'Profile a singe call to the function.'

def __call__(self, *args, **kw):

    self.ncalls += 1
    if TraceFuncCoverage.tracing:
        return self.fn(*args, **kw)
    try:
        TraceFuncCoverage.tracing = True
        return self.tracer.runfunc(self.fn, *args, **kw)
    finally:
        TraceFuncCoverage.tracing = False
