---
id: practical-python-4.5
source_exercise_id: "4.5"
title: "An Extensibility Problem"
section: "4.2 Inheritance"
source_path: "04_Classes_objects/02_Inheritance.md"
source_repo: "https://github.com/dabeaz-course/practical-python"
source_commit: "93dca856b41c61a0a0f85ae334116e4c125629ea"
student_visible_solution: false
has_private_solution: false
skip: false
---

# Exercise 4.5: An Extensibility Problem

> Source: Practical Python Programming, `04_Classes_objects/02_Inheritance.md`.

### Exercise 4.5: An Extensibility Problem

Suppose that you wanted to modify the `print_report()` function to
support a variety of different output formats such as plain-text,
HTML, CSV, or XML.  To do this, you could try to write one gigantic
function that did everything.  However, doing so would likely lead to
an unmaintainable mess.  Instead, this is a perfect opportunity to use
inheritance instead.

To start, focus on the steps that are involved in a creating a table.
At the top of the table is a set of table headers.  After that, rows
of table data appear.  Let's take those steps and put them into
their own class.  Create a file called `tableformat.py` and define the
following class:

```python
# tableformat.py

class TableFormatter:
    def headings(self, headers):
        '''
        Emit the table headings.
        '''
        raise NotImplementedError()

    def row(self, rowdata):
        '''
        Emit a single row of table data.
        '''
        raise NotImplementedError()
```

This class does nothing, but it serves as a kind of design specification for
additional classes that will be defined shortly.  A class like this is
sometimes called an "abstract base class."

Modify the `print_report()` function so that it accepts a
`TableFormatter` object as input and invokes methods on it to produce
the output.  For example, like this:

```python
# report.py
...

def print_report(reportdata, formatter):
    '''
    Print a nicely formatted table from a list of (name, shares, price, change) tuples.
    '''
    formatter.headings(['Name','Shares','Price','Change'])
    for name, shares, price, change in reportdata:
        rowdata = [ name, str(shares), f'{price:0.2f}', f'{change:0.2f}' ]
        formatter.row(rowdata)
```

Since you added an argument to print_report(), you're going to need to modify the
`portfolio_report()` function as well.  Change it so that it creates a `TableFormatter`
like this:

```python
# report.py

import tableformat

...
def portfolio_report(portfoliofile, pricefile):
    '''
    Make a stock report given portfolio and price data files.
    '''
    # Read data files
    portfolio = read_portfolio(portfoliofile)
    prices = read_prices(pricefile)

    # Create the report data
    report = make_report_data(portfolio, prices)

    # Print it out
    formatter = tableformat.TableFormatter()
    print_report(report, formatter)
```

Run this new code:

```python
>>> ================================ RESTART ================================
>>> import report
>>> report.portfolio_report('Data/portfolio.csv', 'Data/prices.csv')
... crashes ...
```

It should immediately crash with a `NotImplementedError` exception.  That's not
too exciting, but it's exactly what we expected.  Continue to the next part.

## 关联来源

- [[summaries/02_Inheritance]]