earwigbot/earwigbot/lazy.py

# -*- coding: utf-8  -*-
#
# Copyright (C) 2009-2014 Ben Kurtovic <ben.kurtovic@gmail.com>
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.

"""
Implements a hierarchy of importing classes as defined in `PEP 302
<http://www.python.org/dev/peps/pep-0302/>`_ to load modules in a safe yet lazy
manner, so that they can be referred to by name but are not actually loaded
until they are used (i.e. their attributes are read or modified).
"""

from imp import acquire_lock, release_lock
import sys
from types import ModuleType

__all__ = ["LazyImporter"]

def _getattribute(self, attr):
    _load(self)
    return self.__getattribute__(attr)

def _setattr(self, attr, value):
    _load(self)
    self.__setattr__(attr, value)

def _load(self):
    type(self).__getattribute__ = ModuleType.__getattribute__
    type(self).__setattr__ = ModuleType.__setattr__
    reload(self)


class _LazyModule(type):
    def __new__(cls, name):
        acquire_lock()
        try:
            if name not in sys.modules:
                attributes = {
                    "__name__": name,
                    "__getattribute__": _getattribute,
                    "__setattr__": _setattr
                }
                parents = (ModuleType,)
                klass = type.__new__(cls, "module", parents, attributes)
                sys.modules[name] = klass(name)
                if "." in name:  # Also ensure the parent exists
                    _LazyModule(name.rsplit(".", 1)[0])
            return sys.modules[name]
        finally:
            release_lock()


class LazyImporter(object):
    """An importer for modules that are loaded lazily.

    This inserts itself into :py:data:`sys.meta_path`, storing a dictionary of
    :py:class:`_LazyModule`\ s (which is added to with :py:meth:`new`).
    """
    def __init__(self):
        self._modules = {}
        sys.meta_path.append(self)

    def new(self, name):
        module = _LazyModule(name)
        self._modules[name] = module
        return module

    def find_module(self, fullname, path=None):
        if fullname in self._modules and fullname not in sys.modules:
            return self

    def load_module(self, fullname):
        return self._modules.pop(fullname)