Class Get Attribute Property

When it is time to get the object's attribute, LW Python looks in the "Values" stack to see where the object is in the "Object Store." Then it looks for the attribute in the object and returns the corresponding value. More formally,

For example, suppose the following are true:

• expression state at time 67 = "call_function_begin"
• the expression at time 67 = self.age
• Value Stack at time 67 = [ [ Python reference 0, [ ] ], [ [ ], [ ] ] ]
• Python Object Store at time 67 = [ Python object: [ entry "__class_name__": "Person", [ entry "first_name": "John", [ entry "last_name": "Smith", [ entry "age": 25, [ ] ] ] ] ], [ ] ]

Here is a table view of the "Object Store:"

Index	Type	Contents
0	class instance	__class_name__	Person
		first_name	John
		last_name	Smith
		age	25

Then we claim that

the element at index 0 of stack (Python Object Store at time 67) = Python object: [ entry "__class_name__": "Person", [ entry "first_name": "John", [ entry "last_name": "Smith", [ entry "age": 25, [ ] ] ] ] ]

Here is a table view of the object at index 0:

Attribute	Value
__class_name__	Person
first_name	John
last_name	Smith
age	25

Then we conclude that:

Return Value at time (67 + 1) = value at "age" in map [ entry "__class_name__": "Person", [ entry "first_name": "John", [ entry "last_name": "Smith", [ entry "age": 25, [ ] ] ] ] ]

We claim that:

value at "age" in map [ entry "__class_name__": "Person", [ entry "first_name": "John", [ entry "last_name": "Smith", [ entry "age": 25, [ ] ] ] ] ] = 25

We conclude that:

Return Value at time (67 + 1) = 25

Which simplifies to:

Return Value at time 68 = 25

See here for the full proof.

The object is in index 0 of the "Object Store" and the "age" in this object is 25. Try stepping through the simulator to see LW Python set the return value.

Comments

Please log in to add comments