---
id: practical-python-4.7
source_exercise_id: "4.7"
title: "Polymorphism in Action"
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 4.7: Polymorphism in Action

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

### Exercise 4.7: Polymorphism in Action

A major feature of object-oriented programming is that you can
plug an object into a program and it will work without having to
change any of the existing code.  For example, if you wrote a program
that expected to use a `TableFormatter` object, it would work no
matter what kind of `TableFormatter` you actually gave it.  This
behavior is sometimes referred to as "polymorphism."

One potential problem is figuring out how to allow a user to pick out
the formatter that they want.  Direct use of the class names such as
`TextTableFormatter` is often annoying.  Thus, you might consider some
simplified approach.  Perhaps you embed an `if-`statement into the
code like this:

```python
def portfolio_report(portfoliofile, pricefile, fmt='txt'):
    '''
    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
    if fmt == 'txt':
        formatter = tableformat.TextTableFormatter()
    elif fmt == 'csv':
        formatter = tableformat.CSVTableFormatter()
    elif fmt == 'html':
        formatter = tableformat.HTMLTableFormatter()
    else:
        raise RuntimeError(f'Unknown format {fmt}')
    print_report(report, formatter)
```

In this code, the user specifies a simplified name such as `'txt'` or
`'csv'` to pick a format.  However, is putting a big `if`-statement in
the `portfolio_report()` function like that the best idea?  It might
be better to move that code to a general purpose function somewhere
else.

In the `tableformat.py` file, add a function `create_formatter(name)`
that allows a user to create a formatter given an output name such as
`'txt'`, `'csv'`, or `'html'`.  Modify `portfolio_report()` so that it
looks like this:

```python
def portfolio_report(portfoliofile, pricefile, fmt='txt'):
    '''
    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.create_formatter(fmt)
    print_report(report, formatter)
```

Try calling the function with different formats to make sure it's working.

## 关联来源

- [[summaries/02_Inheritance]]