Previously, using _ in a @deal.ensure contract validator would lead to this error:

$ python3 -m deal lint
dumbpw/pwgen.py
  32:4 DEAL002 ensure contract must have `result` arg
    lambda _: len(_.result) == _.length,
    ^

This change fixes that false positive by detecting when the arg name is _ and passing the lint check if _.result is anywhere in the validator body.

Before this change, the prover attempts to handle the case where deal_solver cannot be imported by setting deal_solver = None. However, instantiation of the DealTheorem class a few lines later depends on deal_solver. This results in an unhandled exception because None does not have a "Theorem" attribute:

$ python3 -m deal lint
Traceback (most recent call last):
  File "/Users/ryan/.local/share/asdf/installs/python/3.10.2/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/Users/ryan/.local/share/asdf/installs/python/3.10.2/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/Users/ryan/src/me/extinfo/.venv/lib/python3.10/site-packages/deal/__main__.py", line 6, in <module>
    sys.exit(main(sys.argv[1:]))
  File "/Users/ryan/src/me/extinfo/.venv/lib/python3.10/site-packages/deal/_cli/_main.py", line 37, in main
    commands = get_commands()
  File "/Users/ryan/src/me/extinfo/.venv/lib/python3.10/site-packages/deal/_cli/_main.py", line 16, in get_commands
    from ._prove import ProveCommand
  File "/Users/ryan/src/me/extinfo/.venv/lib/python3.10/site-packages/deal/_cli/_prove.py", line 26, in <module>
    class DealTheorem(deal_solver.Theorem):
AttributeError: 'NoneType' object has no attribute 'Theorem'

After this change, a special exception is raised to get_commands() so that when deal_solver import fails, the ProveCommand can be set to an empty Command (because the prover cannot run at all without the solver).

The deal linter decorates functions that call sys.exit() with @deal.raises(SystemExit). SystemExit inherits from BaseException, which makes it incompatible with Exception:

dumbpw/cli.py:14:14: error: Argument 1 to "raises" has incompatible type
"Type[SystemExit]"; expected "Type[Exception]"  [arg-type]
    @deal.raises(SystemExit)
                 ^
Found 1 error in 1 file (checked 14 source files)

This change annotates the raises() decorator to expect BaseException so that SystemExit can be included without an incompatible type error from the type checker.

It would be helpful for python3 -m deal stub /path/to/a/file.py to detect whether file.py already has deal annotations and generate itself based on them. This could be a neat way to separate deal contracts into their own file rather than intrude on the code, sort of like it's possible to do with type hints.

Most of vaa usage is for short signature. I doubt anyone really uses schemas. So, let's reimplement simple signature logic on deal side and make vaa optional.

Attempting to run the "deal in 30s" example:

# the result is always non-negative
@deal.post(lambda result: result >= 0)
# the function has no side-effects
@deal.pure
def count(items: List[str], item: str) -> int:
    return items.count(item)

# generate test function
test_count = deal.cases(count)

fails with the following error run under pytest:

[nix-shell:~/code/nixpkgs]$ pytest deal_test.py
======================================================== test session starts ========================================================
platform darwin -- Python 3.9.11, pytest-7.0.1, pluggy-1.0.0
rootdir: /Users/panashe/code/nixpkgs
plugins: hypothesis-6.38.0
collected 1 item

deal_test.py F                                                                                                                [100%]

============================================================= FAILURES ==============================================================
____________________________________________________________ test_count _____________________________________________________________

>   ???

/nix/store/3mnlyndr9s9r0v49ynahldczkawqa076-python3.9-deal-4.21.1/lib/python3.9/site-packages/deal/_testing.py:329:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
/nix/store/3mnlyndr9s9r0v49ynahldczkawqa076-python3.9-deal-4.21.1/lib/python3.9/site-packages/deal/_testing.py:328: in <lambda>
    return self._wrap(lambda case: case())
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = TestCase(args=(), kwargs={'items': [], 'item': ''}, func=<function count at 0x10fe74af0>, exceptions=(), check_types=True)
result = 0

    def _check_result(self, result: typing.Any) -> None:
        if not self.check_types:
            return
>       memo = typeguard._CallMemo(
            func=self.func,
            args=self.args,
            kwargs=self.kwargs,
        )
E       AttributeError: 'NoneType' object has no attribute '_CallMemo'

/nix/store/3mnlyndr9s9r0v49ynahldczkawqa076-python3.9-deal-4.21.1/lib/python3.9/site-packages/deal/_testing.py:68: AttributeError
------------------------------------------------------- Captured stdout call --------------------------------------------------------

You can reproduce this example by temporarily adding @reproduce_failure('6.38.0', b'AAAA') as a decorator on your test case
====================================================== short test summary info ======================================================
FAILED deal_test.py::test_count - AttributeError: 'NoneType' object has no attribute '_CallMemo'
========================================================= 1 failed in 0.81s =========================================================

This does not fail if you just call deal.cases(count)(). Looking into the failure, this is attempting to call typeguard when the dependency is not available. In the constructor, self.check_types defaults to True when not passed, but there is no explicit check that typeguard exists before using it, even though it should optional.

Related #44

That's the first step to get the typing support. The next step is to run mypy on the source code.

And probably fix some problems, annotate some missing functions, etc.