1 files changed, 126 insertions, 0 deletions
diff --git a/Demo/metaclasses/Trace.py b/Demo/metaclasses/Trace.py
new file mode 100644
index 0000000..ed3944f
--- /dev/null
+++ b/Demo/metaclasses/Trace.py
@@ -0,0 +1,126 @@
+"""Tracing metaclass."""
+
+import types
+
+class TraceMetaClass:
+    """Metaclass for tracing.
+
+    Classes defined using this metaclass have an automatic tracing
+    feature -- by setting the __trace_output__ instance (or class)
+    variable to a file object, trace messages about all calls are
+    written to the file.  The trace formatting can be changed by
+    defining a suitable __trace_call__ method.
+
+    """
+
+    __inited = 0
+
+    def __init__(self, name, bases, dict):
+	self.__name__ = name
+	self.__bases__ = bases
+	self.__dict = dict
+	# XXX Can't define __dict__, alas
+	self.__inited = 1
+
+    def __getattr__(self, name):
+	try:
+	    return self.__dict[name]
+	except KeyError:
+	    for base in self.__bases__:
+		try:
+		    return getattr(base, name)
+		except AttributeError:
+		    pass
+	    raise AttributeError, name
+
+    def __setattr__(self, name, value):
+	if not self.__inited:
+	    self.__dict__[name] = value
+	else:
+	    self.__dict[name] = value
+
+    def __call__(self, *args, **kw):
+	inst = TracingInstance()
+	inst.__meta_init__(self)
+	try:
+	    init = inst.__getattr__('__init__')
+	except AttributeError:
+	    init = lambda: None
+	apply(init, args, kw)
+	return inst
+
+    __trace_output__ = None
+
+class TracingInstance:
+    """Helper class to represent an instance of a tracing class."""
+
+    def __trace_call__(self, fp, fmt, *args):
+	fp.write((fmt+'\n') % args)
+
+    def __meta_init__(self, klass):
+	self.__class = klass
+
+    def __getattr__(self, name):
+	# Invoked for any attr not in the instance's __dict__
+	try:
+	    raw = self.__class.__getattr__(name)
+	except AttributeError:
+	    raise AttributeError, name
+	if type(raw) != types.FunctionType:
+	    return raw
+	# It's a function
+	fullname = self.__class.__name__ + "." + name
+	if not self.__trace_output__ or name == '__trace_call__':
+	    return NotTracingWrapper(fullname, raw, self)
+	else:
+	    return TracingWrapper(fullname, raw, self)
+
+class NotTracingWrapper:
+    def __init__(self, name, func, inst):
+	self.__name__ = name
+	self.func = func
+	self.inst = inst
+    def __call__(self, *args, **kw):
+	return apply(self.func, (self.inst,) + args, kw)
+
+class TracingWrapper(NotTracingWrapper):
+    def __call__(self, *args, **kw):
+	self.inst.__trace_call__(self.inst.__trace_output__,
+				 "calling %s, inst=%s, args=%s, kw=%s",
+				 self.__name__, self.inst, args, kw)
+	try:
+	    rv = apply(self.func, (self.inst,) + args, kw)
+	except:
+	    t, v, tb = sys.exc_info()
+	    self.inst.__trace_call__(self.inst.__trace_output__,
+				     "returning from %s with exception %s: %s",
+				     self.__name__, t, v)
+	    raise t, v, tb
+	else:
+	    self.inst.__trace_call__(self.inst.__trace_output__,
+				     "returning from %s with value %s",
+				     self.__name__, rv)
+	    return rv
+
+Traced = TraceMetaClass('Traced', (), {'__trace_output__': None})
+
+
+def _test():
+    import sys
+    class C(Traced):
+	def __init__(self, x=0): self.x = x
+	def m1(self, x): self.x = x
+	def m2(self, y): return self.x + y
+    C.__trace_output__ = sys.stdout
+    x = C(4321)
+    print x
+    print x.x
+    print x.m1(100)
+    print x.m1(10)
+    print x.m2(33)
+    print x.m1(5)
+    print x.m2(4000)
+    print x.x
+
+if __name__ == '__main__':
+    _test()