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""" 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