Pretty often SQL is used to calculate different metrics or to aggregate values and measurements. Alongside with aggregating functions there are windows functions, which are used widely. 

weakref module allows to create "weak" references to objects. It's not enough to have a weak reference to keep an objects alive - if weak references are the only references the object has, then garbage collector will remove the object and reuse the memory for other objects. However, while the object isn't deleted, a weak reference can return it even if there are no regular references to the objects.

Python is a dynamic programming language. Here you're allowed to do whatever you wish with objects and their instances - you can add attributes, remove them etc.