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Change reversed() to [::-1] for speed #13

wants to merge 1 commit into from

Change reversed() to [::-1] for speed #13

wants to merge 1 commit into from


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@scragg0x scragg0x commented Oct 2, 2012

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Skimming your PR made me curious... [::-1] only replaces reversed().

Why did you drop the ''.join() from your version?

I haven't dug into the surrounding code so I'm unaware of the overall calling context.

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Ahh, such an old commit of mine. It would appear that I just neglected it. Good thing this wasn't pulled.

@davidism davidism closed this Apr 10, 2016
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I timed both versions and there was no significant change, so just going to close this and leave it as is.

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Interestingly, I just did a bench and reversed it much faster anyway with larger lists. 25x faster when using the example gist.

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That benchmark is incorrect - reversed() doesn't actually reverse anything when called; it just provides an iterator that starts at the end of the collection

In [1]: lst = ['x'] * 1000
In [2]: %timeit lst[::-1]
100000 loops, best of 3: 4.37 µs per loop
In [3]: %timeit list(reversed(lst))
100000 loops, best of 3: 8.27 µs per loop

With strings:

In [1]: s = 'x' * 1000
In [2]: %timeit s[::-1]
1000000 loops, best of 3: 883 ns per loop
In [3]: %timeit ''.join(reversed(s))
10000 loops, best of 3: 20.7 µs per loop

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@ThiefMaster I see, thanks for clarifying. I figured I missed something to have such a large disparity.

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Now that I understand what's happening here, I'd vote to reconsider and merge this.

BTW, thanks @ThiefMaster for taking the time to explain, I hadn't realized that reversed() only returns an iterator.

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After a (very silly) discussion about this micro-optimization,, it looks like it would be better to replace the whole thing with the following, which outperforms everything else on py2 and py3 and doesn't require compat imports.

import struct

to_int64 = struct.Struct('>Q').pack

def int_to_bytes(num):
    return to_int64(num).lstrip(b'\x00')

timeit int_to_bytes(ts)
1000000 loops, best of 3: 398 ns per loop

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@davidism davidism added this to the 1.0.0 milestone Sep 28, 2018
netbsd-srcmastr pushed a commit to NetBSD/pkgsrc that referenced this pull request Nov 10, 2018
Version 1.1.0

Released 2018-10-26

-   Change default signing algorithm back to SHA-1. (`#113`_)
-   Added a default SHA-512 fallback for users who used the yanked 1.0.0
    release which defaulted to SHA-512. (`#114`_)
-   Add support for fallback algorithms during deserialization to
    support changing the default in the future without breaking existing
    signatures. (`#113`_)
-   Changed capitalization of packages back to lowercase as the change
    in capitalization broke some tooling. (`#113`_)

.. _#113: pallets/itsdangerous#113
.. _#114: pallets/itsdangerous#114

Version 1.0.0

Released 2018-10-18


*Note*: This release was yanked from PyPI because it changed the default
algorithm to SHA-512. This decision was reverted in 1.1.0 and it remains
at SHA1.

-   Drop support for Python 2.6 and 3.3.
-   Refactor code from a single module to a package. Any object in the
    API docs is still importable from the top-level ``itsdangerous``
    name, but other imports will need to be changed. A future release
    will remove many of these compatibility imports. (`#107`_)
-   Optimize how timestamps are serialized and deserialized. (`#13`_)
-   ``base64_decode`` raises ``BadData`` when it is passed invalid data.
-   Ensure value is bytes when signing to avoid a ``TypeError`` on
    Python 3. (`#29`_)
-   Add a ``serializer_kwargs`` argument to ``Serializer``, which is
    passed to ``dumps`` during ``dump_payload``. (`#36`_)
-   More compact JSON dumps for unicode strings. (`#38`_)
-   Use the full timestamp rather than an offset, allowing dates before
    2011. (`#46`_)
-   Detect a ``sep`` character that may show up in the signature itself
    and raise a ``ValueError``. (`#62`_)
-   Use a consistent signature for keyword arguments for
    ``Serializer.load_payload`` in subclasses. (`#74`_, `#75`_)
-   Change default intermediate hash from SHA-1 to SHA-512. (`#80`_)
-   Convert JWS exp header to an int when loading. (`#99`_)

.. _#13: pallets/itsdangerous#13
.. _#27: pallets/itsdangerous#27
.. _#29: pallets/itsdangerous#29
.. _#36: pallets/itsdangerous#36
.. _#38: pallets/itsdangerous#38
.. _#46: pallets/itsdangerous#46
.. _#62: pallets/itsdangerous#62
.. _#74: pallets/itsdangerous#74
.. _#75: pallets/itsdangerous#75
.. _#80: pallets/itsdangerous#80
.. _#99: pallets/itsdangerous#99
.. _#107: pallets/itsdangerous#107
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4 participants