Bastion

Bastionification utility.

A bastion (for another object – the ‘original’) is an object that has the same methods as the original but does not give access to its instance variables. Bastions have a number of uses, but the most obvious one is to provide code executing in restricted mode with a safe interface to an object implemented in unrestricted mode.

The bastionification routine has an optional second argument which is a filter function. Only those methods for which the filter method (called with the method name as argument) returns true are accessible. The default filter method returns true unless the method name begins with an underscore.

There are a number of possible implementations of bastions. We use a ‘lazy’ approach where the bastion’s __getattr__() discipline does all the work for a particular method the first time it is used. This is usually fastest, especially if the user doesn’t call all available methods. The retrieved methods are stored as instance variables of the bastion, so the overhead is only occurred on the first use of each method.

Detail: the bastion class has a __repr__() discipline which includes the repr() of the original object. This is precomputed when the bastion is created.

Functions

Bastion(object[, filter, name, bastionclass])

Create a bastion for an object, using an optional filter.

Classes

BastionClass(get, name)	Helper class used by the Bastion() function.
MethodType	alias of instancemethod