On Wed, Jan 1, 2014 at 11:43 AM, Keith Winston <keithw...@gmail.com> wrote:
> Thanks Danny, I don't understand the re-persisted part, but I'll look into
> it.
Shelf.close calls Shelf.sync to flush the cache. Here's what it does:
>>> print(inspect.getsource(shelve.Shelf.sync))
def sync(self):
if self.writeback and self.cache:
self.writeback = False
for key, entry in self.cache.items():
self[key] = entry
self.writeback = True
self.cache = {}
if hasattr(self.dict, 'sync'):
self.dict.sync()
If the writeback cache is enabled, then loading a key also caches the
item in a dict. `sync` writes all of the cached items back and clears
the cache (for which it should be using `self.cache.clear()`).
It also syncs the underlying database if it has a `sync` attribute,
which the gdbm database [1] requires. Per its man page, "gdbm defaults
to no-sync mode" [2]. So actually the "f" flag for opening in fast
mode is obsolete [3].
1. If you're using a Debian-family Linux, install the
packages python-gdbm and python3-gdbm.
2. http://linux.die.net/man/3/gdbm
3. http://docs.python.org/3/library/dbm#dbm.gnu.open
Have a look at what gets stored in the database. First lets create a
database and add a list:
>>> import shelve, dbm
>>> sh = shelve.open('class_shelve')
>>> sh['alist'] = [1, 2, 3]
You could leave the Shelf open and use its underlying `sh.dict`, but
I'll reopen the database instead:
>>> sh.close()
>>> db = dbm.open('class_shelve')
>>> data = db['alist']
>>> data
b'\x80\x03]q\x00(K\x01K\x02K\x03e.'
shelve serialized the list to a byte string with the pickle protocol.
This is a program that instructs the pickle VM to build the list. You
can manually load the list with pickle.loads:
>>> import pickle
>>> pickle.loads(data)
[1, 2, 3]
You can disassemble the pickle with pickletools.dis, if you're curious:
>>> import pickletools
>>> pickletools.dis(data)
0: \x80 PROTO 3
2: ] EMPTY_LIST
3: q BINPUT 0
5: ( MARK
6: K BININT1 1
8: K BININT1 2
10: K BININT1 3
12: e APPENDS (MARK at 5)
13: . STOP
highest protocol among opcodes = 2
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