    """ImportSpy

    Keeps track of modules not imported directly by Webware for Python.

    This module helps save the filepath of every module which is imported.
    This is used by the `AutoReloadingAppServer` (see doc strings for more
    information) to restart the server if any source files change.

    Other than keeping track of the filepaths, the behaviour of this module
    loader is identical to Python's default behaviour.

    If the system supports FAM (file alteration monitor) and python-fam is
    installed, then the need for reloading can be monitored very effectively
    with the use of ImportSpy. Otherwise, ImportSpy will not have much benefit.

    Note that ImportSpy is based on the new import hooks of Python described
    in PEP 302. It is possible to suppress the use of ImportSpy by setting
    `UseImportSpy` in AppServer.config to False.

    """

    from os.path import isdir
    from sys import path_hooks, path_importer_cache

    class ImportSpy(object):
        """New style import tracker."""

        _imp = None

        def __init__(self, path=None):
            """Create importer."""
            assert self._imp
            if path and isdir(path):
                self.path = path
            else:
                raise ImportError

        def find_module(self, fullname):
            """Replaces imp.find_module."""
            try:
                self.file, self.filename, self.info = self._imp.find_module(
                    fullname.rsplit('.', 1)[-1], [self.path])
            except ImportError:
                pass
            else:
                return self

        def load_module(self, fullname):
            """Replaces imp.load_module."""
            mod = self._imp.load_module(fullname, self.file, self.filename, self.info)
            if mod:
                mod.__loader__ = self
            return mod

    def activate(impManager):
        """Activate ImportSpy."""
        assert not ImportSpy._imp
        ImportSpy._imp = impManager
        path_hooks.append(ImportSpy)
        path_importer_cache.clear()
        impManager.recordModules()