Fixed unhelpful error message in from_thread_run functions.

newsfragments/1244.bugfix.rst

@@ -0,0 +1 @@
+Added a helpful error message if an async function is passed to `trio.from_thread.run_sync` or a sync function to `trio.from_thread.run`.

trio/_core/_run.py

@@ -17,7 +17,6 @@
 from sniffio import current_async_library_cvar
 
 import attr
-from async_generator import isasyncgen
 from sortedcontainers import SortedDict
 from outcome import Error, Value, capture
 
@@ -36,7 +35,7 @@
 )
 from .. import _core
 from .._deprecate import deprecated
-from .._util import Final, NoPublicConstructor
+from .._util import Final, NoPublicConstructor, coroutine_or_error
 
 _NO_SEND = object()
 
@@ -1247,86 +1246,7 @@ def spawn_impl(self, async_fn, args, nursery, name, *, system_task=False):
         # Call the function and get the coroutine object, while giving helpful
         # errors for common mistakes.
         ######
-
-        def _return_value_looks_like_wrong_library(value):
-            # Returned by legacy @asyncio.coroutine functions, which includes
-            # a surprising proportion of asyncio builtins.
-            if isinstance(value, collections.abc.Generator):
-                return True
-            # The protocol for detecting an asyncio Future-like object
-            if getattr(value, "_asyncio_future_blocking", None) is not None:
-                return True
-            # This janky check catches tornado Futures and twisted Deferreds.
-            # By the time we're calling this function, we already know
-            # something has gone wrong, so a heuristic is pretty safe.
-            if value.__class__.__name__ in ("Future", "Deferred"):
-                return True
-            return False
-
-        try:
-            coro = async_fn(*args)
-        except TypeError:
-            # Give good error for: nursery.start_soon(trio.sleep(1))
-            if isinstance(async_fn, collections.abc.Coroutine):
-                raise TypeError(
-                    "Trio was expecting an async function, but instead it got "
-                    "a coroutine object {async_fn!r}\n"
-                    "\n"
-                    "Probably you did something like:\n"
-                    "\n"
-                    "  trio.run({async_fn.__name__}(...))            # incorrect!\n"
-                    "  nursery.start_soon({async_fn.__name__}(...))  # incorrect!\n"
-                    "\n"
-                    "Instead, you want (notice the parentheses!):\n"
-                    "\n"
-                    "  trio.run({async_fn.__name__}, ...)            # correct!\n"
-                    "  nursery.start_soon({async_fn.__name__}, ...)  # correct!"
-                    .format(async_fn=async_fn)
-                ) from None
-
-            # Give good error for: nursery.start_soon(future)
-            if _return_value_looks_like_wrong_library(async_fn):
-                raise TypeError(
-                    "Trio was expecting an async function, but instead it got "
-                    "{!r} – are you trying to use a library written for "
-                    "asyncio/twisted/tornado or similar? That won't work "
-                    "without some sort of compatibility shim."
-                    .format(async_fn)
-                ) from None
-
-            raise
-
-        # We can't check iscoroutinefunction(async_fn), because that will fail
-        # for things like functools.partial objects wrapping an async
-        # function. So we have to just call it and then check whether the
-        # return value is a coroutine object.
-        if not isinstance(coro, collections.abc.Coroutine):
-            # Give good error for: nursery.start_soon(func_returning_future)
-            if _return_value_looks_like_wrong_library(coro):
-                raise TypeError(
-                    "start_soon got unexpected {!r} – are you trying to use a "
-                    "library written for asyncio/twisted/tornado or similar? "
-                    "That won't work without some sort of compatibility shim."
-                    .format(coro)
-                )
-
-            if isasyncgen(coro):
-                raise TypeError(
-                    "start_soon expected an async function but got an async "
-                    "generator {!r}".format(coro)
-                )
-
-            # Give good error for: nursery.start_soon(some_sync_fn)
-            raise TypeError(
-                "Trio expected an async function, but {!r} appears to be "
-                "synchronous".format(
-                    getattr(async_fn, "__qualname__", async_fn)
-                )
-            )
-
-        ######
-        # Set up the Task object
-        ######
+        coro = coroutine_or_error(async_fn, *args)
 
         if name is None:
             name = async_fn
@@ -1353,6 +1273,9 @@ async def python_wrapper(orig_coro):
             LOCALS_KEY_KI_PROTECTION_ENABLED, system_task
         )
 
