---
id: practical-python-7.11
source_exercise_id: "7.11"
title: "Class Methods in Practice"
section: "7.5 Decorated Methods"
source_path: "07_Advanced_Topics/05_Decorated_methods.md"
source_repo: "https://github.com/dabeaz-course/practical-python"
source_commit: "93dca856b41c61a0a0f85ae334116e4c125629ea"
student_visible_solution: false
has_private_solution: true
skip: false
---

# Exercise 7.11: Class Methods in Practice

> Source: Practical Python Programming, `07_Advanced_Topics/05_Decorated_methods.md`.

### Exercise 7.11: Class Methods in Practice

In your `report.py` and `portfolio.py` files, the creation of a `Portfolio`
object is a bit muddled.  For example, the `report.py` program has code like this:

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

    portfolio = [ Stock(**d) for d in portdicts ]
    return Portfolio(portfolio)
```

and the `portfolio.py` file defines `Portfolio()` with an odd initializer
like this:

```python
class Portfolio:
    def __init__(self, holdings):
        self.holdings = holdings
    ...
```

Frankly, the chain of responsibility is all a bit confusing because the
code is scattered.    If a `Portfolio` class is supposed to contain
a list of `Stock` instances, maybe you should change the class to be a bit more clear.
Like this:

```python
# portfolio.py

import stock

class Portfolio:
    def __init__(self):
        self.holdings = []

    def append(self, holding):
        if not isinstance(holding, stock.Stock):
            raise TypeError('Expected a Stock instance')
        self.holdings.append(holding)
    ...
```

If you want to read a portfolio from a CSV file, maybe you should make a
class method for it:

```python
# portfolio.py

import fileparse
import stock

class Portfolio:
    def __init__(self):
        self.holdings = []

    def append(self, holding):
        if not isinstance(holding, stock.Stock):
            raise TypeError('Expected a Stock instance')
        self.holdings.append(holding)

    @classmethod
    def from_csv(cls, lines, **opts):
        self = cls()
        portdicts = fileparse.parse_csv(lines,
                                        select=['name','shares','price'],
                                        types=[str,int,float],
                                        **opts)

        for d in portdicts:
            self.append(stock.Stock(**d))

        return self
```

To use this new Portfolio class, you can now write code like this:

```
>>> from portfolio import Portfolio
>>> with open('Data/portfolio.csv') as lines:
...     port = Portfolio.from_csv(lines)
...
>>>
```

Make these changes to the `Portfolio` class and modify the `report.py`
code to use the class method.

[Contents](../Contents.md) \| [Previous (7.4 Decorators)](04_Function_decorators.md) \| [Next (8 Testing and Debugging)](../08_Testing_debugging/00_Overview.md)

## 关联来源

- [[summaries/05_Decorated_methods]]