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
Detail: the bastion class has a __repr__() discipline which includes
the repr() of the original object. This is precomputed when the
bastion is created.
||Helper class used by the Bastion() function.|
You could subclass this and pass the subclass as the bastionclass
argument to the Bastion() function, as long as the constructor has
the same signature (a get() function and a name for the object).
||Methods defined here:|
- __getattr__(self, name)
- Get an as-yet undefined attribute value.
This calls the get() function that was passed to the
constructor. The result is stored as an instance variable so
that the next time the same attribute is requested,
__getattr__() won't be invoked.
If the get() function raises an exception, this is simply
passed on -- exceptions are not cached.
- __init__(self, get, name)
get - a function that gets the attribute value (by name)
name - a human-readable name for the original object
(suggestion: use repr(object))
- Return a representation string.
This includes the name passed in to the constructor, so that
if you print the bastion during debugging, at least you have
some idea of what it is.
- Bastion(object, filter=<function <lambda>>, name=None, bastionclass=<class Bastion.BastionClass>)
- Create a bastion for an object, using an optional filter.
See the Bastion module's documentation for background.
object - the original object
filter - a predicate that decides whether a function name is OK;
by default all names are OK that don't start with '_'
name - the name of the object; default repr(object)
bastionclass - class used to create the bastion; default BastionClass
||__all__ = ['BastionClass', 'Bastion']|