Relevance of typename in namedtuple

Question

from collections import namedtuple

Point = namedtuple('whatsmypurpose',['x','y'])
p = Point(11,22)
print(p)

Output:

whatsmypurpose(x=11,y=22)

What’s the relevance/use of 'whatsmypurpose'?

Asked By: Phoenix

||

Source

Answer 1

namedtuple() is a factory function for tuple subclasses. Here, 'whatsmypurpose'is the type name. When you create a named tuple, a class with this name (whatsmypurpose) gets created internally.

You can notice this by using the verbose argument like:

Point=namedtuple('whatsmypurpose',['x','y'], verbose=True)

Also you can try type(p) to verify this.

Answered By: haraprasadj

Answer 2

'whatsmypurpose' gives the new subclass its type name. From the docs:

collections.namedtuple(typename, field_names, verbose=False,rename=False)
Returns a new tuple subclass named typename.

Here is an example:

>>> from collections import namedtuple
>>> Foo = namedtuple('Foo', ['a', 'b'])
>>> type(Foo)
<class 'type'>
>>> a = Foo(a = 1, b = 2)
>>> a
Foo(a=1, b=2)
>>> Foo = namedtuple('whatsmypurpose', ['a', 'b'])
>>> a = Foo(a = 1, b = 2)
>>> a
whatsmypurpose(a=1, b=2)
>>>

Set the verbose parameter to True and you can see the complete whatsmypurpose class definition.

>>> Foo = namedtuple('whatsmypurpose', ['a', 'b'], verbose=True)
from builtins import property as _property, tuple as _tuple
from operator import itemgetter as _itemgetter
from collections import OrderedDict

class whatsmypurpose(tuple):
    'whatsmypurpose(a, b)'

    __slots__ = ()

    _fields = ('a', 'b')

    def __new__(_cls, a, b):
        'Create new instance of whatsmypurpose(a, b)'
        return _tuple.__new__(_cls, (a, b))

    @classmethod
    def _make(cls, iterable, new=tuple.__new__, len=len):
        'Make a new whatsmypurpose 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 _replace(_self, **kwds):
        'Return a new whatsmypurpose object replacing specified fields with new values'
        result = _self._make(map(kwds.pop, ('a', 'b'), _self))
        if kwds:
            raise ValueError('Got unexpected field names: %r' % list(kwds))
        return result

    def __repr__(self):
        'Return a nicely formatted representation string'
        return self.__class__.__name__ + '(a=%r, b=%r)' % self

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

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

    a = _property(_itemgetter(0), doc='Alias for field number 0')

    b = _property(_itemgetter(1), doc='Alias for field number 1')

Answered By: wwii

Answer 3

Consider:

class MyClass(tuple):
   pass

This creates a type, which is a tuple subclass, and it has a name MyClass.__name__ == "MyClass". namedtuple is a type factory, it also creates tuple subclasses, but in this functional API you have to pass the name explicitly.

When you assign the returned type to a different name:

Point = namedtuple('whatsmypurpose',['x','y'])

It is similar to doing this:

class whatsmypurpose(tuple):
    ... # extra stuff here to setup slots, field names, etc

Point = whatsmypurpose
del whatsmypurpose

In both cases you are just aliasing a different name to the type.

Usually you would assign to the same name as used for the type name. If you are bothered that repeating the same string is not DRY, then you can use the declarative API in typing.NamedTuple instead of that functional API in collections. _{Then you can be bothered by needing to annotate types, instead.}

Answered By: wim

Relevance of typename in namedtuple

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