Add matcher for exceptions in asyncio future

doc/tutorial.rst

@@ -89,6 +89,39 @@ assure that the right issue was found::
     # assert_that(23, raises(IOError))
 
 
+Asserting exceptions from async methods
+---------------------------------------
+
+An async method does not directly return the result or raise an exception but
+instead returns a Future-object that represent the async operation that can
+later be resolved with the `await` keyword. The
+:py:func:`~hamcrest.core.core.future.resolved` utility function can be used to
+wait for a future to be done but without retrieving the value or raising the
+exception. The :py:func:`~hamcrest.core.core.future.future_raising` matcher can
+be used with any future object but combined lets you assert that calling some
+async method, and waiting for the result, causes an exception to be raised.
+
+This is best used together with an async test runner like IsolatedAsyncioTestCase or pytest-asyncio::
+
+    async def parse(input: str):
+        ...
+
+    class Test(unittest.IsolatedAsyncioTestCase):
+
+        async def testParse(self):
+            future = parse("some bad data")
+            assert_that(await resolved(future), future_raising(ValueError))
+
+But it's possible to use with an async unware runner by explicitly running the event loop in the test::
+
+    class Test(unittest.TestCase):
+        def test_parse(self):
+            async def test():
+                future = parse("some bad data")
+                assert_that(await resolved(future), future_raising(ValueError))
+
+            asyncio.get_event_loop().run_until_complete(test())
+
 
 Predefined matchers
 -------------------

src/hamcrest/core/core/future.py

@@ -0,0 +1,137 @@
+import sys
+import re
+import asyncio
+from typing import (
+    Optional,
+    Type,
+    TypeVar,
+    Union,
+    Awaitable,
+)
+
+from hamcrest.core.base_matcher import BaseMatcher
+from hamcrest.core.description import Description
+from hamcrest.core.matcher import Matcher
+
+__author__ = "David Keijser"
+__copyright__ = "Copyright 2021 hamcrest.org"
+__license__ = "BSD, see License.txt"
+
+T = TypeVar("T")
+
+if sys.version_info > (3, 9):
+    # Same as used in typeshed for asyncio.ensure_future
+    FutureT = asyncio.Future[T]
+    FutureLike = Union[asyncio.Future[T], Awaitable[T]]
+else:
+    # Future is not a parametrised type in earlier version of python
+    FutureT = asyncio.Future
+    FutureLike = Union[asyncio.Future, Awaitable]
+
+
+class FutureRaising(BaseMatcher[asyncio.Future]):
+    def __init__(
+        self,
+        expected: Type[Exception],
+        pattern: Optional[str] = None,
+        matching: Optional[Matcher] = None,
+    ) -> None:
+        self.pattern = pattern
+        self.matcher = matching
+        self.expected = expected
+
+    def _matches(self, future: asyncio.Future) -> bool:
+        if not asyncio.isfuture(future):
+            return False
+
+        if not future.done():
+            return False
+
+        if future.cancelled():
+            return False
+
+        exc = future.exception()
+        if exc is None:
+            return False
+
+        if isinstance(exc, self.expected):
+            if self.pattern is not None:
+                if re.search(self.pattern, str(exc)) is None:
+                    return False
+            if self.matcher is not None:
+                if not self.matcher.matches(exc):
+                    return False
+            return True
+
+        return False
+
+    def describe_to(self, description: Description) -> None:
+        description.append_text("Expected a completed future with exception %s" % self.expected)
+
+    def describe_mismatch(self, future: asyncio.Future, description: Description) -> None:
+        if not asyncio.isfuture(future):
+            description.append_text("%s is not a future" % future)
+            return
+
+        if not future.done():
+            description.append_text("%s is not completed yet" % future)
+            return
+
+        if future.cancelled():
+            description.append_text("%s is cancelled" % future)
+            return
+
+        exc = future.exception()
+        if exc is None:
+            description.append_text("No exception raised.")
+        elif isinstance(exc, self.expected):
+            if self.pattern is not None or self.matcher is not None:
+                description.append_text("Correct assertion type raised, but ")
+                if self.pattern is not None:
+                    description.append_text('the expected pattern ("%s") ' % self.pattern)
+                if self.pattern is not None and self.matcher is not None:
+                    description.append_text("and ")
+                if self.matcher is not None:
+                    description.append_description_of(self.matcher)
+                    description.append_text(" ")
+                description.append_text('not found. Exception message was: "%s"' % str(exc))
+        else:
+            description.append_text("%r of type %s was raised instead" % (exc, type(exc)))
+
+    def describe_match(self, future: asyncio.Future, match_description: Description) -> None:
+        exc = future.exception()
+        match_description.append_text("%r of type %s was raised." % (exc, type(exc)))
+
+
+def future_raising(
+    exception: Type[Exception], pattern=None, matching=None
+) -> Matcher[asyncio.Future]:
+    """Matches a future with the expected exception.
+
+    :param exception:  The class of the expected exception
+    :param pattern:    Optional regular expression to match exception message.
+    :param matching:   Optional Hamcrest matchers to apply to the exception.
+
+    Expects the actual to be an already resolved future. The :py:func:`~hamcrest:core.core.future.resolved` helper can be used to wait for a future to resolve.
+    Optional argument pattern should be a string containing a regular expression.  If provided,
+    the string representation of the actual exception - e.g. `str(actual)` - must match pattern.
+
+    Examples::
+
+        assert_that(somefuture, future_exception(ValueError))
+        assert_that(
+            await resolved(async_http_get()),
+            future_exception(HTTPError, matching=has_properties(status_code=500)
+        )
+    """
+    return FutureRaising(exception, pattern, matching)
+
+
+async def resolved(obj: FutureLike) -> FutureT:
+    """Wait for an async operation to finish and return a resolved future object with the result.
+
+    :param obj: A future like object or an awaitable object.
+    """
+    fut = asyncio.ensure_future(obj)
+    await asyncio.wait([fut])
+    return fut

