Method chaining in python example

method_chaining.py

import copy
from collections import namedtuple, Sequence


Item = namedtuple('Item', 'name, price')

items = [
    Item('apple', 10.0),
    Item('banana', 12.0),
    Item('orange', 8.0),
    Item('coconut', 50.0),
]


class Query(Sequence):
    def __init__(self, items):
        self.items = items

    def __getitem__(self, i):
        return self.items[i]

    def __len__(self):
        return len(self.items)

    def _clone(self):
        #return Query(self.items)
        return copy.deepcopy(self)

    def first(self):
        q = self._clone()
        return next(iter(q), None)

    def last(self):
        q = self._clone()
        try:
            return q[-1]
        except KeyError:
            return None

    def values(self, attr):
        q = self._clone()
        return [getattr(o, attr) for o in q]

    def filter(self, cond):
        """
        Filter by condition.

        Condition must be a function taking one argument (an object),
        and returning True or False.
        """
        q = self._clone()
        return Query([o for o in q if cond(o)])

    @property
    def total(self):
        return sum(self.values('price'))


if __name__ == "__main__":
    q = Query(items)
    q2 = Query(items)
    assert id(q.items) != id(q2.items)

    assert q.first() == items[0]
    assert q.last() == items[-1]

    assert q.values('name') == ['apple', 'banana', 'orange', 'coconut']
    assert q.values('price') == [10.0, 12.0, 8.0, 50.0]

    assert q.filter(lambda x: x.price > 30).values('name') == ['coconut']
    assert q.filter(lambda x: x.price > 30).total == 50.0

    assert q.total == 80.0