coding-mentor-agent/kb/python-course-kb-practical-python/wiki/exercises/4-7-polymorphism-in-action.md

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id	source_exercise_id	title	section	source_path	source_repo	source_commit	student_visible_solution	has_private_solution	skip
practical-python-4.7	4.7	Polymorphism in Action	4.2 Inheritance	04_Classes_objects/02_Inheritance.md	https://github.com/dabeaz-course/practical-python	93dca856b41c61a0a0f85ae334116e4c125629ea	false	false	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:

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:

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