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Bump the default pickle protocol in shelve #78385

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serhiy-storchaka opened this issue Jul 24, 2018 · 5 comments
Closed

Bump the default pickle protocol in shelve #78385

serhiy-storchaka opened this issue Jul 24, 2018 · 5 comments
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3.10 stdlib type-feature

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@serhiy-storchaka
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@serhiy-storchaka serhiy-storchaka commented Jul 24, 2018

BPO 34204
Nosy @rhettinger, @vstinner, @avassalotti, @ambv, @serhiy-storchaka, @ZackerySpytz, @marco-c
PRs
  • #19639
  • #22751
  • Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.

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    GitHub fields:

    assignee = None
    closed_at = <Date 2020-10-29.09:46:33.380>
    created_at = <Date 2018-07-24.09:16:42.390>
    labels = ['type-feature', 'library', '3.10']
    title = 'Bump the default pickle protocol in shelve'
    updated_at = <Date 2020-10-29.09:46:33.379>
    user = 'https://github.com/serhiy-storchaka'

    bugs.python.org fields:

    activity = <Date 2020-10-29.09:46:33.379>
    actor = 'cheryl.sabella'
    assignee = 'none'
    closed = True
    closed_date = <Date 2020-10-29.09:46:33.380>
    closer = 'cheryl.sabella'
    components = ['Library (Lib)']
    creation = <Date 2018-07-24.09:16:42.390>
    creator = 'serhiy.storchaka'
    dependencies = []
    files = []
    hgrepos = []
    issue_num = 34204
    keywords = ['patch']
    message_count = 5.0
    messages = ['322281', '370137', '379812', '379813', '379861']
    nosy_count = 7.0
    nosy_names = ['rhettinger', 'vstinner', 'alexandre.vassalotti', 'lukasz.langa', 'serhiy.storchaka', 'ZackerySpytz', 'marco-c']
    pr_nums = ['19639', '22751']
    priority = 'normal'
    resolution = 'fixed'
    stage = 'resolved'
    status = 'closed'
    superseder = None
    type = 'enhancement'
    url = 'https://bugs.python.org/issue34204'
    versions = ['Python 3.10']

    @serhiy-storchaka
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    @serhiy-storchaka serhiy-storchaka commented Jul 24, 2018

    The default pickle protocol is 4 now. But shelve still uses the pickle protocol 3. Shouldn't it be bumped? Shouldn't shelve use pickle.DEFAULT_PROTOCOL by default?

    Disadvantages:

    1. This will make shelve files incompatible with Python 3.3 by default.

    2. Protocol 4 adds 9 bytes of overhead in comparison with protocol 3. This can be too large for the shelve containing a lot of small objects. Maybe strip redundant frame header for small pickles?

    @serhiy-storchaka serhiy-storchaka added 3.8 stdlib type-feature labels Jul 24, 2018
    @vstinner
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    @vstinner vstinner commented May 27, 2020

    I wrote a short script to see the impact of file size depending on the protocol:
    ---

    import shelve
    import os.path
    
    print("== Short value ==")
    for proto in (0, 1, 2, 3, 4, 5):
        filename = 'shelve-picklev%s' % proto
        with shelve.open(filename, protocol=proto) as db:
            assert db._protocol == proto
            for x in range(1000):
                db[str(x)] = str(x)
        print(f'Protocol {proto}: {os.path.getsize(filename)} bytes')
        os.unlink(filename)
    print()
    print("== Large value ==")
    large_value = [str(x) for x in range(1000)]
    for proto in (0, 1, 2, 3, 4, 5):
        filename = 'shelve-picklev%s' % proto
        with shelve.open(filename, protocol=proto) as db:
            assert db._protocol == proto
            for x in range(10):
                db[str(x)] = large_value
        print(f'Protocol {proto}: {os.path.getsize(filename)} bytes')
        os.unlink(filename)

    Output with Python 3.9.0b1 (on Fedora 32):
    ---
    == Short value ==
    Protocol 0: 90112 bytes
    Protocol 1: 94208 bytes
    Protocol 2: 94208 bytes
    Protocol 3: 94208 bytes
    Protocol 4: 94208 bytes
    Protocol 5: 94208 bytes

    == Large value ==
    Protocol 0: 139264 bytes
    Protocol 1: 139264 bytes
    Protocol 2: 139264 bytes
    Protocol 3: 139264 bytes
    Protocol 4: 98304 bytes
    Protocol 5: 98304 bytes
    ---

    For short string values, protocol 0 produces smaller files than protocol 1 and higher.

    For large value, protocol 4 and higher produce smaller files than protocol 3 and lower.

    @marco-c
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    @marco-c marco-c mannequin commented Oct 28, 2020

    I've opened #22751 to fix this, I know there was already a PR, but it seems to have been abandoned.

    @ZackerySpytz
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    @ZackerySpytz ZackerySpytz mannequin commented Oct 28, 2020

    It has not been abandoned.

    @ZackerySpytz ZackerySpytz mannequin added 3.10 and removed 3.8 labels Oct 28, 2020
    @miss-islington
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    @miss-islington miss-islington commented Oct 29, 2020

    New changeset df59273 by Zackery Spytz in branch 'master':
    bpo-34204: Use pickle.DEFAULT_PROTOCOL in shelve (GH-19639)
    df59273

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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