coding-mentor-agent/kb/python-course-kb-practical-python/wiki/exercises/5-3-the-role-of-classes.md

id, source_exercise_id, title, section, source_path, source_repo, source_commit, student_visible_solution, has_private_solution, skip

id	source_exercise_id	title	section	source_path	source_repo	source_commit	student_visible_solution	has_private_solution	skip
practical-python-5.3	5.3	The role of classes	5.1 Dictionaries Revisited	05_Object_model/01_Dicts_revisited.md	https://github.com/dabeaz-course/practical-python	93dca856b41c61a0a0f85ae334116e4c125629ea	false	false	false

Exercise 5.3: The role of classes

Source: Practical Python Programming, 05_Object_model/01_Dicts_revisited.md.

Exercise 5.3: 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:

>>> goog.__class__
... look at output ...
>>> ibm.__class__
... look at output ...
>>>

Try calling a method on the instances:

>>> 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:

>>> Stock.__dict__['cost']
... look at output ...
>>>

Try calling the cost() method directly through the dictionary:

>>> Stock.__dict__['cost'](goog)
49010.0
>>> Stock.__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.

Try adding a new attribute to the Stock class:

>>> Stock.foo = 42
>>>

Notice how this new attribute now shows up on all of the instances:

>>> goog.foo
42
>>> ibm.foo
42
>>>

However, notice that it is not part of the instance dictionary:

>>> goog.__dict__
... look at output and notice there is no 'foo' attribute ...
>>>

The reason you can access the foo attribute on instances is that Python always checks the class dictionary if it can't find something on the instance itself.

Note: This part of the exercise illustrates something known as a class variable. Suppose, for instance, you have a class like this:

class Foo(object):
     a = 13                  # Class variable
     def __init__(self,b):
         self.b = b          # Instance variable

In this class, the variable a, assigned in the body of the class itself, is a "class variable." It is shared by all of the instances that get created. For example:

>>> f = Foo(10)
>>> g = Foo(20)
>>> f.a          # Inspect the class variable (same for both instances)
13
>>> g.a
13
>>> f.b          # Inspect the instance variable (differs)
10
>>> g.b
20
>>> Foo.a = 42   # Change the value of the class variable
>>> f.a
42
>>> g.a
42
>>>

关联来源

• summaries/01_Dicts_revisited