Demo/metaclasses/Meta.py

   1 """Generic metaclass.
   2
   3 XXX This is very much a work in progress.
   4
   5 """
   6
   7 import types
   8
   9 class MetaMethodWrapper:
  10
  11     def __init__(self, func, inst):
  12         self.func = func
  13         self.inst = inst
  14         self.__name__ = self.func.__name__
  15
  16     def __call__(self, *args, **kw):
  17         return apply(self.func, (self.inst,) + args, kw)
  18
  19 class MetaHelper:
  20
  21     __methodwrapper__ = MetaMethodWrapper # For derived helpers to override
  22
  23     def __helperinit__(self, formalclass):
  24         self.__formalclass__ = formalclass
  25
  26     def __getattr__(self, name):
  27         # Invoked for any attr not in the instance's __dict__
  28         try:
  29             raw = self.__formalclass__.__getattr__(name)
  30         except AttributeError:
  31             try:
  32                 ga = self.__formalclass__.__getattr__('__usergetattr__')
  33             except (KeyError, AttributeError):
  34                 raise AttributeError, name
  35             return ga(self, name)
  36         if type(raw) != types.FunctionType:
  37             return raw
  38         return self.__methodwrapper__(raw, self)
  39
  40 class MetaClass:
  41
  42     """A generic metaclass.
  43
  44     This can be subclassed to implement various kinds of meta-behavior.
  45
  46     """
  47
  48     __helper__ = MetaHelper             # For derived metaclasses to override
  49
  50     __inited = 0
  51
  52     def __init__(self, name, bases, dict):
  53         try:
  54             ga = dict['__getattr__']
  55         except KeyError:
  56             pass
  57         else:
  58             dict['__usergetattr__'] = ga
  59             del dict['__getattr__']
  60         self.__name__ = name
  61         self.__bases__ = bases
  62         self.__realdict__ = dict
  63         self.__inited = 1
  64
  65     def __getattr__(self, name):
  66         try:
  67             return self.__realdict__[name]
  68         except KeyError:
  69             for base in self.__bases__:
  70                 try:
  71                     return base.__getattr__(name)
  72                 except AttributeError:
  73                     pass
  74             raise AttributeError, name
  75
  76     def __setattr__(self, name, value):
  77         if not self.__inited:
  78             self.__dict__[name] = value
  79         else:
  80             self.__realdict__[name] = value
  81
  82     def __call__(self, *args, **kw):
  83         inst = self.__helper__()
  84         inst.__helperinit__(self)
  85         try:
  86             init = inst.__getattr__('__init__')
  87         except AttributeError:
  88             init = lambda: None
  89         apply(init, args, kw)
  90         return inst
  91
  92
  93 Meta = MetaClass('Meta', (), {})
  94
  95
  96 def _test():
  97     class C(Meta):
  98         def __init__(self, *args):
  99             print "__init__, args =", args
 100         def m1(self, x):
 101             print "m1(x=%r)" % (x,)
 102     print C
 103     x = C()
 104     print x
 105     x.m1(12)
 106     class D(C):
 107         def __getattr__(self, name):
 108             if name[:2] == '__': raise AttributeError, name
 109             return "getattr:%s" % name
 110     x = D()
 111     print x.foo
 112     print x._foo
 113 ##     print x.__foo
 114 ##     print x.__foo__
 115
 116
 117 if __name__ == '__main__':
 118     _test()