code/train/Python/0006716_declarative.py

"""
Declarative objects.

Declarative objects have a simple protocol: you can use classes in
lieu of instances and they are equivalent, and any keyword arguments
you give to the constructor will override those instance variables.
(So if a class is received, we'll simply instantiate an instance with
no arguments).

You can provide a variable __unpackargs__ (a list of strings), and if
the constructor is called with non-keyword arguments they will be
interpreted as the given keyword arguments.

If __unpackargs__ is ('*', name), then all the arguments will be put
in a variable by that name.

You can define a __classinit__(cls, new_attrs) method, which will be
called when the class is created (including subclasses).  Note: you
can't use super() in __classinit__ because the class isn't bound to a
name.  As an analog to __classinit__, Declarative adds
__instanceinit__ which is called with the same argument (new_attrs).
This is like __init__, but after __unpackargs__ and other factors have
been taken into account.

If __mutableattributes__ is defined as a sequence of strings, these
attributes will not be shared between superclasses and their
subclasses.  E.g., if you have a class variable that contains a list
and you append to that list, changes to subclasses will effect
superclasses unless you add the attribute here.

Also defines classinstancemethod, which acts as either a class method
or an instance method depending on where it is called.
"""

from __future__ import generators

__all__ = ('classinstancemethod', 'DeclarativeMeta', 'Declarative')

import copy

try:
    import itertools
    counter = itertools.count()
except ImportError:
    def _counter():
        i = 0
        while 1:
            i += 1
            yield i
    counter = _counter()

class classinstancemethod(object):
    """
    Acts like a class method when called from a class, like an
    instance method when called by an instance.  The method should
    take two arguments, 'self' and 'cls'; one of these will be None
    depending on how the method was called.
    """

    def __init__(self, func):
        self.func = func

    def __get__(self, obj, type=None):
        return _methodwrapper(self.func, obj=obj, type=type)

class _methodwrapper(object):

    def __init__(self, func, obj, type):
        self.func = func
        self.obj = obj
        self.type = type

    def __call__(self, *args, **kw):
        assert not kw.has_key('self') and not kw.has_key('cls'), (
            "You cannot use 'self' or 'cls' arguments to a "
            "classinstancemethod")
        return self.func(*((self.obj, self.type) + args), **kw)

    def __repr__(self):
        if self.obj is None:
            return ('<bound class method %s.%s>'
                    % (self.type.__name__, self.func.func_name))
        else:
            return ('<bound method %s.%s of %r>'
                    % (self.type.__name__, self.func.func_name, self.obj))


class DeclarativeMeta(type):

    def __new__(meta, class_name, bases, new_attrs):
        cls = type.__new__(meta, class_name, bases, new_attrs)
        if new_attrs.has_key('__classinit__'):
            cls.__classinit__ = staticmethod(cls.__classinit__.im_func)
        cls.__classinit__(cls, new_attrs)
        return cls

class Declarative(object):

    __unpackargs__ = ()

    __mutableattributes__ = ()

    __metaclass__ = DeclarativeMeta

    def __classinit__(cls, new_attrs):
        cls.declarative_count = counter.next()
        for name in cls.__mutableattributes__:
            if not new_attrs.has_key(name):
                setattr(cls, copy.copy(getattr(cls, name)))

    def __instanceinit__(self, new_attrs):
        for name, value in new_attrs.items():
            setattr(self, name, value)
        if not new_attrs.has_key('declarative_count'):
            self.declarative_count = counter.next()

    def __init__(self, *args, **kw):
        if self.__unpackargs__ and self.__unpackargs__[0] == '*':
            assert len(self.__unpackargs__) == 2, \
                   "When using __unpackargs__ = ('*', varname), you must only provide a single variable name (you gave %r)" % self.__unpackargs__
            name = self.__unpackargs__[1]
            if kw.has_key(name):
                raise TypeError(
                    "keyword parameter '%s' was given by position and name"
                    % name)
            kw[name] = args
        else:
            if len(args) > len(self.__unpackargs__):
                raise TypeError(
                    '%s() takes at most %i arguments (%i given)'
                    % (self.__class__.__name__,
                       len(self.__unpackargs__),
                       len(args)))
            for name, arg in zip(self.__unpackargs__, args):
                if kw.has_key(name):
                    raise TypeError(
                        "keyword parameter '%s' was given by position and name"
                        % name)
                kw[name] = arg
        if kw.has_key('__alsocopy'):
            for name, value in kw['__alsocopy'].items():
                if not kw.has_key(name):
                    if name in self.__mutableattributes__:
                        value = copy.copy(value)
                    kw[name] = value
            del kw['__alsocopy']
        self.__instanceinit__(kw)

    def __call__(self, *args, **kw):
        kw['__alsocopy'] = self.__dict__
        return self.__class__(*args, **kw)

    def singleton(self, cls):
        if self:
            return self
        name = '_%s__singleton' % cls.__name__
        if not hasattr(cls, name):
            setattr(cls, name, cls(declarative_count=cls.declarative_count))
        return getattr(cls, name)
    singleton = classinstancemethod(singleton)

    def __repr__(self, cls):
        if self:
            name = '%s object' % self.__class__.__name__
            v = self.__dict__.copy()
        else:
            name = '%s class' % cls.__name__
            v = cls.__dict__.copy()
        if v.has_key('declarative_count'):
            name = '%s %i' % (name, v['declarative_count'])
            del v['declarative_count']
        # @@: simplifying repr:
        #v = {}
        names = v.keys()
        args = []
        for n in self._repr_vars(names):
            args.append('%s=%r' % (n, v[n]))
        if not args:
            return '<%s>' % name
        else:
            return '<%s %s>' % (name, ' '.join(args))

    def _repr_vars(dictNames):
        names = [n for n in dictNames
                 if not n.startswith('_')
                 and n != 'declarative_count']
        names.sort()
        return names
    _repr_vars = staticmethod(_repr_vars)

    __repr__ = classinstancemethod(__repr__)