+        ######
+        # Set up the Task object
+        ######
         task = Task._create(
             coro=coro,
             parent_nursery=nursery,

trio/_core/tests/test_run.py

@@ -15,7 +15,11 @@
 import sniffio
 import pytest
 
-from .tutil import slow, check_sequence_matches, gc_collect_harder
+from .tutil import (
+    slow, check_sequence_matches, gc_collect_harder,
+    ignore_coroutine_never_awaited_warnings
+)
+
 from ... import _core
 from ..._threads import to_thread_run_sync
 from ..._timeouts import sleep, fail_after
@@ -33,24 +37,6 @@ async def sleep_forever():
     return await _core.wait_task_rescheduled(lambda _: _core.Abort.SUCCEEDED)
 
 
-# Some of our tests need to leak coroutines, and thus trigger the
-# "RuntimeWarning: coroutine '...' was never awaited" message. This context
-# manager should be used anywhere this happens to hide those messages, because
-# when expected they're clutter.
-@contextmanager
-def ignore_coroutine_never_awaited_warnings():
-    with warnings.catch_warnings():
-        warnings.filterwarnings(
-            "ignore", message="coroutine '.*' was never awaited"
-        )
-        try:
-            yield
-        finally:
-            # Make sure to trigger any coroutine __del__ methods now, before
-            # we leave the context manager.
-            gc_collect_harder()
-
-
 def test_basic():
     async def trivial(x):
         return x
@@ -1696,8 +1682,6 @@ async def test_current_effective_deadline(mock_clock):
     assert _core.current_effective_deadline() == inf
 
 
-# @coroutine is deprecated since python 3.8, which is fine with us.
-@pytest.mark.filterwarnings("ignore:.*@coroutine.*:DeprecationWarning")
 def test_nice_error_on_bad_calls_to_run_or_spawn():
     def bad_call_run(*args):
         _core.run(*args)
@@ -1709,59 +1693,22 @@ async def main():
 
         _core.run(main)
 
-    class Deferred:
-        "Just kidding"
-
-    with ignore_coroutine_never_awaited_warnings():
-        for bad_call in bad_call_run, bad_call_spawn:
-
-            async def f():  # pragma: no cover
-                pass
-
-            with pytest.raises(TypeError) as excinfo:
-                bad_call(f())
-            assert "expecting an async function" in str(excinfo.value)
-
-            import asyncio
-
-            @asyncio.coroutine
-            def generator_based_coro():  # pragma: no cover
-                yield from asyncio.sleep(1)
-
-            with pytest.raises(TypeError) as excinfo:
-                bad_call(generator_based_coro())
-            assert "asyncio" in str(excinfo.value)
+    for bad_call in bad_call_run, bad_call_spawn:
 
-            with pytest.raises(TypeError) as excinfo:
-                bad_call(asyncio.Future())
-            assert "asyncio" in str(excinfo.value)
-
-            with pytest.raises(TypeError) as excinfo:
-                bad_call(lambda: asyncio.Future())
-            assert "asyncio" in str(excinfo.value)
-
-            with pytest.raises(TypeError) as excinfo:
-                bad_call(Deferred())
-            assert "twisted" in str(excinfo.value)
-
-            with pytest.raises(TypeError) as excinfo:
-                bad_call(lambda: Deferred())
-            assert "twisted" in str(excinfo.value)
-
-            with pytest.raises(TypeError) as excinfo:
-                bad_call(len, [1, 2, 3])
-            assert "appears to be synchronous" in str(excinfo.value)
+        async def f():  # pragma: no cover
+            pass
 
-            async def async_gen(arg):  # pragma: no cover
-                yield
+        with pytest.raises(TypeError, match="expecting an async function"):
+            bad_call(f())
 
