Expert Python Programming - Third Edition

Public and private variables

For global variables that are mutable and freely available through imports, a lowercase letter with an underscore should be used when they do not need to be protected. If a variable shouldn't be used and modified outside of its origin module we consider it a private member of that module. A leading underscore, in that case, can mark the variable as a private element of the package, as shown in the following code:

_observers = []def add_observer(observer):    _observers.append(observer)def get_observers():    """Makes sure _observers cannot be modified."""    return tuple(_observers)

Variables that are located in functions, and methods, follow the same rules as public variables and are never marked as private since they ...