Symbolic Constants¶

Overview¶

It is often useful to define names which will be treated as constants. twisted.python.constants provides APIs for defining such symbolic constants with minimal overhead and some useful features beyond those afforded by the common Python idioms for this task.

This document will explain how to use these APIs and what circumstances they might be helpful in.

Constant Names¶

Constants which have no value apart from their name and identity can be defined by subclassing Names . Consider this example, in which some HTTP request method constants are defined.

from twisted.python.constants import NamedConstant, Names
class METHOD(Names):
    """
    Constants representing various HTTP request methods.
    """
    GET = NamedConstant()
    PUT = NamedConstant()
    POST = NamedConstant()
    DELETE = NamedConstant()

Only direct subclasses of Names are supported (i.e., you cannot subclass METHOD to add new constants the collection).

Given this definition, constants can be looked up by name using attribute access on the METHOD object:

>>> METHOD.GET
<METHOD=GET>
>>> METHOD.PUT
<METHOD=PUT>
>>>

If it’s necessary to look up constants from a string (e.g. based on user input of some sort), a safe way to do it is using lookupByName :

>>> METHOD.lookupByName('GET')
<METHOD=GET>
>>> METHOD.lookupByName('__doc__')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "twisted/python/constants.py", line 145, in lookupByName
    raise ValueError(name)
ValueError: __doc__
>>>

As demonstrated, it is safe because any name not associated with a constant (even those special names initialized by Python itself) will result in ValueError being raised, not some other object not intended to be used the way the constants are used.

The constants can also be enumerated using the iterconstants method:

>>> list(METHOD.iterconstants())
[<METHOD=GET>, <METHOD=PUT>, <METHOD=POST>, <METHOD=DELETE>]
>>>

Constants can be compared for equality or identity:

>>> METHOD.GET is METHOD.GET
True
>>> METHOD.GET == METHOD.GET
True
>>> METHOD.GET is METHOD.PUT
False
>>> METHOD.GET == METHOD.PUT
False
>>>

Ordered comparisons (and therefore sorting) also work. The order is defined to be the same as the instantiation order of the constants:

>>> from twisted.python.constants import NamedConstant, Names
>>> class Letters(Names):
...   a = NamedConstant()
...   b = NamedConstant()
...   c = NamedConstant()
...
>>> Letters.a < Letters.b < Letters.c
True
>>> Letters.a > Letters.b
False
>>> sorted([Letters.b, Letters.a, Letters.c])
[<Letters=a>, <Letters=b>, <Letters=c>]
>>>

A subclass of Names may define class methods to implement custom functionality. Consider this definition of METHOD :

from twisted.python.constants import NamedConstant, Names
class METHOD(Names):
    """
    Constants representing various HTTP request methods.
    """
    GET = NamedConstant()
    PUT = NamedConstant()
    POST = NamedConstant()
    DELETE = NamedConstant()

    @classmethod
    def isIdempotent(cls, method):
        """
        Return True if the given method is side-effect free, False otherwise.
        """
        return method is cls.GET

This functionality can be used as any class methods are used:

>>> METHOD.isIdempotent(METHOD.GET)
True
>>> METHOD.isIdempotent(METHOD.POST)
False
>>>

Constants With Values¶

Constants with a particular associated value are supported by the Values base class. Consider this example, in which some HTTP status code constants are defined.

from twisted.python.constants import ValueConstant, Values
class STATUS(Values):
    """
    Constants representing various HTTP status codes.
    """
    OK = ValueConstant("200")
    FOUND = ValueConstant("302")
    NOT_FOUND = ValueConstant("404")

As with Names , constants are accessed as attributes of the class object:

>>> STATUS.OK
<STATUS=OK>
>>> STATUS.FOUND
<STATUS=FOUND>
>>>

Additionally, the values of the constants can be accessed using the value attribute of one these objects:

>>> STATUS.OK.value
'200'
>>>

As with Names , constants can be looked up by name:

>>> STATUS.lookupByName('NOT_FOUND')
<STATUS=NOT_FOUND>
>>>

Constants on a Values subclass can also be looked up by value:

>>> STATUS.lookupByValue('404')
<STATUS=NOT_FOUND>
>>> STATUS.lookupByValue('500')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "twisted/python/constants.py", line 244, in lookupByValue
      raise ValueError(value)
ValueError: 500
>>>

Multiple constants may have the same value. If they do, lookupByValue will find the one which is defined first.

Iteration is also supported:

>>> list(STATUS.iterconstants())
[<STATUS=OK>, <STATUS=FOUND>, <STATUS=NOT_FOUND>]
>>>

Constants can be compared for equality, identity and ordering:

>>> STATUS.OK == STATUS.OK
True
>>> STATUS.OK is STATUS.OK
True
>>> STATUS.OK is STATUS.NOT_FOUND
False
>>> STATUS.OK == STATUS.NOT_FOUND
False
>>> STATUS.NOT_FOUND > STATUS.OK
True
>>> STATUS.FOUND < STATUS.OK
False
>>>

Note that like Names , Values are ordered by instantiation order, not by value, though either order is the same in the above example.

