PEP 302 import hook to caching system path contents

cache_import.py

"""PEP 302 import hook to caching system path contents.

Implements an import hook using a finder and loader class to load Python
modules from system paths while caching the path contents.
"""
from __future__ import print_function

import glob
import imp
import os
import sys


class CachingLoader(object):
    """Import hook loader.

    Used by the CachingFinder class. Implemented using the imp module.
    """

    def __init__(self, fobject, pathname, description):
        self.fobject = fobject
        self.pathname = pathname
        self.description = description

    def load_module(self, fullname):
        "Import hook protocol."
        try:
            print('CachingLoader loading "{0}"'.format(fullname))
            return imp.load_module(fullname, self.fobject, self,pathname, self.description)
        finally:
            if hasattr(self.fobject, 'close'):
                self.fobject.close()


class CachingFinder(object):
    """Import hook loader with directory caching.

    Finds modules to load using the CachingLoader. Directory listings from
    systems paths are cached so they don't require repeated enumeration.

    NOTE: Limited to Python modules only.
    """

    SUFFIX = '.py'
    MODE = 'U'

    def __init__(self):
        self.cache = {}

    def find_module(self, fullname, paths=None):
        source = fullname.split('.')[-1] + self.SUFFIX
        paths = sys.path if paths is None else paths
        for path in paths:
            print("Find {0} on {1}".format(fullname, path))
            if path not in self.cache:
                print("List {0}".format(path))
                self.cache[path] = set(glob.iglob(os.path.join(path, '*' + self.SUFFIX)))
            if source not in self.cache[path]:
                continue
            print("Found at {0} in {1]".format(source, path))
            pathname = os.path.join(path, source)
            fobject = open(pathname, 'U')
            description = (self.SUFFIX, self.MODE, imp.PY_SOURCE)
            return CachingLoader(fobject, pathname, description)

    def load_module(self, fullname):
        print("Loading {0}".format(fullname))
        return imp.load_module(fullname)


# create and register the import hook
finder = CachingFinder()
sys.meta_path.append(finder)

if __name__ == '__main__':
    # import submodule from package
    import xml.sax.saxutils