Demo/metaclasses/Trace.py

   1 """Tracing metaclass.
   2
   3 XXX This is very much a work in progress.
   4
   5 """
   6
   7 import types, sys
   8
   9 class TraceMetaClass:
  10     """Metaclass for tracing.
  11
  12     Classes defined using this metaclass have an automatic tracing
  13     feature -- by setting the __trace_output__ instance (or class)
  14     variable to a file object, trace messages about all calls are
  15     written to the file.  The trace formatting can be changed by
  16     defining a suitable __trace_call__ method.
  17
  18     """
  19
  20     __inited = 0
  21
  22     def __init__(self, name, bases, dict):
  23         self.__name__ = name
  24         self.__bases__ = bases
  25         self.__dict = dict
  26         # XXX Can't define __dict__, alas
  27         self.__inited = 1
  28
  29     def __getattr__(self, name):
  30         try:
  31             return self.__dict[name]
  32         except KeyError:
  33             for base in self.__bases__:
  34                 try:
  35                     return base.__getattr__(name)
  36                 except AttributeError:
  37                     pass
  38             raise AttributeError, name
  39
  40     def __setattr__(self, name, value):
  41         if not self.__inited:
  42             self.__dict__[name] = value
  43         else:
  44             self.__dict[name] = value
  45
  46     def __call__(self, *args, **kw):
  47         inst = TracingInstance()
  48         inst.__meta_init__(self)
  49         try:
  50             init = inst.__getattr__('__init__')
  51         except AttributeError:
  52             init = lambda: None
  53         apply(init, args, kw)
  54         return inst
  55
  56     __trace_output__ = None
  57
  58 class TracingInstance:
  59     """Helper class to represent an instance of a tracing class."""
  60
  61     def __trace_call__(self, fp, fmt, *args):
  62         fp.write((fmt+'\n') % args)
  63
  64     def __meta_init__(self, klass):
  65         self.__class = klass
  66
  67     def __getattr__(self, name):
  68         # Invoked for any attr not in the instance's __dict__
  69         try:
  70             raw = self.__class.__getattr__(name)
  71         except AttributeError:
  72             raise AttributeError, name
  73         if type(raw) != types.FunctionType:
  74             return raw
  75         # It's a function
  76         fullname = self.__class.__name__ + "." + name
  77         if not self.__trace_output__ or name == '__trace_call__':
  78             return NotTracingWrapper(fullname, raw, self)
  79         else:
  80             return TracingWrapper(fullname, raw, self)
  81
  82 class NotTracingWrapper:
  83     def __init__(self, name, func, inst):
  84         self.__name__ = name
  85         self.func = func
  86         self.inst = inst
  87     def __call__(self, *args, **kw):
  88         return apply(self.func, (self.inst,) + args, kw)
  89
  90 class TracingWrapper(NotTracingWrapper):
  91     def __call__(self, *args, **kw):
  92         self.inst.__trace_call__(self.inst.__trace_output__,
  93                                  "calling %s, inst=%s, args=%s, kw=%s",
  94                                  self.__name__, self.inst, args, kw)
  95         try:
  96             rv = apply(self.func, (self.inst,) + args, kw)
  97         except:
  98             t, v, tb = sys.exc_info()
  99             self.inst.__trace_call__(self.inst.__trace_output__,
 100                                      "returning from %s with exception %s: %s",
 101                                      self.__name__, t, v)
 102             raise t, v, tb
 103         else:
 104             self.inst.__trace_call__(self.inst.__trace_output__,
 105                                      "returning from %s with value %s",
 106                                      self.__name__, rv)
 107             return rv
 108
 109 Traced = TraceMetaClass('Traced', (), {'__trace_output__': None})
 110
 111
 112 def _test():
 113     global C, D
 114     class C(Traced):
 115         def __init__(self, x=0): self.x = x
 116         def m1(self, x): self.x = x
 117         def m2(self, y): return self.x + y
 118         __trace_output__ = sys.stdout
 119     class D(C):
 120         def m2(self, y): print "D.m2(%s)" % `y`; return C.m2(self, y)
 121         __trace_output__ = None
 122     x = C(4321)
 123     print x
 124     print x.x
 125     print x.m1(100)
 126     print x.m1(10)
 127     print x.m2(33)
 128     print x.m1(5)
 129     print x.m2(4000)
 130     print x.x
 131
 132     print C.__init__
 133     print C.m2
 134     print D.__init__
 135     print D.m2
 136
 137     y = D()
 138     print y
 139     print y.m1(10)
 140     print y.m2(100)
 141     print y.x
 142
 143 if __name__ == '__main__':
 144     _test()
 145