green >>> print Colors.blue.enumname 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 Ordered comparisons between enumeration values are *not* supported. Enums are not integers! >>> 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 You also cannot compare enumeration values directly against their integer equivalents... >>> 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 ...unless you are doing equality comparisons. >>> Colors.blue == 3 False >>> Colors.green != 3 True 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 for example. >>> 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 class, passing in the integer value for the item you want. >>> Colors(1) >>> Colors(2) >>> Colors(3) >>> Colors(1) is Colors.red True The Enum class 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.enumname 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.enumname, '->', apples[color] red -> red delicious green -> granny smith 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 emphasis the fact that enumeration values are singletons, much like ``None``. Acknowledgments =============== The ``flufl.enum`` implementation is based on an example by Jeremy Hylton. It has been modified and extended by Barry Warsaw for use in the `GNU Mailman`_ project. Ben Finney is the author of the earlier enumeration PEP 354. .. _`PEP 354`: http://www.python.org/dev/peps/pep-0354/ .. _enum: http://cheeseshop.python.org/pypi/enum/ .. _`GNU Mailman`: http://www.list.org