"""
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various reasons (cache is out of date, permissions are bad, wrong python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
"""
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)
class PickleCache:
"""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):
"""Read data from pickle cache.
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)
if not havePython22OrGreater:
if v:
print 'Python version is too old for this. Returning None.'
return None
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, 'rb')
except IOError, e:
if v:
print 'Cannot open cache file: %s: %s.' % (
e.__class__.__name__, e)
else:
try:
if v:
print 'about to load'
dict = load(file)
except EOFError:
if v:
print 'EOFError - not loading'
shouldDeletePickle = 1
except Exception, exc:
print 'WARNING: %s: %s: %s' % (
self.__class__.__name__, exc.__class__, exc)
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']
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)
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, 'wb')
except IOError, e:
if v:
print 'error. not writing. %s: %s' % (
e.__class__.__name__, e)
else:
while 1:
dump(dict, file, 1)
file.close()
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
_reader = PickleCacheReader()
readPickleCache = _reader.read
_writer = PickleCacheWriter()
writePickleCache = _writer.write