Is this use of isinstance pythonic/“good”?

Question

A side effect of this question is that I was lead to this post, which states:

Whenever isinstance is used, control flow forks; one type of object goes dow

Answer 1

To elaborate further on the comment I made under Justin's answer, I would keep his code for __iadd__ (i.e., so MyTime objects can only be added to other MyTime objects) and rewrite __init__ in this way:

def __init__(self, **params):
    if params.get('sec'):
        t = params['sec']
        self.h = t/3600
        t %= 3600
        self.m = t/60
        t %= 60
        self.s = t
    elif params.get('time'):
        t = params['time']
        self.h = t.h
        self.m = t.m
        self.s = t.s
    else:
        if params:
            raise TypeError("__init__() got unexpected keyword argument '%s'" % params.keys()[0])
        else:
            raise TypeError("__init__() expected keyword argument 'sec' or 'time'")

# example usage
t1 = MyTime(sec=30)
t2 = MyTime(sec=60)
t2 += t1 
t3 = MyTime(time=t1)

I just tried to pick short keyword arguments, but you may want to get more descriptive than I did.