    """ImportManager

    Manages imported modules and protects against concurrent imports.

    Keeps lists of all imported Python modules and templates as well as other
    config files used by Webware for Python. For modules which are not directly
    imported, ImportManager can use ImportSpy to keep track of them. This can
    be used to detect changes in source files, templates or config files in order
    to reload them automatically by the AutoReloadingAppServer. The use of
    ImportSpy can be suppressed with the``UseImportSpy`` setting.

    """

    from Common import *
    import os
    import sys
    import imp

    try: # backward compatibility for Python < 2.3

        True, False
    except NameError:
        True, False = 1, 0

    try: # backward compatibility for Python < 2.4

        set
    except NameError: # fallback for Python < 2.4

        try:
            from sets import Set as set
        except ImportError: # fallback for Python < 2.3

            from UserDict import UserDict
            class set(UserDict):
                def add(self, item):
                    self.data[item] = item
                def remove(self, item):
                    del self._dict[item]

    class ImportLock:
        """Lock for multi-threaded imports.

        Provides a lock for protecting against concurrent imports. This is
        necessary because WebKit is multithreaded and uses its own import hook.

        This class abstracts the difference between using the Python interpreter's
        global import lock, and using our own RLock. The global lock is the correct
        solution, but is only available in Python since version 2.2.3. If it's not
        available, we fall back to using an RLock (which is not as good, but better
        than nothing).

        """

        def __init__(self):
            """Create the lock.

            Aliases the `acquire` and `release` methods to
            `imp.acquire_lock` and `imp.release_lock` (if available),
            or to acquire and release our own RLock.

            """
            if hasattr(imp, 'acquire_lock'):
                self.acquire = imp.acquire_lock
                self.release = imp.release_lock
            else: # fallback for Python < 2.3

                from threading import RLock
                self._lock = RLock()
                self.acquire = self._lock.acquire
                self.release = self._lock.release

    class ImportManager(Object):
        """The import manager.

        Keeps track of the Python modules and other system files that have been
        imported and are used by Webware.

        """

        _imp = _spy = None

        def __init__(self):
            """Create import hook."""
            assert not self._imp, "Only one instance of ImportManager is possible."
            self._imp = True
            Object.__init__(self)
            self._lock = ImportLock()
            self._fileList = {}
            self._moduleFiles = {}
            self._otherFiles = set()
            self._notifyHook = None

        def load_module(self, name, file, filename, info):
            """Replaces imp.load_module."""
            try:
                try:
                    self._lock.acquire()
                    mod = imp.load_module(name, file, filename, info)
                finally:
                    self._lock.release()
                self.recordModule(mod)
            except:
                # Also record filepaths which weren't successfully loaded, which

                # may happen due to a syntax error in a servlet, because we also

                # want to know when such a file is modified:

                self.recordFile(filename)
                raise
            return mod

        def find_module(self, name, path=None):
            """Replaces imp.find_module."""
            return imp.find_module(name, path)

        def activateImportSpy(self):
            """Activate ImportSpy to keep track of modules loaded elsewhere."""
            if not self._spy:
                from ImportSpy import activate
                self._spy = activate(self)
            return self._spy

        def fileList(self, update=True):
            """Return the list of tracked files."""
            if not self._spy and update:
                # Update list of files of imported modules

                moduleNames = []
                for modname in sys.modules:
                    if not self._moduleFiles.has_key(modname):
                        moduleNames.append(modname)
                if moduleNames:
                    self.recordModules(moduleNames)
            return self._fileList

        def notifyOfNewFiles(self, hook):
            """Register notification hook.

            Called by someone else to register that they'd like to be know
            when a new file is imported.

            """
            self._notifyHook = hook

        def watchFile(self, path, modname=None, getmtime=os.path.getmtime):
            """Add more files to watch without importing them."""
            modtime = getmtime(path)
            self._fileList[path] = modtime
            if modname:
                self._moduleFiles[modname] = path
            else:
                self._otherFiles.add(path)
            # send notification that this file was imported

            if self._notifyHook:
                self._notifyHook(path, modtime)

        def recordModule(self, mod, isfile=os.path.isfile):
            """Record a module."""
            modname = getattr(mod, '__name__', None)
            if not modname or not sys.modules.has_key(modname):
                return
            fileList = self._fileList
            # __orig_file__ is used for PSP, Kid and Cheetah templates; we want

            # to record the source filenames, not the auto-generated modules:

            f = getattr(mod, '__orig_file__', None)
            if f and not fileList.has_key(f):
                try:
                    if isfile(f):
                        self.watchFile(f, modname)
                except OSError:
                    pass
            else:
                f = getattr(mod, '__file__', None)
                if f and not fileList.has_key(f):
                    # record the .py file corresponding to each .pyc or .pyo

                    if f[-4:].lower() in ['.pyc', '.pyo']:
                        f = f[:-1]
                    try:
                        if isfile(f):
                            self.watchFile(f, modname)
                        else:
                            self.watchFile(os.path.join(f, '__init__.py'))
                    except OSError:
                        pass

        def recordModules(self, moduleNames=None):
            """Record a list of modules (or all modules)."""
            if moduleNames is None:
                moduleNames = sys.modules.keys()
            for modname in moduleNames:
                mod = sys.modules[modname]
                self.recordModule(mod)

        def recordFile(self, filename, isfile=os.path.isfile):
            """Record a file."""
            if isfile(filename):
                self.watchFile(filename)

        def fileUpdated(self, filename, update=True, getmtime=os.path.getmtime):
            """Check whether file has been updated."""
            fileList = self.fileList(update)
            try:
                mtime = fileList[filename]
            except KeyError:
                return True
            try:
                newmtime = getmtime(filename)
            except OSError:
                return True
            if mtime < newmtime:
                fileList[filename] = newmtime
                for modname, modfile in self._moduleFiles.items():
                    if modfile == filename:
                        break
                else:
                    return True # it's not a module, we must reload

                mod = sys.modules.get(modname, None)
                if not mod or not getattr(mod, '__donotreload__', None):
                    return True # it's a module that needs to be reloaded

            return False

        def updatedFile(self, update=True, getmtime=os.path.getmtime):
            """Check whether one of the files has been updated."""
            fileList = self.fileList(update)
            for filename, mtime in fileList.items():
                try:
                    newmtime = getmtime(filename)
                except OSError:
                    return filename
                if mtime < newmtime:
                    fileList[filename] = newmtime
                    for modname, modfile in self._moduleFiles.items():
                        if modfile == filename:
                            break
                    else:
                        return filename # it's not a module, we must reload

                    mod = sys.modules.get(modname, None)
                    if not mod or not getattr(mod, '__donotreload__', None):
                        return filename # it's a module that needs to be reloaded

            return False

        def delModules(self, includePythonModules=False, excludePrefixes=[]):
            """Delete imported modules.

            Deletes all the modules that the ImportSpy has ever imported unless
            they are part of WebKit. This in support of DebugAppServer's useful
            (yet imperfect) support for AutoReload.

            """
            moduleFiles = self._moduleFiles
            moduleNames = moduleFiles.keys()
            fileList = self._fileList
            for modname in moduleNames:
                if modname == __name__:
                    continue
                filename = self._moduleFiles[modname]
                if not includePythonModules:
                    if not filename or filename.startswith(sys.prefix):
                        continue
                for prefix in excludePrefixes:
                    if modname.startswith(prefix):
                        break
                else:
                    try:
                        del sys.modules[modname]
                    except KeyError:
                        pass
                    try:
                        del moduleFiles[modname]
                    except KeyError:
                        pass
                    try:
                        del fileList[filename]
                    except KeyError:
                        pass