python-mastery/Exercises/ex6_3.md

\[ [Index](index.md) | [Exercise 6.2](ex6_2.md) | [Exercise 6.4](ex6_4.md) \]

# Exercise 6.3

*Objectives:*

- Learn how to inspect the internals of functions

*Files Modified:* `structure.py`

## (a) Inspecting functions

Define a simple function:

```python
>>> def add(x,y):
       'Adds two things'
       return x+y

>>>
```

Do a `dir()` on the function to look at its attributes.

```python
>>> dir(add)
... look at the result ...
>>>
```

Get some basic information such as the function name, defining module name, and documentation string.

```python
>>> add.__name__
'add'
>>> add.__module__
'__main__'
>>> add.__doc__
'Adds two things'
>>>
```

The `__code__` attribute of a function has low-level information about
the function implementation.  See if you can look at this and
determine the number of required arguments and names of local
variables.

## (b) Using the inspect module

Use the inspect module to get calling information about the function:

```python
>>> import inspect
>>> sig = inspect.signature(add)
>>> sig
<Signature (x, y)>
>>> sig.parameters
mappingproxy(OrderedDict([('x', <Parameter "x">), ('y', <Parameter "y">)]))
>>> tuple(sig.parameters)
('x', 'y')
>>>
```

## (c) Putting it Together

In [Exercise 6.1](ex6_1.md), you created a class `Structure`
that defined a generalized `__init__()`, `__setattr__()`, and `__repr__()`
method.   That class required a user to define a `_fields` class
variable like this:

```python
class Stock(Structure):
    _fields = ('name','shares','price')
```

The problem with this class is that the `__init__()` function didn't
have a useful argument signature for the purposes of help and
keyword argument passing.  In [Exercise 6.2](ex6_2.md), you 
did a sneaky trick involving a special `self._init()` function. For example:

```python
class Stock(Structure):
    _fields = ('name', 'shares', 'price')
    def __init__(self, name, shares, price):
        self._init()
    ...
```

This gave a useful signature, but now the class is just weird because
the user has to provide both the `_fields` variable and the `__init__()` method.

Your task is to eliminate the `_fields` variable using some function
inspection techniques.  First, notice that you can get the argument
signature from `Stock` as follows:

```python
>>> import inspect
>>> sig = inspect.signature(Stock)
>>> tuple(sig.parameters)
('name', 'shares', 'price')
>>>
```

Perhaps you could set the `_fields` variable from the argument signature
of `__init__()`.   Add a class method `set_fields(cls)` to `Structure` that
inspects the `__init__()` function, and sets the `_fields`
variable appropriately.   You should use your new function like this:

```python
class Stock(Structure):
    def __init__(self, name, shares, price):
        self._init()

    ...

Stock.set_fields()
```

The resulting class should work the same way as before:

```python
>>> s = Stock(name='GOOG', shares=100, price=490.1)
>>> s
Stock('GOOG',100,490.1)
>>> s.shares = 50
>>> s.share = 50
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "structure.py", line 12, in __setattr__
    raise AttributeError('No attribute %s' % name)
AttributeError: No attribute share
>>> 
```

Verify the slightly modified `Stock` class with your unit tests again.  There will still
be failures, but nothing should change from the previous exercise.

At this point, it's all still a bit "hacky", but you're making 
progress.  You have a Stock structure class with a useful `__init__()`
function, there is a useful representation string, and the
`__setattr__()` method restricts the set of attribute names.   The
extra step of having to invoke `set_fields()` is a bit odd, but we'll
get back to that.

\[ [Solution](soln6_3.md) | [Index](index.md) | [Exercise 6.2](ex6_2.md) | [Exercise 6.4](ex6_4.md) \]

----
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