import sys

class Tracer:
    def __init__(self, clazz=None):
        self.clazz = clazz

    def set_trace(self):
        print('sys.settrace(%s.trace)' % self.__class__.__name__)
        sys.settrace(self.trace)
        f = sys._getframe().f_back
        while f:
            f.f_trace = self.trace
            f = f.f_back

    def trace(self, f, event, arg):
        print('%s:%s at %s:%d' % (self.__class__.__name__, event,
                                        f.f_code.co_name, f.f_lineno))
        if self.clazz and event == 'call':
            self.clazz().set_trace()
        return self.trace

class New_tracer(Tracer): pass

def foo():
    bar()
    lineno = 25

def bar(): pass

Tracer(New_tracer).set_trace()
foo()