Memory diagram

To understand the bat snatching bugs you will need to understand how the program objects are related in memory. Hopefully this diagram will help with that.

Many lines have been left off the diagram to keep it readable, but at least one connection of each type is shown.

It is important to understand the difference between the solid lines and the dotted lines. The solid lines represent a standard Python object reference where a name directly refers to an object. The dotted lines show where it is possible to make an indirect reference by using a list index number, i.e. to use the number to access the object by using the number as an index into the list cave_system.rooms. In both cases the object at the end of the arrow can be referenced, the difference is whether it requires a subscript to do it or not.

The symbol, ⏚, is used to signify None, i.e. that the object reference is undefined.

One striking feature of the diagram is that only two objects in it have names, cave_system and player, and all the others are referenced through cave_system. That is one reason it is forgivable to have cave_system as a global variable in our program: it is effectively the single point of access for all the objects.