Importing Python packages in-memory

Student Robotics needs to import python packages written by the students into a simulator. Currently the simulator downloads a ZIP of the python package to the temp directory, then uses the inherent ability of python to transparently treat a zip file as a package to import the code. This has caused us problems, mainly as some schools don't allow students to write to %TEMP%!!!

We are now planning to run the simulator as a web service - Students will save their code in an online IDE and then request a simulation to be run. A physics engine will be plugged in, and some nifty Javascript used to draw a little virtual robot driving around a virtual arena. This means that we (Student Robotics) are running untrusted code on our servers by design. In order to carefully manage our sanity we will be running the students code in its own process as a severely crippled user. This includes banning writing to disk.

We can't use the standard python zip imports as the zipfile module only supports opening files directly off disk. I have implemented some import hooks to add the ability to import files from a zip file held in memory. Exceptions aren't handled, that's up to the caller.

EDIT: Compile before exec so I can set the filename of the code object correctly.

import zipfile, StringIO, imp, sys, os.path

f = StringIO.StringIO()
f.write(open("", "rb").read())

zip = zipfile.ZipFile(f)

modules = dict([(os.path.splitext(z.filename)[0],
                    for z in zip. infolist() \
                    if os.path.splitext(z.filename)[1] == ".py"])

class Loader:
    def __init__(self, fullname, contents):
        self.fullname = fullname
        self.contents = contents

    def load_module(self, fullname):
        if fullname in sys.modules:
            return sys.modules[fullname]

        mod = sys.modules.setdefault(fullname, imp.new_module(fullname))
        mod.__file__ = "<%s>" % fullname
        mod.__loader__ = self

        code = compile(self.contents, mod.__file__, "exec")

        exec code in mod.__dict__
        return mod

class Finder:
    def __init__(self, modules):
        self.modules = modules

    def find_module(self, fullname, path=None):
        if (fullname in self.modules) and (path == None):
            return Loader(fullname, self.modules[fullname])

        return None


def trace(frame, event, arg):
    if event == "call":
        print frame.f_code.co_filename
    return None


print "**************************"

import robot

f = robot.Face()
Posted on 04 May 2009
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