A Python 2.7 programming tutorial |  |
A Python 2.7 programming tutorial | |
To really understand what is going on inside a running
Python program, let's follow the creation of an instance
of the SnailRun class from the preceding
section.
Just for review, let's assume you are using conversational mode, and you create a variable like this:
>>> badPi = 3.00
Whenever you start up Python, it creates the “global namespace” to hold the names and values you define. After the statement above, here's how it looks.
Next, suppose you type in the class definition as above.
As it happens, a class is a namespace too—it is a
container for methods. So the global namespace now
has two names in it: the variable badPi
and the class SnailRun. Here is a picture
of the world after you define the class:
Next, create an instance of class SnailRun
like this:
>>> j1 = SnailRun ( 'Judy', 87.3 )
Here is the sequence of operations:
Python creates a new instance namespace. This
namespace is initially a copy of the
class's namespace: it contains the two
methods .__init__() and .show().
The constructor method starts execution with these arguments:
The name self is bound to the
instance namespace.
The name snailName is bound to the
string value 'Judy'.
The name finishTime is bound to
the float value 87.3.
This statement in the constructor
self.name = snailName
creates a new
attribute .name in the instance
namespace, and assigns it the value 'Judy'.
The next statement in the constructor creates an
attribute named .time in the instance
namespace, and binds it to the value 87.3.
The constructor completes, and back in conversational
mode, in the global namespace, variable j1 is bound to the instance namespace.
Here's a picture of the world after all this:
