earwigbot/earwigbot/lazy.py

# -*- coding: utf-8  -*-
#
# Copyright (C) 2009-2015 Ben Kurtovic <ben.kurtovic@gmail.com>
#
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# 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.
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# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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# SOFTWARE.

"""
Implements a hierarchy of importing classes as defined in `PEP 302
<https://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 importlib
import sys
from threading import RLock
from types import ModuleType

__all__ = ["LazyImporter"]

_real_get = ModuleType.__getattribute__

def _create_failing_get(exc):
    def _fail(self, attr):
        raise exc
    return _fail

def _mock_get(self, attr):
    with _real_get(self, "_lock"):
        if _real_get(self, "_unloaded"):
            type(self)._unloaded = False
            try:
                importlib.reload(self)
            except ImportError as exc:
                type(self).__getattribute__ = _create_failing_get(exc)
                del type(self)._lock
                raise
            type(self).__getattribute__ = _real_get
            del type(self)._lock
        return _real_get(self, attr)


class _LazyModule(type):
    def __new__(cls, name):
        acquire_lock()
        try:
            if name not in sys.modules:
                attributes = {
                    "__name__": name,
                    "__getattribute__": _mock_get,
                    "_unloaded": True,
                    "_lock": RLock()
                }
                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:
    """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)