Sanjeev Kumar's Blog

magic of collections namedtuple and make the object to be used in a manner similar to a tuple or list

5/6/2015

OMFG, just one of my discovery how we can have class object behave like its of list type when it is actually not list.

From PMOTW, namedtuple instances are just as memory efficient as regular tuples because they do not have per-instance dictionaries. Each kind of namedtuple is represented by its own class, created by using the namedtuple() factory function. The arguments are the name of the new class and a string containing the names of the elements.

import collections as _collections

Point = _collections.namedtuple("Point", ("x y z"))

print Point(4,5,6)

>> Point(x=4, y=5, z=6)

collections.namedtuple docs: https://docs.python.org/2/library/collections.html

In the below code we make use of namedtuple and make class object behave like a list or tuple using __getitem__ and update the value of arguments using __setitem__ .

class PointLocation(object):
        def __init__(self, x, y, z):
                self._x = x
                self._y = y
                self._z = z
                self._location = Point(x, y, z)

        def __repr__(self):
                return "%s(%r, %r, %r)" % (
                        self.__class__.__name__,
                        self._x,
                        self._y,
                        self._z,
                        )

        def __getitem__(self, key):
                """ to make the object to be used in a manner similar to a tuple or list
               """
                return self._location.__getitem__(key)

        def __setitem__(self, key, value):
                """ update the value of argument like a dictionary
               """
                try:
                        self._location = self._location._replace(**{'xyz'[key]: value})
                except IndexError:
                        raise IndexError("%s takes %s has arguments. You are trying to update %s argument." % (
                                self.__class__.__name__, 
                                len(self._location), key)
                                )

print PointLocation(4,5,6)

pointLocationObj = PointLocation(4,5,6)

for axis in pointLocationObj:
        print axis

>> PointLocation(4, 5, 6)

4
5
6

Recipe for Named tuple on ActiveState: http://code.activestate.com/recipes/500261/
my discussion with Martijin Pieters on SO: http://stackoverflow.com/questions/30075560/how-to-set-value-of-class-object-argument-as-if-its-a-dict-using-setitem
regarding setting and updating the value of argument.

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