-            with pytest.raises(TypeError) as excinfo:
-                bad_call(async_gen, 0)
-            msg = "expected an async function but got an async generator"
-            assert msg in str(excinfo.value)
+        async def async_gen(arg):  # pragma: no cover
+            yield arg
 
-            # Make sure no references are kept around to keep anything alive
-            del excinfo
+        with pytest.raises(
+            TypeError,
+            match="expected an async function but got an async generator"
+        ):
+            bad_call(async_gen, 0)
 
 
 def test_calling_asyncio_function_gives_nice_error():

trio/_core/tests/tutil.py

@@ -3,6 +3,8 @@
 import os
 
 import pytest
+import warnings
+from contextlib import contextmanager
 
 import gc
 
@@ -52,6 +54,24 @@ def gc_collect_harder():
         gc.collect()
 
 
+# Some of our tests need to leak coroutines, and thus trigger the
+# "RuntimeWarning: coroutine '...' was never awaited" message. This context
+# manager should be used anywhere this happens to hide those messages, because
+# when expected they're clutter.
+@contextmanager
+def ignore_coroutine_never_awaited_warnings():
+    with warnings.catch_warnings():
+        warnings.filterwarnings(
+            "ignore", message="coroutine '.*' was never awaited"
+        )
+        try:
+            yield
+        finally:
+            # Make sure to trigger any coroutine __del__ methods now, before
+            # we leave the context manager.
+            gc_collect_harder()
+
+
 # template is like:
 #   [1, {2.1, 2.2}, 3] -> matches [1, 2.1, 2.2, 3] or [1, 2.2, 2.1, 3]
 def check_sequence_matches(seq, template):

trio/_threads.py

@@ -3,12 +3,14 @@
 from itertools import count
 
 import attr
+import inspect
 import outcome
 
 import trio
 
 from ._sync import CapacityLimiter
 from ._core import enable_ki_protection, disable_ki_protection, RunVar, TrioToken
+from ._util import coroutine_or_error
 
 # Global due to Threading API, thread local storage for trio token
 TOKEN_LOCAL = threading.local()
@@ -365,6 +367,7 @@ def from_thread_run(afn, *args, trio_token=None):
             which would otherwise cause a deadlock.
         AttributeError: if no ``trio_token`` was provided, and we can't infer
             one from context.
+        TypeError: if ``afn`` is not an asynchronous function.
 
     **Locating a Trio Token**: There are two ways to specify which
     `trio.run` loop to reenter:
@@ -380,7 +383,8 @@ def from_thread_run(afn, *args, trio_token=None):
     def callback(q, afn, args):
         @disable_ki_protection
         async def unprotected_afn():
-            return await afn(*args)
+            coro = coroutine_or_error(afn, *args)
+            return await coro
 
         async def await_in_trio_thread_task():
             q.put_nowait(await outcome.acapture(unprotected_afn))
@@ -403,13 +407,11 @@ def from_thread_run_sync(fn, *args, trio_token=None):
     Raises:
         RunFinishedError: if the corresponding call to `trio.run` has
             already completed.
-        Cancelled: if the corresponding call to `trio.run` completes
-            while ``afn(*args)`` is running, then ``afn`` is likely to raise
-            :exc:`trio.Cancelled`, and this will propagate out into
         RuntimeError: if you try calling this from inside the Trio thread,
             which would otherwise cause a deadlock.
         AttributeError: if no ``trio_token`` was provided, and we can't infer
             one from context.
+        TypeError: if ``fn`` is an async function.
 
     **Locating a Trio Token**: There are two ways to specify which
     `trio.run` loop to reenter:
@@ -425,7 +427,17 @@ def from_thread_run_sync(fn, *args, trio_token=None):
     def callback(q, fn, args):
         @disable_ki_protection
         def unprotected_fn():
-            return fn(*args)
+            ret = fn(*args)
+
+            if inspect.iscoroutine(ret):
+                # Manually close coroutine to avoid RuntimeWarnings
+                ret.close()
+                raise TypeError(
+                    "Trio expected a sync function, but {!r} appears to be "
+                    "asynchronous".format(getattr(fn, "__qualname__", fn))
+                )
+
+            return ret
 
         res = outcome.capture(unprotected_fn)
         q.put_nowait(res)