trio101: never yield inside a nursery or cancel scope

flake8_trio.py

@@ -11,7 +11,7 @@
 
 import ast
 import tokenize
-from typing import Any, Generator, Iterable, List, Optional, Tuple, Type, Union
+from typing import Any, Generator, Iterable, List, Optional, Set, Tuple, Type, Union
 
 # CalVer: YY.month.patch, e.g. first release of July 2022 == "22.7.1"
 __version__ = "22.7.1"
@@ -40,13 +40,24 @@ class Visitor(ast.NodeVisitor):
     def __init__(self) -> None:
         super().__init__()
         self.problems: List[Error] = []
+        self.safe_yields: Set[ast.Yield] = set()
 
     def visit_With(self, node: ast.With) -> None:
         self.check_for_trio100(node)
+        self.check_for_trio101(node)
         self.generic_visit(node)
 
     def visit_AsyncWith(self, node: ast.AsyncWith) -> None:
         self.check_for_trio100(node)
+        self.check_for_trio101(node)
+        self.generic_visit(node)
+
+    def visit_FunctionDef(self, node: ast.FunctionDef) -> None:
+        self.trio101_mark_yields_safe(node)
+        self.generic_visit(node)
+
+    def visit_AsyncFunctionDef(self, node: ast.AsyncFunctionDef) -> None:
+        self.trio101_mark_yields_safe(node)
         self.generic_visit(node)
 
     def check_for_trio100(self, node: Union[ast.With, ast.AsyncWith]) -> None:
@@ -58,6 +69,36 @@ def check_for_trio100(self, node: Union[ast.With, ast.AsyncWith]) -> None:
                     make_error(TRIO100, item.lineno, item.col_offset, (call,))
                 )
 
+    def trio101_mark_yields_safe(
+        self, node: Union[ast.FunctionDef, ast.AsyncFunctionDef]
+    ) -> None:
+        if any(
+            isinstance(d, ast.Name)
+            and d.id in ("contextmanager", "asynccontextmanager")
+            for d in node.decorator_list
+        ):
+            self.safe_yields.update(
+                {x for x in ast.walk(node) if isinstance(x, ast.Yield)}
+            )
+
+    def check_for_trio101(self, node: Union[ast.With, ast.AsyncWith]) -> None:
+        for item in (i.context_expr for i in node.items):
+            call = is_trio_call(
+                item,
+                "open_nursery",
+                "fail_after",
+                "fail_at",
+                "move_on_after",
+                "move_at",
+            )
+            if call and any(
+                isinstance(x, ast.Yield) and x not in self.safe_yields
+                for x in ast.walk(node)
+            ):
+                self.problems.append(
+                    make_error(TRIO101, item.lineno, item.col_offset, (call,))
+                )
+
 
 class Plugin:
     name = __name__
@@ -83,3 +124,4 @@ def run(self) -> Generator[Tuple[int, int, str, Type[Any]], None, None]:
 
 
 TRIO100 = "TRIO100: {} context contains no checkpoints, add `await trio.sleep(0)`"
+TRIO101 = "TRIO101: {} never yield inside a nursery or cancel scope"

tests/test_flake8_trio.py

@@ -9,7 +9,7 @@
 from hypothesis import HealthCheck, given, settings
 from hypothesmith import from_grammar
 
-from flake8_trio import TRIO100, Error, Plugin, Visitor, make_error
+from flake8_trio import TRIO100, TRIO101, Error, Plugin, Visitor, make_error
 
 
 class Flake8TrioTestCase(unittest.TestCase):
@@ -40,6 +40,13 @@ def test_trio100_py39(self):
             make_error(TRIO100, 14, 8, ("trio.move_on_after",)),
         )
 
+    def test_trio101(self):
+        self.assert_expected_errors(
+            "trio101.py",
+            make_error(TRIO101, 7, 9, ("trio.open_nursery",)),
+            make_error(TRIO101, 12, 15, ("trio.open_nursery",)),
+        )
+
 
 @pytest.mark.fuzz
 class TestFuzz(unittest.TestCase):

tests/trio101.py

@@ -0,0 +1,25 @@
+from contextlib import asynccontextmanager, contextmanager
+
+import trio
+
+
+def foo0():
+    with trio.open_nursery() as _:
+        yield 1
+
+
+async def foo1():
+    async with trio.open_nursery() as _:
+        yield 1
+
+
+@contextmanager
+def foo2():
+    with trio.open_nursery() as _:
+        yield 1
+
+
+@asynccontextmanager
+async def foo3():
+    async with trio.open_nursery() as _:
+        yield 1