As with Names , a subclass of Values can define custom methods:

from twisted.python.constants import ValueConstant, Values
class STATUS(Values):
    """
    Constants representing various HTTP status codes.
    """
    OK = ValueConstant("200")
    NO_CONTENT = ValueConstant("204")
    NOT_MODIFIED = ValueConstant("304")
    NOT_FOUND = ValueConstant("404")

    @classmethod
    def hasBody(cls, status):
        """
        Return True if the given status is associated with a response body,
        False otherwise.
        """
        return status not in (cls.NO_CONTENT, cls.NOT_MODIFIED)

This functionality can be used as any class methods are used:

>>> STATUS.hasBody(STATUS.OK)
True
>>> STATUS.hasBody(STATUS.NO_CONTENT)
False
>>>

Constants As Flags¶

Integers are often used as a simple set for constants. The values for these constants are assigned as powers of two so that bits in the integer can be set to represent them. Individual bits are often called flags . Flags supports this use-case, including allowing constants with particular bits to be set, for interoperability with other tools.

POSIX filesystem access control is traditionally done using a bitvector defining which users and groups may perform which operations on a file. This state might be represented using Flags as follows:

from twisted.python.constants import FlagConstant, Flags
class Permission(Flags):
    """
    Constants representing user, group, and other access bits for reading,
    writing, and execution.
    """
    OTHER_EXECUTE = FlagConstant()
    OTHER_WRITE = FlagConstant()
    OTHER_READ = FlagConstant()
    GROUP_EXECUTE = FlagConstant()
    GROUP_WRITE = FlagConstant()
    GROUP_READ = FlagConstant()
    USER_EXECUTE = FlagConstant()
    USER_WRITE = FlagConstant()
    USER_READ = FlagConstant()

As for the previous types of constants, these can be accessed as attributes of the class object:

>>> Permission.USER_READ
<Permission=USER_READ>
>>> Permission.USER_WRITE
<Permission=USER_WRITE>
>>> Permission.USER_EXECUTE
<Permission=USER_EXECUTE>
>>>

These constant objects also have a value attribute giving their integer value:

>>> Permission.USER_READ.value
256
>>>

These constants can be looked up by name or value:

>>> Permission.lookupByName('USER_READ') is Permission.USER_READ
True
>>> Permission.lookupByValue(256) is Permission.USER_READ
True
>>>

Constants can also be combined using the logical operators & (and ), | (or ), and ^ (exclusive or ).

>>> Permission.USER_READ | Permission.USER_WRITE
<Permission={USER_READ,USER_WRITE}>
>>> (Permission.USER_READ | Permission.USER_WRITE) & Permission.USER_WRITE
<Permission=USER_WRITE>
>>> (Permission.USER_READ | Permission.USER_WRITE) ^ Permission.USER_WRITE
<Permission=USER_READ>
>>>

These combined constants can be deconstructed via iteration:

>>> mode = Permission.USER_READ | Permission.USER_WRITE
>>> list(mode)
[<Permission=USER_READ>, <Permission=USER_WRITE>]
>>> Permission.USER_READ in mode
True
>>> Permission.USER_EXECUTE in mode
False
>>>

They can also be inspected via boolean operations:

>>> Permission.USER_READ & mode
<Permission=USER_READ>
>>> bool(Permission.USER_READ & mode)
True
>>> Permission.USER_EXECUTE & mode
<Permission={}>
>>> bool(Permission.USER_EXECUTE & mode)
False
>>>

The unary operator ~ (not ) is also defined:

>>> ~Permission.USER_READ
<Permission={GROUP_EXECUTE,GROUP_READ,GROUP_WRITE,OTHER_EXECUTE,OTHER_READ,OTHER_WRITE,USER_EXECUTE,USER_WRITE}>
>>>

Constants created using these operators also have a value attribute.

>>> (~Permission.USER_WRITE).value
383
>>>

Note the care taken to ensure the ~ operator is applied first and the value attribute is looked up second.

A Flags subclass can also define methods, just as a Names or Values subclass may. For example, Permission might benefit from a method to format a flag as a string in the traditional style. Consider this addition to that class:

from twisted.python import filepath
from twisted.python.constants import FlagConstant, Flags
class Permission(Flags):
    ...

    @classmethod
    def format(cls, permissions):
        """
        Format permissions flags in the traditional 'rwxr-xr-x' style.
        """
        return filepath.Permissions(permissions.value).shorthand()

Use this like any other class method:

>>> Permission.format(Permission.USER_READ | Permission.USER_WRITE | Permission.GROUP_READ | Permission.OTHER_READ)
'rw-r--r--'
>>>

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