tests/hamcrest_unit_test/core/future_test.py

@@ -0,0 +1,184 @@
+import sys
+
+import pytest
+import asyncio
+from hamcrest import has_properties
+from hamcrest.core.core.future import resolved, future_raising
+from hamcrest_unit_test.matcher_test import MatcherTest
+
+if __name__ == "__main__":
+    sys.path.insert(0, "..")
+    sys.path.insert(0, "../..")
+
+
+__author__ = "David Keijser"
+__copyright__ = "Copyright 2023 hamcrest.org"
+__license__ = "BSD, see License.txt"
+
+
+async def no_exception(*args, **kwargs):
+    return
+
+
+async def raise_exception(*args, **kwargs):
+    raise AssertionError(str(args) + str(kwargs))
+
+
+async def raise_exception_with_properties(**kwargs):
+    err = AssertionError("boom")
+    for k, v in kwargs.items():
+        setattr(err, k, v)
+    raise err
+
+
+# From python 3.8 this could be simplified by using unittest.IsolatedAsyncioTestCase
+class FutureExceptionTest(MatcherTest):
+    def testMatchesIfFutureHasTheExactExceptionExpected(self):
+        async def test():
+            self.assert_matches(
+                "Right exception",
+                future_raising(AssertionError),
+                await resolved(raise_exception()),
+            )
+
+        asyncio.get_event_loop().run_until_complete(test())
+
+    def testDoesNotMatchIfActualIsNotAFuture(self):
+        async def test():
+            self.assert_does_not_match("Not a future", future_raising(TypeError), 23)
+
+        asyncio.get_event_loop().run_until_complete(test())
+
+    def testDoesNotMatchIfFutureIsNotDone(self):
+        future = asyncio.Future()
+        self.assert_does_not_match("Unresolved future", future_raising(TypeError), future)
+
+    def testDoesNotMatchIfFutureIsCancelled(self):
+        future = asyncio.Future()
+        future.cancel()
+        self.assert_does_not_match("Cancelled future", future_raising(TypeError), future)
+
+    @pytest.mark.skipif(
+        not (3, 0) <= sys.version_info < (3, 7), reason="Message differs between Python versions"
+    )
+    def testDoesNotMatchIfFutureHasTheWrongExceptionTypePy3(self):
+        async def test():
+            self.assert_does_not_match(
+                "Wrong exception", future_raising(IOError), await resolved(raise_exception())
+            )
+            expected_message = (
+                "AssertionError('(){}',) of type <class 'AssertionError'> was raised instead"
+            )
+            self.assert_mismatch_description(
+                expected_message, future_raising(TypeError), await resolved(raise_exception())
+            )
+
+        asyncio.get_event_loop().run_until_complete(test())
+
+    @pytest.mark.skipif(sys.version_info < (3, 7), reason="Message differs between Python versions")
+    def testDoesNotMatchIfFutureHasTheWrongExceptionTypePy37(self):
+        async def test():
+            self.assert_does_not_match(
+                "Wrong exception", future_raising(IOError), await resolved(raise_exception())
+            )
+            expected_message = (
+                "AssertionError('(){}') of type <class 'AssertionError'> was raised instead"
+            )
+            self.assert_mismatch_description(
+                expected_message, future_raising(TypeError), await resolved(raise_exception())
+            )
+
+        asyncio.get_event_loop().run_until_complete(test())
+
+    def testMatchesIfFutureHasASubclassOfTheExpectedException(self):
+        async def test():
+            self.assert_matches(
+                "Subclassed Exception",
+                future_raising(Exception),
+                await resolved(raise_exception()),
+            )
+
+        asyncio.get_event_loop().run_until_complete(test())
