The Bastion Module

The Bastion module, shown in Example 14-7, allows you to control how a given object is used. It can be used to pass objects from unrestricted parts of your application to code running in restricted mode.

To create a restricted instance, simply call the Bastion wrapper. By default, all instance variables are hidden, as well as all methods that start with an underscore.

Example 14-7. Using the Bastion Module

File: bastion-example-1.py import Bastion class Sample: value = 0 def _set(self, value): self.value = value def setvalue(self, value): if 10 < value <= 20: self._set(value) else: raise ValueError, "illegal value" def getvalue(self): return self.value # # try it s = Sample() s._set(100) # cheat print s.getvalue() s = Bastion.Bastion(Sample()) s._set(100) # attempt to cheat print s.getvalue() 100 Traceback (innermost last): ... AttributeError: _set

You can control which functions to publish. In Example 14-8, the internal method can be called from outside, but the getvalue no longer works.

Example 14-8. Using the Bastion Module with a Non-Standard Filter

File: bastion-example-2.py import Bastion class Sample: value = 0 def _set(self, value): self.value = value def setvalue(self, value): if 10 < value <= 20: self._set(value) else: raise ValueError, "illegal value" def getvalue(self): return self.value # # try it def is_public(name): return name[:3] != "get" s = Bastion.Bastion(Sample(), is_public) s._set(100) # this works print s.getvalue() # but not this 100 Traceback (innermost last): ... AttributeError: getvalue