[webkit-dev] Moving to Python 3

[Philippe Normand]

Hi folks,

Now that Catalina is released, can we move on to some of the proposed
changes discussed in this thread?

Philippe

On Fri, 2019-07-12 at 12:18 -0700, Jonathan Bedard 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.
> 
> 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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