twisted/python/constants.py

# -*- test-case-name: twisted.python.test.test_constants -*-
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.

"""
Symbolic constant support, including collections and constants with text,
numeric, and bit flag values.
"""

from __future__ import division, absolute_import

__all__ = [
    'NamedConstant', 'ValueConstant', 'FlagConstant',
    'Names', 'Values', 'Flags']

from functools import partial
from itertools import count
from operator import and_, or_, xor

_unspecified = object()
_constantOrder = partial(next, count())


class _Constant(object):
    """
    @ivar _index: A C{int} allocated from a shared counter in order to keep
        track of the order in which L{_Constant}s are instantiated.

    @ivar name: A C{str} giving the name of this constant; only set once the
        constant is initialized by L{_ConstantsContainer}.

    @ivar _container: The L{_ConstantsContainer} subclass this constant belongs
        to; C{None} until the constant is initialized by that subclass.
    """
    def __init__(self):
        self._container = None
        self._index = _constantOrder()


    def __repr__(self):
        """
        Return text identifying both which constant this is and which
        collection it belongs to.
        """
        return "<%s=%s>" % (self._container.__name__, self.name)


    def __lt__(self, other):
        """
        Implements C{<}.  Order is defined by instantiation order.

        @param other: An object.

        @return: C{NotImplemented} if C{other} is not a constant belonging to
            the same container as this constant, C{True} if this constant is
            defined before C{other}, otherwise C{False}.
        """
        if (
            not isinstance(other, self.__class__) or
            not self._container == other._container
        ):
            return NotImplemented
        return self._index < other._index


    def __le__(self, other):
        """
        Implements C{<=}.  Order is defined by instantiation order.

        @param other: An object.

        @return: C{NotImplemented} if C{other} is not a constant belonging to
            the same container as this constant, C{True} if this constant is
            defined before or equal to C{other}, otherwise C{False}.
        """
        if (
            not isinstance(other, self.__class__) or
            not self._container == other._container
        ):
            return NotImplemented
        return self is other or self._index < other._index


    def __gt__(self, other):
        """
        Implements C{>}.  Order is defined by instantiation order.

        @param other: An object.

        @return: C{NotImplemented} if C{other} is not a constant belonging to
            the same container as this constant, C{True} if this constant is
            defined after C{other}, otherwise C{False}.
        """
        if (
            not isinstance(other, self.__class__) or
            not self._container == other._container
        ):
            return NotImplemented
        return self._index > other._index


    def __ge__(self, other):
        """
        Implements C{>=}.  Order is defined by instantiation order.

        @param other: An object.

        @return: C{NotImplemented} if C{other} is not a constant belonging to
            the same container as this constant, C{True} if this constant is
            defined after or equal to C{other}, otherwise C{False}.
        """
        if (
            not isinstance(other, self.__class__) or
            not self._container == other._container
        ):
            return NotImplemented
        return self is other or self._index > other._index


    def _realize(self, container, name, value):
        """
        Complete the initialization of this L{_Constant}.

        @param container: The L{_ConstantsContainer} subclass this constant is
            part of.

        @param name: The name of this constant in its container.

        @param value: The value of this constant; not used, as named constants
            have no value apart from their identity.
        """
        self._container = container
        self.name = name


class _ConstantsContainerType(type):
    """
    L{_ConstantsContainerType} is a metaclass for creating constants container
    classes.
    """
    def __new__(self, name, bases, attributes):
        """
        Create a new constants container class.

        If C{attributes} includes a value of C{None} for the C{"_constantType"}
        key, the new class will not be initialized as a constants container and
        it will behave as a normal class.

        @param name: The name of the container class.
        @type name: L{str}

        @param bases: A tuple of the base classes for the new container class.
        @type bases: L{tuple} of L{_ConstantsContainerType} instances

        @param attributes: The attributes of the new container class, including
            any constants it is to contain.
        @type attributes: L{dict}
        """
        cls = super(_ConstantsContainerType, self).__new__(
            self, name, bases, attributes)

