Plexnet gadgets - espians

Plexnet gadgets

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This page documents bits of interesting, mainly Python, code that have been produced in coming up with Espra alpha 1.

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Chainable Stateful Coroutines (disco)

These are basically generators that have internal state. They act like miniature classes in a way. 

@worker()
def cat(self):	 
   """A demonstration ``cat`` disco worker.
      Yields out what it gets in.
   """
   while True: 
      value = yield self.get()
      yield value

As you can see, cat has a simple state storage ("self"), an input interface ("yield self.get()"), and an output interface ("yield value"). The idea is that you can chain the coroutines together, like in unix piping: 

counter(2, 10) | cat() | square() | sum()
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Capabilities (capabilities)

How do you restrict the user's environment so that they can't write to the filesystem? Tav came up with a system that didn't quite work, so he asked on python-dev and got given a beautiful recipe that he was able to hash out into a working secure system. Obviously the only way to test such a thing is to try to get people to crack it; but so far it's proving impregnable.

The problem with passing open('test.txt', 'r') around is... 

# First system opens a file, and passes it to another system
f = open('test.txt', 'r')
# Second system can reopen the file in write mode!
type(f)('test.txt', 'w')

Even with a closure-based system you can get the file type by inspecting func_closure[0].cell_contents. So Tav used the recipe to come up with safe.py. You do "from safe import FileReader", and your environment can no longer write to the disc. 

>>> from safe import FileReader
>>> f = FileReader('test.txt')
>>> f.getter.func_closure
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'function' object has no attribute 'func_closure'
>>> license
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.5/site.py", line 291, in __repr__
    self.__setup()
  File "/usr/local/lib/python2.5/site.py", line 277, in __setup
    fp = file(filename, "rU")
NameError: global name 'file' is not defined
>>>

Note that this merely demonstrates the core capability. Obviously it's not a good thing to lack even the ability to import all the time, so this is useful for when you want to make sure that code is executed in a restricted environment—when it's been passed, for example, from an external unverifiably secure source.

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Custom Importing (pimp)

Not sure what the benefits of pimp are.

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