Demo/metaclasses/Trace.py

"""Tracing metaclass.

XXX This is very much a work in progress.

"""

import types, sys

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 base.__getattr__(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():
    global C, D
    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
	__trace_output__ = sys.stdout
    class D(C):
	def m2(self, y): print "D.m2(%s)" % `y`; return C.m2(self, y)
	__trace_output__ = None
    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

    print C.__init__
    print C.m2
    print D.__init__
    print D.m2

    y = D()
    print y
    print y.m1(10)
    print y.m2(100)
    print y.x

if __name__ == '__main__':
    _test()