        # Only realize constants in concrete _ConstantsContainer subclasses.
        # Ignore intermediate base classes.
        constantType = getattr(cls, '_constantType', None)
        if constantType is None:
            return cls

        constants = []
        for (name, descriptor) in attributes.items():
            if isinstance(descriptor, cls._constantType):
                if descriptor._container is not None:
                    raise ValueError(
                        "Cannot use %s as the value of an attribute on %s" % (
                            descriptor, cls.__name__))
                constants.append((descriptor._index, name, descriptor))

        enumerants = {}
        for (index, enumerant, descriptor) in sorted(constants):
            value = cls._constantFactory(enumerant, descriptor)
            descriptor._realize(cls, enumerant, value)
            enumerants[enumerant] = descriptor

        # Save the dictionary which contains *only* constants (distinct from
        # any other attributes the application may have given the container)
        # where the class can use it later (eg for lookupByName).
        cls._enumerants = enumerants

        return cls


# In Python3 metaclasses are defined using a C{metaclass} keyword argument in
# the class definition. This would cause a syntax error in Python2.
# So we use L{type} to introduce an intermediate base class with the desired
# metaclass.
# See:
# * http://docs.python.org/2/library/functions.html#type
# * http://docs.python.org/3/reference/datamodel.html#customizing-class-creation
class _ConstantsContainer(_ConstantsContainerType('', (object,), {})):
    """
    L{_ConstantsContainer} is a class with attributes used as symbolic
    constants.  It is up to subclasses to specify what kind of constants are
    allowed.

    @cvar _constantType: Specified by a L{_ConstantsContainer} subclass to
        specify the type of constants allowed by that subclass.

    @cvar _enumerants: A C{dict} mapping the names of constants (eg
        L{NamedConstant} instances) found in the class definition to those
        instances.
    """

    _constantType = None

    def __new__(cls):
        """
        Classes representing constants containers are not intended to be
        instantiated.

        The class object itself is used directly.
        """
        raise TypeError("%s may not be instantiated." % (cls.__name__,))


    @classmethod
    def _constantFactory(cls, name, descriptor):
        """
        Construct the value for a new constant to add to this container.

        @param name: The name of the constant to create.

        @param descriptor: An instance of a L{_Constant} subclass (eg
            L{NamedConstant}) which is assigned to C{name}.

        @return: L{NamedConstant} instances have no value apart from identity,
            so return a meaningless dummy value.
        """
        return _unspecified


    @classmethod
    def lookupByName(cls, name):
        """
        Retrieve a constant by its name or raise a C{ValueError} if there is no
        constant associated with that name.

        @param name: A C{str} giving the name of one of the constants defined
            by C{cls}.

        @raise ValueError: If C{name} is not the name of one of the constants
            defined by C{cls}.

        @return: The L{NamedConstant} associated with C{name}.
        """
        if name in cls._enumerants:
            return getattr(cls, name)
        raise ValueError(name)


    @classmethod
    def iterconstants(cls):
        """
        Iteration over a L{Names} subclass results in all of the constants it
        contains.

        @return: an iterator the elements of which are the L{NamedConstant}
            instances defined in the body of this L{Names} subclass.
        """
        constants = cls._enumerants.values()

        return iter(
            sorted(constants, key=lambda descriptor: descriptor._index))


class NamedConstant(_Constant):
    """
    L{NamedConstant} defines an attribute to be a named constant within a
    collection defined by a L{Names} subclass.

    L{NamedConstant} is only for use in the definition of L{Names}
    subclasses.  Do not instantiate L{NamedConstant} elsewhere and do not
    subclass it.
    """


class Names(_ConstantsContainer):
    """
    A L{Names} subclass contains constants which differ only in their names and
    identities.
    """
    _constantType = NamedConstant


class ValueConstant(_Constant):
    """
    L{ValueConstant} defines an attribute to be a named constant within a
    collection defined by a L{Values} subclass.

    L{ValueConstant} is only for use in the definition of L{Values} subclasses.
    Do not instantiate L{ValueConstant} elsewhere and do not subclass it.
    """
    def __init__(self, value):
        _Constant.__init__(self)
        self.value = value


class Values(_ConstantsContainer):
    """
    A L{Values} subclass contains constants which are associated with arbitrary
    values.
    """
    _constantType = ValueConstant

    @classmethod
    def lookupByValue(cls, value):
        """
        Retrieve a constant by its value or raise a C{ValueError} if there is
        no constant associated with that value.

        @param value: The value of one of the constants defined by C{cls}.

        @raise ValueError: If C{value} is not the value of one of the constants
            defined by C{cls}.

