    """
    PickleCache provides tools for keeping fast-loading cached versions of
    files so that subsequent loads are faster. This is similar to how Python
    silently caches .pyc files next to .py files.

    The typical scenario is that you have a type of text file that gets
    "translated" to Pythonic data (dictionaries, tuples, instances, ints,
    etc.). By caching the Python data on disk in pickle format, you can
    avoid the expensive translation on subsequent reads of the file.

    Two real life cases are MiscUtils.DataTable, which loads and represents
    comma-separated files, and MiddleKit which has an object model file.
    So for examples on using this module, load up the following files and
    search for "Pickle":
        Webware/MiscUtils/DataTable.py
        MiddleKit/Core/Model.py

    The cached file is named the same as the original file with
    '.pickle.cache' suffixed. The utility of '.pickle' is to denote the file
    format and the utilty of '.cache' is to provide '*.cache' as a simple
    pattern that can be removed, ignored by backup scripts, etc.

    The treatment of the cached file is silent and friendly just like
    Python's approach to .pyc files. If it cannot be read or written for
    various reasons (cache is out of date, permissions are bad, wrong python
    version, etc.), then it will be silently ignored.

    GRANULARITY

    In constructing the test suite, I discovered that if the source file is
    newly written less than 1 second after the cached file, then the fact
    that the source file is newer will not be detected and the cache will
    still be used. I believe this is a limitation of the granularity of
    os.path.getmtime(). If anyone knows of a more granular solution, please
    let me know.

    This would only be a problem in programmatic situations where the source
    file was rapidly being written and read. I think that's fairly rare.

    PYTHON VERSION

    These operations do nothing if you don't have Python 2.2 or greater.

    SEE ALSO
        http://www.python.org/doc/current/lib/module-pickle.html

    - wordwrap bar --------------------------------------------------------
    """

    verbose = 0

    import os, sys, time
    from types import DictType
    from pprint import pprint
    try:
        from cPickle import load, dump
    except ImportError:
        from pickle import load, dump

    havePython22OrGreater = sys.version_info[0]>2 or (sys.version_info[0]==2 and sys.version_info[1]>=2)

    s ="""
    def readPickleCache(filename, pickleVersion=1, source=None, verbose=None):
        return _reader.read(filename, pickleVersion, source, verbose)

    def writePickleCache(data, filename, pickleVersion=1, source=None, verbose=None):
        return _writer.write(data, filename, pickleVersion, source, verbose)
    """

    class PickleCache:
        """
        Just a simple abstract base class for PickleCacheReader and
        PickleCacheWriter.
        """
        verbose = verbose

        def picklePath(self, filename):
            return filename + '.pickle.cache'

    class PickleCacheReader(PickleCache):

        def read(self, filename, pickleVersion=1, source=None, verbose=None):
            """
            Returns the data from the pickle cache version of the filename, if it can read. Otherwise returns None which also indicates that writePickleCache() should be subsequently called after the original file is read.
            """
            if verbose is None:
                v = self.verbose
            else:
                v = verbose
            if v: print '>> PickleCacheReader.read() - verbose is on'
            assert filename

            if not os.path.exists(filename):
                #if v: print 'cannot find %r' % filename
                open(filename) # to get a properly constructed IOError

            if not havePython22OrGreater:
                #if v: print 'Python version is too old for this. Returning None.'
                return None

            didReadPickle = 0
            shouldDeletePickle = 0

            data = None

            picklePath = self.picklePath(filename)
            if os.path.exists(picklePath):
                if os.path.getmtime(picklePath)<os.path.getmtime(filename):
                    #if v: print 'cache is out of date'
                    shouldDeletePickle = 1
                else:
                    try:
                        #if v: print 'about to open for read %r' % picklePath
                        file = open(picklePath)
                    except IOError, e:
                        #if v: print 'cannot open cache file: %s: %s' % (e.__class__.__name__, e)
                        pass
                    else:
                        try:
                            #if v: print 'about to load'
                            dict = load(file)
                        except EOFError:
                            #if v: print 'EOFError - not loading'
                            shouldDeletePickle = 1
                        else:
                            file.close()
                            #if v: print 'finished reading'
                            assert isinstance(dict, DictType), 'type=%r dict=%r' % (type(dict), dict)
                            for key in ('source', 'data', 'pickle version', 'python version'):
                                assert dict.has_key(key), key
                            if source and dict['source']!=source:
                                #if v: print 'not from required source (%s): %s' % (source, dict['source'])
                                shouldDeletePickle = 1
                            elif dict['pickle version']!=pickleVersion:
                                #if v: print 'pickle version (%i) does not match expected (%i)' % (dict['pickle version'], pickleVersion)
                                shouldDeletePickle = 1
                            elif dict['python version']!=sys.version_info:
                                #if v: print 'python version %s does not match current %s' % (dict['python version'], sys.version_info)
                                shouldDeletePickle = 1
                            else:
                                #if v: print 'all tests pass. accepting data'
                                if v>1:
                                    print 'display full dict:'
                                    pprint(dict)
                                data = dict['data']
                                didReadPickle = 1

            # delete the pickle file if suggested by previous conditions
            if shouldDeletePickle:
                try:
                    #if v: print 'attempting to remove pickle cache file'
                    os.remove(picklePath)
                except OSError, e:
                    if v: print 'failed to remove: %s: %s' % (e.__class__.__name__, e)
                    pass

            if v: print 'done reading data'; print

            return data

    class PickleCacheWriter(PickleCache):

        writeSleepInterval = 0.1

        def write(self, data, filename, pickleVersion=1, source=None, verbose=None):
            if verbose is None:
                v = self.verbose
            else:
                v = verbose
            if v: print '>> PickleCacheWriter.write() - verbose is on'
            assert filename
            sourceTimestamp = os.path.getmtime(filename)

            if not havePython22OrGreater:
                #if v: print 'Python version is too old for this. Returning None.'
                return None

            picklePath = self.picklePath(filename)
            dict = {
                'source': source,
                'python version': sys.version_info,
                'pickle version': pickleVersion,
                'data': data,
            }
            if v>1:
                print 'display full dict:'
                pprint(dict)
            try:
                if v: print 'about to open for write %r' % picklePath
                file = open(picklePath, 'w')
            except IOError, e:
                if v: print 'error. not writing. %s: %s' % (e.__class__.__name__, e)
            else:
                while 1:
                    dump(dict, file, 1)   # 1 = binary format
                    file.close()
                    # make sure the cache has a newer timestamp, otherwise the cache will
                    # just get ignored and rewritten next time.
                    if os.path.getmtime(picklePath)==sourceTimestamp:
                        if v: print 'timestamps are identical. sleeping %0.2f seconds' % self.writeSleepInterval
                        time.sleep(self.writeSleepInterval)
                        file = open(picklePath, 'w')
                    else:
                        break

            if v: print 'done writing data'; print

    # define module level convenience functions, readPickleCache and writePickleCache:

    _reader = PickleCacheReader(); readPickleCache  = _reader.read
    _writer = PickleCacheWriter(); writePickleCache = _writer.write