+
+    def testDoesNotMatchIfFutureDoesNotHaveException(self):
+        async def test():
+            self.assert_does_not_match(
+                "No exception", future_raising(ValueError), await resolved(no_exception())
+            )
+
+        asyncio.get_event_loop().run_until_complete(test())
+
+    def testDoesNotMatchExceptionIfRegularExpressionDoesNotMatch(self):
+        async def test():
+            self.assert_does_not_match(
+                "Bad regex",
+                future_raising(AssertionError, "Phrase not found"),
+                await resolved(raise_exception()),
+            )
+            self.assert_mismatch_description(
+                '''Correct assertion type raised, but the expected pattern ("Phrase not found") not found. Exception message was: "(){}"''',
+                future_raising(AssertionError, "Phrase not found"),
+                await resolved(raise_exception()),
+            )
+
+        asyncio.get_event_loop().run_until_complete(test())
+
+    def testMatchesRegularExpressionToStringifiedException(self):
+        async def test():
+            self.assert_matches(
+                "Regex",
+                future_raising(AssertionError, "(3, 1, 4)"),
+                await resolved(raise_exception(3, 1, 4)),
+            )
+
+            self.assert_matches(
+                "Regex",
+                future_raising(AssertionError, r"([\d, ]+)"),
+                await resolved(raise_exception(3, 1, 4)),
+            )
+
+        asyncio.get_event_loop().run_until_complete(test())
+
+    def testMachesIfExceptionMatchesAdditionalMatchers(self):
+        async def test():
+            self.assert_matches(
+                "Properties",
+                future_raising(AssertionError, matching=has_properties(prip="prop")),
+                await resolved(raise_exception_with_properties(prip="prop")),
+            )
+
+        asyncio.get_event_loop().run_until_complete(test())
+
+    def testDoesNotMatchIfAdditionalMatchersDoesNotMatch(self):
+        async def test():
+            self.assert_does_not_match(
+                "Bad properties",
+                future_raising(AssertionError, matching=has_properties(prop="prip")),
+                await resolved(raise_exception_with_properties(prip="prop")),
+            )
+            self.assert_mismatch_description(
+                '''Correct assertion type raised, but an object with a property 'prop' matching 'prip' not found. Exception message was: "boom"''',
+                future_raising(AssertionError, matching=has_properties(prop="prip")),
+                await resolved(raise_exception_with_properties(prip="prop")),
+            )
+
+        asyncio.get_event_loop().run_until_complete(test())
+
+    def testDoesNotMatchIfNeitherPatternOrMatcherMatch(self):
+        async def test():
+            self.assert_does_not_match(
+                "Bad pattern and properties",
+                future_raising(
+                    AssertionError, pattern="asdf", matching=has_properties(prop="prip")
+                ),
+                await resolved(raise_exception_with_properties(prip="prop")),
+            )
+            self.assert_mismatch_description(
+                '''Correct assertion type raised, but the expected pattern ("asdf") and an object with a property 'prop' matching 'prip' not found. Exception message was: "boom"''',
+                future_raising(
+                    AssertionError, pattern="asdf", matching=has_properties(prop="prip")
+                ),
+                await resolved(raise_exception_with_properties(prip="prop")),
+            )
+
+        asyncio.get_event_loop().run_until_complete(test())