coding-mentor-agent/kb/python-course-kb-practical-python/wiki/exercises/6-2-supporting-iteration.md

id, source_exercise_id, title, section, source_path, source_repo, source_commit, student_visible_solution, has_private_solution, skip

id	source_exercise_id	title	section	source_path	source_repo	source_commit	student_visible_solution	has_private_solution	skip
practical-python-6.2	6.2	Supporting Iteration	6.1 Iteration Protocol	06_Generators/01_Iteration_protocol.md	https://github.com/dabeaz-course/practical-python	93dca856b41c61a0a0f85ae334116e4c125629ea	false	false	false

Exercise 6.2: Supporting Iteration

Source: Practical Python Programming, 06_Generators/01_Iteration_protocol.md.

Exercise 6.2: Supporting Iteration

On occasion, you might want to make one of your own objects support iteration--especially if your object wraps around an existing list or other iterable. In a new file portfolio.py, define the following class:

# portfolio.py

class Portfolio:

    def __init__(self, holdings):
        self._holdings = holdings

    @property
    def total_cost(self):
        return sum([s.cost for s in self._holdings])

    def tabulate_shares(self):
        from collections import Counter
        total_shares = Counter()
        for s in self._holdings:
            total_shares[s.name] += s.shares
        return total_shares

This class is meant to be a layer around a list, but with some extra methods such as the total_cost property. Modify the read_portfolio() function in report.py so that it creates a Portfolio instance like this:

# report.py
...

import fileparse
from stock import Stock
from portfolio import Portfolio

def read_portfolio(filename):
    '''
    Read a stock portfolio file into a list of dictionaries with keys
    name, shares, and price.
    '''
    with open(filename) as file:
        portdicts = fileparse.parse_csv(file,
                                        select=['name','shares','price'],
                                        types=[str,int,float])

    portfolio = [ Stock(d['name'], d['shares'], d['price']) for d in portdicts ]
    return Portfolio(portfolio)
...

Try running the report.py program. You will find that it fails spectacularly due to the fact that Portfolio instances aren't iterable.

>>> import report
>>> report.portfolio_report('Data/portfolio.csv', 'Data/prices.csv')
... crashes ...

Fix this by modifying the Portfolio class to support iteration:

class Portfolio:

    def __init__(self, holdings):
        self._holdings = holdings

    def __iter__(self):
        return self._holdings.__iter__()

    @property
    def total_cost(self):
        return sum([s.shares*s.price for s in self._holdings])

    def tabulate_shares(self):
        from collections import Counter
        total_shares = Counter()
        for s in self._holdings:
            total_shares[s.name] += s.shares
        return total_shares

After you've made this change, your report.py program should work again. While you're at it, fix up your pcost.py program to use the new Portfolio object. Like this:

# pcost.py

import report

def portfolio_cost(filename):
    '''
    Computes the total cost (shares*price) of a portfolio file
    '''
    portfolio = report.read_portfolio(filename)
    return portfolio.total_cost
...

Test it to make sure it works:

>>> import pcost
>>> pcost.portfolio_cost('Data/portfolio.csv')
44671.15
>>>

关联来源

• summaries/01_Iteration_protocol