Add a test for circular imports in the project (#6375)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

tests/test_circular_imports.py

@@ -0,0 +1,83 @@
+"""Tests for circular imports in all local packages and modules.
+
+This ensures all internal packages can be imported right away without
+any need to import some other module before doing so.
+
+This module is based on an idea that pytest uses for self-testing:
+* https://github.com/sanitizers/octomachinery/blob/be18b54/tests/circular_imports_test.py
+* https://github.com/pytest-dev/pytest/blob/d18c75b/testing/test_meta.py
+* https://twitter.com/codewithanthony/status/1229445110510735361
+"""  # noqa: E501
+import os
+import pkgutil
+import subprocess
+import sys
+from itertools import chain
+from pathlib import Path
+from types import ModuleType
+from typing import Generator, List
+
+import pytest
+
+import aiohttp
+
+
+def _find_all_importables(pkg: ModuleType) -> List[str]:
+    """Find all importables in the project.
+
+    Return them in order.
+    """
+    return sorted(
+        set(
+            chain.from_iterable(
+                _discover_path_importables(Path(p), pkg.__name__)
+                # FIXME: Unignore after upgrading to `mypy > 0.910`. The fix
+                # FIXME: is in the `master` branch of upstream since Aug 4,
+                # FIXME: 2021 but has not yet been included in any releases.
+                # Refs:
+                # * https://github.com/python/mypy/issues/1422
+                # * https://github.com/python/mypy/pull/9454
+                for p in pkg.__path__  # type: ignore[attr-defined]
+            ),
+        ),
+    )
+
+
+def _discover_path_importables(
+    pkg_pth: Path,
+    pkg_name: str,
+) -> Generator[str, None, None]:
+    """Yield all importables under a given path and package."""
+    for dir_path, _d, file_names in os.walk(pkg_pth):
+        pkg_dir_path = Path(dir_path)
+
+        if pkg_dir_path.parts[-1] == "__pycache__":
+            continue
+
+        if all(Path(_).suffix != ".py" for _ in file_names):
+            continue
+
+        rel_pt = pkg_dir_path.relative_to(pkg_pth)
+        pkg_pref = ".".join((pkg_name,) + rel_pt.parts)
+        yield from (
+            pkg_path
+            for _, pkg_path, _ in pkgutil.walk_packages(
+                (str(pkg_dir_path),),
+                prefix=f"{pkg_pref}.",
+            )
+        )
+
+
+@pytest.mark.parametrize(
+    "import_path",
+    _find_all_importables(aiohttp),
+)
+def test_no_warnings(import_path: str) -> None:
+    """Verify that exploding importables doesn't explode.
+
+    This is seeking for any import errors including ones caused
+    by circular imports.
+    """
+    imp_cmd = sys.executable, "-W", "error"
+
+    subprocess.check_call(imp_cmd)