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Exercises/ex6_3.md

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+\[ [Index](index.md) | [Exercise 6.2](ex6_2.md) | [Exercise 6.4](ex6_4.md) \]
+
+# 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 link:ex6_1.html[Exercise 6.1], 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 link:ex6_2.html[Exercise 6.2], 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.
+
+\[ [Solution](soln6_3.md) | [Index](index.md) | [Exercise 6.2](ex6_2.md) | [Exercise 6.4](ex6_4.md) \]
+
+----
+`>>>` Advanced Python Mastery  
+`...` A course by [dabeaz](https://www.dabeaz.com)  
+`...` Copyright 2007-2023  
+
+![](https://i.creativecommons.org/l/by-sa/4.0/88x31.png). This work is licensed under a [Creative Commons Attribution-ShareAlike 4.0 International License](http://creativecommons.org/licenses/by-sa/4.0/)