---
id: practical-python-4.1
source_exercise_id: "4.1"
title: "Objects as Data Structures"
section: "4.1 Classes"
source_path: "04_Classes_objects/01_Class.md"
source_repo: "https://github.com/dabeaz-course/practical-python"
source_commit: "93dca856b41c61a0a0f85ae334116e4c125629ea"
student_visible_solution: false
has_private_solution: false
skip: false
---

# Exercise 4.1: Objects as Data Structures

> Source: Practical Python Programming, `04_Classes_objects/01_Class.md`.

### Exercise 4.1: Objects as Data Structures

In section 2 and 3, we worked with data represented as tuples and
dictionaries.  For example, a holding of stock could be represented as
a tuple like this:

```python
s = ('GOOG',100,490.10)
```

or as a dictionary like this:

```python
s = { 'name'   : 'GOOG',
      'shares' : 100,
      'price'  : 490.10
}
```

You can even write functions for manipulating such data.  For example:

```python
def cost(s):
    return s['shares'] * s['price']
```

However, as your program gets large, you might want to create a better
sense of organization.  Thus, another approach for representing data
would be to define a class.  Create a file called `stock.py` and
define a class `Stock` that represents a single holding of stock.
Have the instances of `Stock` have `name`, `shares`, and `price`
attributes.  For example:

```python
>>> import stock
>>> a = stock.Stock('GOOG',100,490.10)
>>> a.name
'GOOG'
>>> a.shares
100
>>> a.price
490.1
>>>
```

Create a few more `Stock` objects and manipulate them.  For example:

```python
>>> b = stock.Stock('AAPL', 50, 122.34)
>>> c = stock.Stock('IBM', 75, 91.75)
>>> b.shares * b.price
6117.0
>>> c.shares * c.price
6881.25
>>> stocks = [a, b, c]
>>> stocks
[<stock.Stock object at 0x37d0b0>, <stock.Stock object at 0x37d110>, <stock.Stock object at 0x37d050>]
>>> for s in stocks:
     print(f'{s.name:>10s} {s.shares:>10d} {s.price:>10.2f}')

... look at the output ...
>>>
```

One thing to emphasize here is that the class `Stock` acts like a
factory for creating instances of objects.  Basically, you call
it as a function and it creates a new object for you.  Also, it must
be emphasized that each object is distinct---they each have their
own data that is separate from other objects that have been created.

An object defined by a class is somewhat similar to a dictionary--just
with somewhat different syntax.  For example, instead of writing
`s['name']` or `s['price']`, you now write `s.name` and `s.price`.

## 关联来源

- [[summaries/01_Class]]