Fix lazy imports of objects on Python 3.6.

Author: aaugustin

src/websockets/imports.py

@@ -1,4 +1,3 @@
-import importlib
 import sys
 import warnings
 from typing import Any, Dict, Iterable, Optional
@@ -7,6 +6,27 @@
 __all__ = ["lazy_import"]
 
 
+def import_name(name: str, source: str, namespace: Dict[str, Any]) -> Any:
+    """
+    Import <name> from <source> in <namespace>.
+
+    There are two cases:
+
+    - <name> is an object defined in <source>
+    - <name> is a submodule of source
+
+    Neither __import__ nor importlib.import_module does exactly this.
+    __import__ is closer to the intended behavior.
+
+    """
+    level = 0
+    while source[level] == ".":
+        level += 1
+        assert level < len(source), "importing from parent isn't supported"
+    module = __import__(source[level:], namespace, None, [name], level)
+    return getattr(module, name)
+
+
 def lazy_import(
     namespace: Dict[str, Any],
     aliases: Optional[Dict[str, str]] = None,
@@ -58,8 +78,7 @@ def __getattr__(name: str) -> Any:
             except KeyError:
                 pass
             else:
-                module = importlib.import_module(source, package)
-                return getattr(module, name)
+                return import_name(name, source, namespace)
 
             assert deprecated_aliases is not None  # mypy cannot figure this out
             try:
@@ -72,8 +91,7 @@ def __getattr__(name: str) -> Any:
                     DeprecationWarning,
                     stacklevel=2,
                 )
-                module = importlib.import_module(source, package)
-                return getattr(module, name)
+                return import_name(name, source, namespace)
 
             raise AttributeError(f"module {package!r} has no attribute {name!r}")
 
@@ -87,9 +105,7 @@ def __dir__() -> Iterable[str]:
     else:  # pragma: no cover
 
         for name, source in aliases.items():
-            module = importlib.import_module(source, package)
-            namespace[name] = getattr(module, name)
+            namespace[name] = import_name(name, source, namespace)
 
         for name, source in deprecated_aliases.items():
-            module = importlib.import_module(source, package)
-            namespace[name] = getattr(module, name)
+            namespace[name] = import_name(name, source, namespace)

tests/test_imports.py

@@ -1,3 +1,4 @@
+import sys
 import types
 import unittest
 import warnings
@@ -11,18 +12,30 @@
 
 
 class ImportsTests(unittest.TestCase):
+    def setUp(self):
+        self.mod = types.ModuleType("tests.test_imports.test_alias")
+        self.mod.__package__ = self.mod.__name__
+
     def test_get_alias(self):
-        mod = types.ModuleType("tests.test_imports.test_alias")
-        lazy_import(vars(mod), aliases={"foo": ".."})
+        lazy_import(
+            vars(self.mod),
+            aliases={"foo": "...test_imports"},
+        )
 
-        self.assertEqual(mod.foo, foo)
+        self.assertEqual(self.mod.foo, foo)
 
     def test_get_deprecated_alias(self):
-        mod = types.ModuleType("tests.test_imports.test_alias")
-        lazy_import(vars(mod), deprecated_aliases={"bar": ".."})
+        lazy_import(
+            vars(self.mod),
+            deprecated_aliases={"bar": "...test_imports"},
+        )
 
         with warnings.catch_warnings(record=True) as recorded_warnings:
-            self.assertEqual(mod.bar, bar)
+            self.assertEqual(self.mod.bar, bar)
+
+        # No warnings raised on pre-PEP 526 Python.
+        if sys.version_info[:2] < (3, 7):  # pragma: no cover
+            return
 
         self.assertEqual(len(recorded_warnings), 1)
         warning = recorded_warnings[0].message
@@ -32,20 +45,22 @@ def test_get_deprecated_alias(self):
         self.assertEqual(type(warning), DeprecationWarning)
 
     def test_dir(self):
-        mod = types.ModuleType("tests.test_imports.test_alias")
-        lazy_import(vars(mod), aliases={"foo": ".."}, deprecated_aliases={"bar": ".."})
+        lazy_import(
+            vars(self.mod),
+            aliases={"foo": "...test_imports"},
+            deprecated_aliases={"bar": "...test_imports"},
+        )
 
         self.assertEqual(
-            [item for item in dir(mod) if not item[:2] == item[-2:] == "__"],
+            [item for item in dir(self.mod) if not item[:2] == item[-2:] == "__"],
             ["bar", "foo"],
         )
 
     def test_attribute_error(self):
-        mod = types.ModuleType("tests.test_imports.test_alias")
-        lazy_import(vars(mod))
+        lazy_import(vars(self.mod))
 
         with self.assertRaises(AttributeError) as raised:
-            mod.foo
+            self.mod.foo
 
         self.assertEqual(
             str(raised.exception),