[webkit-dev] Moving to Python 3

[Robert Ma]

Any thoughts on bytes and Unicode strings, especially the string literals
in the code base?

On Fri, Jul 12, 2019 at 3:38 PM Tim Horton <timothy_horton at apple.com> wrote:

>
>
> On Jul 12, 2019, at 12:18 PM, Jonathan Bedard <jbedard at apple.com> wrote:
>
> Hello WebKit developers,
>
> Now that the Catalina developer seeds are available, it is official that
> the new Mac developer tools come with Python 3. As a result, we need to
> continue the ongoing discussion about migrating our Python 2.7 scripts to
> Python 3.
>
> I propose that, over the next 9 months, we do the following:
>
> 1. Make any no-cost Python 3 compatibility changes, in particular
>     - print foo -> print(foo)
>     - import .foo -> import webkitpy.foo
> 2. Convert any scripts not used in automation to Python 3 ASAP (scripts
> like bisect-builds, block-spammers, compare-results)
> 3. Make most Python 3 compatibility changes which sacrifice efficiency,
> subject to a case-by-case audit. These would be things like:
>     - dict.iteritems() -> dict.items()
>     - dict.items() -> list(dict.items())
> 4. Install Python 3 on macOS Sierra and Mojave bots
> 5. Convert peripheral automation scripts to Python 3 1-by-1 (scripts like
> clean-webkit, merge-results-json, webkit-patch)
> 6. Convert testing scripts and webkitpy to Python 3 in a single change
>
> The trouble I foresee us encountering with any scheme which attempts a
> conversion which retains both Python 2.7 and Python 3 compatibility is code
> like this:
>
>     for expectation_string, expectation_enum in
> test_expectations.TestExpectations.EXPECTATIONS.iteritems():
>         ...
>
> In this code, the EXPECTATIONS dictionary is thousands of elements long.
> In Python 2.7, iteritems() gives us an iterator instead of creating a new
> list, like items() would. In Python 3, iteritems() doesn’t exist, but
> items() does, and now gives us an iterator instead of creating a new list.
> The trouble here is that, in this case, creating a new list will be very
> expensive, expensive enough that we might manage to impact the testing run.
> There isn’t really an elegant way around this problem if we want to support
> both Python 2.7 and Python 3, other than defining different code paths for
> each language.
>
>
> The official Python 3 transition documentation has a fairly elegant
> solution to this, actually??
>
> https://legacy.python.org/dev/peps/pep-0469/
>
> See "Migrating to the common subset of Python 2 and 3” — you define
> different iteritems() helpers in the two cases. Seems pretty reasonable to
> me.
>
> There are other small gotchas as well. For example, ‘%’ is no longer a
> protected character, which can actually change the behavior of regexes.
> That’s why I think it’s better to just try and directly convert things
> instead of attempting to be compatible with both Python 2.7 and Python 3.
>
> Jonathan
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