python-mastery/Exercises/ex3_6.md

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# Exercise 3.6

*Objectives:*

- Learn how to customize the behavior of objects by redefining special methods.
- Change the way that user-defined objects get printed
- Make objects comparable
- Create a context manager

*Files Created:* None

*Files Modified:* `stock.py`

## (a) Better output for representing objects

All Python objects have two string representations.  The first
representation is created by string conversion via `str()`
(which is called by `print`).   The string representation is
usually a nicely formatted version of the object meant for humans.
The second representation is a code representation of the object
created by `repr()` (or simply by viewing a value in the
interactive shell).   The code representation typically shows you the
code that you have to type to get the object.  Here is an example
that illustrates using dates:

```python
>>> from datetime import date
>>> d = date(2008, 7, 5)
>>> print(d)              # uses str()
2008-07-05
>>> d    # uses repr()
datetime.date(2008, 7, 5)
>>>
```

There are several techniques for obtaining the `repr()` string
in output:

```python
>>> print('The date is', repr(d))
The date is datetime.date(2008, 7, 5)
>>> print(f'The date is {d!r}')
The date is datetime.date(2008, 7, 5)
>>> print('The date is %r' % d)
The date is datetime.date(2008, 7, 5)
>>> 
```

Modify the `Stock` object that you've created so that
the `__repr__()` method
produces more useful output.  For example:

```python
>>> goog = Stock('GOOG', 100, 490.10)
>>> goog
Stock('GOOG', 100, 490.1)
>>>
```

See what happens when you read a portfolio of stocks and view the
resulting list after you have made these changes.  For example:

```python
>>> import stock, reader
>>> portfolio = reader.read_csv_as_instances('Data/portfolio.csv', stock.Stock)
>>> portfolio
[Stock('AA', 100, 32.2), Stock('IBM', 50, 91.1), Stock('CAT', 150, 83.44), Stock('MSFT', 200, 51.23), 
 Stock('GE', 95, 40.37), Stock('MSFT', 50, 65.1), Stock('IBM', 100, 70.44)]
>>>
```

## (b) Making objects comparable

What happens if you create two identical `Stock` objects and try to compare them? Find out:

```python
>>> a = Stock('GOOG', 100, 490.1)
>>> b = Stock('GOOG', 100, 490.1)
>>> a == b
False
>>>
```

You can fix this by giving the `Stock` class an `__eq__()` method.  For example:

```python
class Stock:
    ...
    def __eq__(self, other):
        return isinstance(other, Stock) and ((self.name, self.shares, self.price) == 
                                             (other.name, other.shares, other.price))
    ...
```

Make this change and try comparing two objects again. 

## (c) A Context Manager

In [Exercise 3.5](ex3_5.md), you made it possible for users to make
nicely formatted tables.  For example:

```python
>>> from tableformat import create_formatter
>>> formatter = create_formatter('text')
>>> print_table(portfolio, ['name','shares','price'], formatter)
      name     shares      price
---------- ---------- ---------- 
        AA        100       32.2
       IBM         50       91.1
       CAT        150      83.44
      MSFT        200      51.23
        GE         95      40.37
      MSFT         50       65.1
       IBM        100      70.44
>>>
```

One issue with the code is that all tables are printed to standard out
(`sys.stdout`).  Suppose you wanted to redirect the output to a file
or some other location.  In the big picture, you might modify all of
the table formatting code to allow a different output file.  However,
in a pinch, you could also solve this with a context manager.

Define the following context manager:

```python
>>> import sys
>>> class redirect_stdout:
        def __init__(self, out_file):
            self.out_file = out_file
        def __enter__(self):
            self.stdout = sys.stdout
            sys.stdout = self.out_file
            return self.out_file
        def __exit__(self, ty, val, tb):
            sys.stdout = self.stdout
```

This context manager works by making a temporary patch to `sys.stdout` to cause
all output to redirect to a different file.  On exit, the patch is reverted. 
Try it out:

```python
>>> from tableformat import create_formatter
>>> formatter = create_formatter('text')
>>> with redirect_stdout(open('out.txt', 'w')) as file:
        tableformat.print_table(portfolio, ['name','shares','price', formatter)
        file.close()

>>> # Inspect the file
>>> print(open('out.txt').read())
      name     shares      price
---------- ---------- ---------- 
        AA        100       32.2
       IBM         50       91.1
       CAT        150      83.44
      MSFT        200      51.23
        GE         95      40.37
      MSFT         50       65.1
       IBM        100      70.44
>>>
```

\[ [Solution](soln3_6.md) | [Index](index.md) | [Exercise 3.5](ex3_5.md) | [Exercise 3.7](ex3_7.md) \]

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