Python does not recognize the type of a namedtuple

Question

I suspect this question has been asked before, but I have not been able to find it, so here goes:

On Python (using 2.7) I create a namedtuple as below:

>>> sgn_tuple = namedtuple('sgnt',['signal','type'])
>>> a = sgn_tuple("aaa","bbb")

Then I want to check the type of t and my results are weird:

>>> type (a)
<class '__main__.sgnt'>
>>> a is tuple
False
>>> a is namedtuple
False
>>> a is sgnt
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'sgnt' is not defined
>>> a is sgn_tuple
False
>>>

Why is this so? I would expect a to be recognized as a tuple type, but it is not. Any hints?

Answer 1

is doesn't check class membership. is checks if two objects have the same id

>>> isinstance(a, tuple)
True

Also type(a) is not tuple, a is a subclass of tuple.

If you type verbose=True you can see how it's made (the text is dynamically generated to create the class):

>>> sgn_tuple = namedtuple('sgnt',['signal','type'],verbose=True)

class sgnt(tuple):
        'sgnt(signal, type)' 

        __slots__ = () 

        _fields = ('signal', 'type') 

        def __new__(_cls, signal, type):
            'Create new instance of sgnt(signal, type)'
            return _tuple.__new__(_cls, (signal, type)) 

        @classmethod
        def _make(cls, iterable, new=tuple.__new__, len=len):
            'Make a new sgnt object from a sequence or iterable'
            result = new(cls, iterable)
            if len(result) != 2:
                raise TypeError('Expected 2 arguments, got %d' % len(result))
            return result 

        def __repr__(self):
            'Return a nicely formatted representation string'
            return 'sgnt(signal=%r, type=%r)' % self 

        def _asdict(self):
            'Return a new OrderedDict which maps field names to their values'
            return OrderedDict(zip(self._fields, self)) 

        __dict__ = property(_asdict) 

        def _replace(_self, **kwds):
            'Return a new sgnt object replacing specified fields with new values'
            result = _self._make(map(kwds.pop, ('signal', 'type'), _self))
            if kwds:
                raise ValueError('Got unexpected field names: %r' % kwds.keys())
            return result 

        def __getnewargs__(self):
            'Return self as a plain tuple.  Used by copy and pickle.'
            return tuple(self) 

        signal = _property(_itemgetter(0), doc='Alias for field number 0')
        type = _property(_itemgetter(1), doc='Alias for field number 1')

That is simply execed by Python. I hope that clears things up.