Hi,
I'm currently implementing PEP 526 [1], which requires additions to the
interpreter from the parser down to the bytecode interpreter. Since
this is a common task, I want to expand the documentation a bit, adding
a "how to extend the interpreter" recipe.
One thing which I wasn't 100% sure about myself is how to test it.
Clearly every addition has to come with a test, preferably written
before implementing anything. But a few things are a bit unclear:
For every change in CPython there is (hopefully) a test case in
lib-python/*/test. So technically there's already a test case when
modifying the interpreter. One reason to add app-level tests is to
avoid re-translations. But other than this convenience, how much
app-level testing to we *require*? Should we test everything up to the
last detail or are we fine with testing the basic functionality and
leaving the rest to the CPython test suite?
A related question is which style of testing — integration-test-style or
unit-test-style — we prefer. Different people — also among the core
developers — have different preferences, so we have a bit of a mix. So
in this concrete example, it is tempting to create a single test like
AppTestFunctionAnnotations.test_simple from
pypy/interpreter/test/test_syntax.py [2] and then add all the
functionality to the interpreter until the test passes (this would be
integration-style testing). From the unit-style testing perspective,
this test is not very good, because it doesn't test only the syntax,
like the file name "test_syntax.py" would suggest. The alternative
would be to add a unit test for every affected part of the interpreter
(parser, AST builder, AST validator, bytecode generation, symtable,
stack effect computation, bytecode interpreter).
What do you think? Feel free to braindump your general thoughts (also
outside of the syntax addition case), so I can extend other relevant
parts of the documentation as well. This will hopefully make it easier
for new (but also more advanced) contributors and ensure a consistent
test suite quality all over our code base.
-Manuel
[1] https://www.python.org/dev/peps/pep-0526/
[2]
https://bitbucket.org/pypy/pypy/src/e30b20325b579b5a92cba08d326d2e385caba2d9/pypy/interpreter/test/test_syntax.py?fileviewer=file-view-default#test_syntax.py-685
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