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+\[ [Index](index.md) | [Exercise 3.8](ex3_8.md) | [Exercise 4.2](ex4_2.md) \]
+
+# Exercise 4.1
+
+*Objectives:*
+
+- Learn more about how objects are represented.
+- Learn how attribute assignment and lookup works.
+- Better understand the role of a class definition
+
+*Files Created:* None
+
+*Files Modified:* None
+
+Start this exercise, by going back to a simple version of the `Stock` class you created. 
+At the interactive prompt, define a 
+new class called `SimpleStock` that looks like this:
+
+```python
+>>> class SimpleStock:
+        def __init__(self, name, shares, price):
+            self.name = name
+            self.shares = shares
+            self.price = price
+        def cost(self):
+            return self.shares * self.price
+
+>>>
+```
+
+Once you have defined this class, create a few instances.
+
+```python
+>>> goog = SimpleStock('GOOG',100,490.10)
+>>> ibm  = SimpleStock('IBM',50, 91.23)
+>>>
+```
+
+## (a) Representation of Instances
+
+At the interactive shell, inspect the underlying dictionaries of the two instances you created:
+
+```python
+>>> goog.__dict__
+... look at the output ...
+>>> ibm.__dict__
+... look at the output ...
+>>>
+```
+
+## (b) Modification of Instance Data
+
+Try setting a new attribute on one of the above instances:
+
+```python
+>>> goog.date = "6/11/2007"
+>>> goog.__dict__
+... look at output ...
+>>> ibm.__dict__
+... look at output ...
+>>>
+```
+
+In the above output, you'll notice that the `goog` instance has
+a attribute `date` whereas the `ibm` instance does not.
+It is important to note that Python really doesn't place any
+restrictions on attributes.  For example, the attributes of an
+instance are not limited to those set up in the `__init__()`
+method.
+
+Instead of setting an attribute, try placing a new value directly into
+the `__dict__` object:
+
+```python
+>>> goog.__dict__['time'] = '9:45am'
+>>> goog.time
+'9:45am'
+>>>
+```
+
+Here, you really notice the fact that an instance is a layer on top of a dictionary.
+
+
+## (c) The role of classes
+
+The definitions that make up a class definition are shared by all
+instances of that class.  Notice, that all instances have a link back
+to their associated class:
+
+```python
+>>> goog.__class__
+... look at output ...
+>>> ibm.__class__
+... look at output ...
+>>> 
+```
+
+Try calling a method on the instances:
+
+```python
+>>> goog.cost()
+49010.0
+>>> ibm.cost()
+4561.5
+>>>
+```
+
+Notice that the name 'cost' is not defined in either `goog.__dict__` or `ibm.__dict__`.   Instead, it is being supplied by the
+class dictionary.  Try this:
+
+```python
+>>> SimpleStock.__dict__['cost']
+... look at output ...
+>>>
+```
+
+Try calling the `cost()` method directly through the dictionary:
+
+```python
+>>> SimpleStock.__dict__['cost'](goog)
+49010.00
+>>> SimpleStock.__dict__['cost'](ibm)
+4561.5
+>>>
+```
+
+Notice how you are calling the function defined in the class definition and how the `self` argument
+gets the instance.
+
+If you add a new value to the class, it becomes a class variable that's visible to all instances.  Try it:
+
+```python
+>>> SimpleStock.spam = 42
+>>> ibm.spam
+42
+>>> goog.spam
+42
+>>>
+```
+
+Observe that `spam` is not part of the instance dictionary.
+
+```python
+>>> ibm.__dict__
+... look at the output ...
+>>>
+```
+
+Instead, it's part of the class dictionary:
+
+```python
+>>> SimpleStock.__dict__['spam']
+42
+>>>
+```
+
+Essentially this is all a class really is--it's a collection of values shared by instances.
+
+
+\[ [Solution](soln4_1.md) | [Index](index.md) | [Exercise 3.8](ex3_8.md) | [Exercise 4.2](ex4_2.md) \]
+
+----
+`>>>` 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/)