import sys
import unittest


class Tracer(object):

    def __init__(self):
        self.call_calls = 0
        self.line_calls = 0

    def trace_func(self, frame, event, arg):
        if event == 'call':
            self.call_calls += 1
            return self.trace_func
        
        elif event == 'line':
            self.line_calls += 1
            # Should disable tracing when None is returned
            return None
        
        else:
            return self.trace_func


class Test(unittest.TestCase):

    def test_tracing(self):

        def method():
            _a = 1
            _b = 1
            _c = 1
            _d = 1

        tracer = Tracer()

        curr_trace_func = sys.gettrace()
        try:
            sys.settrace(tracer.trace_func)

            method()

            if tracer.call_calls != 1:
                self.fail('Expected a single call event. Found: %s' % (tracer.call_calls))
            if tracer.line_calls != 1:
                self.fail('Expected a single line event. Found: %s' % (tracer.line_calls,))
        finally:
            sys.settrace(curr_trace_func)