[Python-Dev] peps: Update PEP 399 to include comments from python-dev.

[R. David Murray]

Antoine Pitrou, 13.04.2011 02:07:
> On Tue, 12 Apr 2011 19:50:34 -0400
> Tres Seaver wrote:
>> Trying to accelerate existing code which doesn't have the coverage is
>> insane:  how can you know that the accelerator doesn't subtly change the
>> semantics without tests?
>
> Well, why do you think tests guarantee that the semantics are the same?
> Tests are not a magic bullet. "Covering" a code path doesn't ensure
> that every possible behaviour is accounted for.

When I suggested we add 100% branch coverage as a recommendation or
requirement to the PEP, I pointed out that it was a place to *start*.
Nobody is saying it guarantees the semantics are the same, that was the
whole point of replacing the statement about semantics with the statement
about test coverage.  When we find places where the two versions don't
match, we will have to (a) decide the compatibility issue[*] and (b) add
tests that enshrine the decision.

As Tres said, if I were *writing* an accelerator, I'd want to start
with 100% branch coverage just to have as good as practical a check on
my implementation as I could.  I'd also try to think of additional tests.

I'm doing this in email (increasing test coverage to 100% before rewriting
algorithms) even though I'm not doing C accelerators.  It just seems
like the sensible thing to do.  (You may think I'm really crazy, since
some of the tests needed to get to 100% branch coverage will be testing
lines of code that I'm removing....but those tests represent particular
edges cases and I want to know that those edge cases continue to pass
after I change the code.)

[*] Maybe the PEP needs to talk about the basis on which those decisions
will be made:  maintaining compatibility across Python implementations.
In other words, a CPython C accelerator can be viewed as *breaking
compatibility with standard Python* if it doesn't implement the documented
interface of the Python version of the module.  (My apologies if this
is in fact already discussed, I didn't reread the PEP to check.)  The
idea is to use the test suite as the check for this, adding tests as
necessary.

--
R. David Murray           http://www.bitdance.com