        @return: The L{ValueConstant} associated with C{value}.
        """
        for constant in cls.iterconstants():
            if constant.value == value:
                return constant
        raise ValueError(value)


def _flagOp(op, left, right):
    """
    Implement a binary operator for a L{FlagConstant} instance.

    @param op: A two-argument callable implementing the binary operation.  For
        example, C{operator.or_}.

    @param left: The left-hand L{FlagConstant} instance.
    @param right: The right-hand L{FlagConstant} instance.

    @return: A new L{FlagConstant} instance representing the result of the
        operation.
    """
    value = op(left.value, right.value)
    names = op(left.names, right.names)
    result = FlagConstant()
    result._realize(left._container, names, value)
    return result


class FlagConstant(_Constant):
    """
    L{FlagConstant} defines an attribute to be a flag constant within a
    collection defined by a L{Flags} subclass.

    L{FlagConstant} is only for use in the definition of L{Flags} subclasses.
    Do not instantiate L{FlagConstant} elsewhere and do not subclass it.
    """
    def __init__(self, value=_unspecified):
        _Constant.__init__(self)
        self.value = value


    def _realize(self, container, names, value):
        """
        Complete the initialization of this L{FlagConstant}.

        This implementation differs from other C{_realize} implementations in
        that a L{FlagConstant} may have several names which apply to it, due to
        flags being combined with various operators.

        @param container: The L{Flags} subclass this constant is part of.

        @param names: When a single-flag value is being initialized, a C{str}
            giving the name of that flag.  This is the case which happens when
            a L{Flags} subclass is being initialized and L{FlagConstant}
            instances from its body are being realized.  Otherwise, a C{set} of
            C{str} giving names of all the flags set on this L{FlagConstant}
            instance.  This is the case when two flags are combined using C{|},
            for example.
        """
        if isinstance(names, str):
            name = names
            names = set([names])
        elif len(names) == 1:
            (name,) = names
        else:
            name = "{" + ",".join(sorted(names)) + "}"
        _Constant._realize(self, container, name, value)
        self.value = value
        self.names = names


    def __or__(self, other):
        """
        Define C{|} on two L{FlagConstant} instances to create a new
        L{FlagConstant} instance with all flags set in either instance set.
        """
        return _flagOp(or_, self, other)


    def __and__(self, other):
        """
        Define C{&} on two L{FlagConstant} instances to create a new
        L{FlagConstant} instance with only flags set in both instances set.
        """
        return _flagOp(and_, self, other)


    def __xor__(self, other):
        """
        Define C{^} on two L{FlagConstant} instances to create a new
        L{FlagConstant} instance with only flags set on exactly one instance
        set.
        """
        return _flagOp(xor, self, other)


    def __invert__(self):
        """
        Define C{~} on a L{FlagConstant} instance to create a new
        L{FlagConstant} instance with all flags not set on this instance set.
        """
        result = FlagConstant()
        result._realize(self._container, set(), 0)
        for flag in self._container.iterconstants():
            if flag.value & self.value == 0:
                result |= flag
        return result


    def __iter__(self):
        """
        @return: An iterator of flags set on this instance set.
        """
        return (self._container.lookupByName(name) for name in self.names)


    def __contains__(self, flag):
        """
        @param flag: The flag to test for membership in this instance
            set.

        @return: C{True} if C{flag} is in this instance set, else
            C{False}.
        """
        # Optimization for testing membership without iteration.
        return bool(flag & self)


    def __nonzero__(self):
        """
        @return: C{False} if this flag's value is 0, else C{True}.
        """
        return bool(self.value)
    __bool__ = __nonzero__


class Flags(Values):
    """
    A L{Flags} subclass contains constants which can be combined using the
    common bitwise operators (C{|}, C{&}, etc) similar to a I{bitvector} from a
    language like C.
    """
    _constantType = FlagConstant

    _value = 1

    @classmethod
    def _constantFactory(cls, name, descriptor):
        """
        For L{FlagConstant} instances with no explicitly defined value, assign
        the next power of two as its value.

        @param name: The name of the constant to create.

        @param descriptor: An instance of a L{FlagConstant} which is assigned
            to C{name}.

        @return: Either the value passed to the C{descriptor} constructor, or
            the next power of 2 value which will be assigned to C{descriptor},
            relative to the value of the last defined L{FlagConstant}.
        """
        if descriptor.value is _unspecified:
            value = cls._value
            cls._value <<= 1
        else:
            value = descriptor.value
            cls._value = value << 1
        return value