Addon: fix wheel loading module separation

The loading of modules from wheels wasn't properly separated from the rest
of Python yet. Now `load_wheel()` properly cleans up after itself, making
it impossible for other code to do `import the_module_from_the_wheel`.

addon/flamenco/wheels/__init__.py

@@ -3,25 +3,31 @@
 """External dependencies loader."""
 
 import contextlib
+import importlib
 from pathlib import Path
 import sys
 import logging
 from types import ModuleType
-from typing import Iterator, Optional
+from typing import Iterator
 
 _my_dir = Path(__file__).parent
 _log = logging.getLogger(__name__)
 
 
-def load_wheel(module_name: str, fname_prefix: str) -> ModuleType:
+def load_wheel(module_name: str, fname_prefix: str = "") -> ModuleType:
     """Loads a wheel from 'fname_prefix*.whl', unless the named module can be imported.
 
     This allows us to use system-installed packages before falling back to the shipped wheels.
     This is useful for development, less so for deployment.
+
+    If `fname_prefix` is the empty string, it will use the module name.
     """
 
+    if not fname_prefix:
+        fname_prefix = _fname_prefix_from_module_name(module_name)
+
     try:
-        module = __import__(module_name)
+        module = importlib.import_module(module_name)
     except ImportError as ex:
         _log.debug("Unable to import %s directly, will try wheel: %s", module_name, ex)
     else:
@@ -42,7 +48,7 @@ def load_wheel(module_name: str, fname_prefix: str) -> ModuleType:
     # this add-on from other add-ons.
     with _sys_path_mod_backup(wheel):
         try:
-            module = __import__(module_name)
+            module = importlib.import_module(module_name)
         except ImportError as ex:
             raise ImportError(
                 "Unable to load %r from %s: %s" % (module_name, wheel, ex)
@@ -53,15 +59,21 @@ def load_wheel(module_name: str, fname_prefix: str) -> ModuleType:
     return module
 
 
-def load_wheel_global(module_name: str, fname_prefix: str) -> ModuleType:
+def load_wheel_global(module_name: str, fname_prefix: str = "") -> ModuleType:
     """Loads a wheel from 'fname_prefix*.whl', unless the named module can be imported.
 
     This allows us to use system-installed packages before falling back to the shipped wheels.
     This is useful for development, less so for deployment.
+
+    If `fname_prefix` is the empty string, it will use the first package from `module_name`.
+    In other words, `module_name="pkg.subpkg"` will result in `fname_prefix="pkg"`.
     """
 
+    if not fname_prefix:
+        fname_prefix = _fname_prefix_from_module_name(module_name)
+
     try:
-        module = __import__(module_name)
+        module = importlib.import_module(module_name)
     except ImportError as ex:
         _log.debug("Unable to import %s directly, will try wheel: %s", module_name, ex)
     else:
@@ -80,7 +92,7 @@ def load_wheel_global(module_name: str, fname_prefix: str) -> ModuleType:
         sys.path.insert(0, wheel_filepath)
 
     try:
-        module = __import__(module_name)
+        module = importlib.import_module(module_name)
     except ImportError as ex:
         raise ImportError(
             "Unable to load %r from %s: %s" % (module_name, wheel, ex)
@@ -92,16 +104,24 @@ def load_wheel_global(module_name: str, fname_prefix: str) -> ModuleType:
 
 @contextlib.contextmanager
 def _sys_path_mod_backup(wheel_file: Path) -> Iterator[None]:
+    """Temporarily inserts a wheel onto sys.path.
+
+    When the context exits, it restores sys.path and sys.modules, so that
+    anything that was imported within the context remains unimportable by other
+    modules.
+    """
     old_syspath = sys.path[:]
+    old_sysmod = sys.modules.copy()
 
     try:
         sys.path.insert(0, str(wheel_file))
         yield
     finally:
-        # Restore without assigning new instances. That way references held by
-        # other code will stay valid.
-
+        # Restore without assigning a new list instance. That way references
+        # held by other code will stay valid.
         sys.path[:] = old_syspath
+        sys.modules.clear()
+        sys.modules.update(old_sysmod)
 
 
 def _wheel_filename(fname_prefix: str) -> Path:
@@ -119,6 +139,10 @@ def _wheel_filename(fname_prefix: str) -> Path:
     return wheels[-1]
 
 
+def _fname_prefix_from_module_name(module_name: str) -> str:
+    return module_name.split(".", 1)[0]
+
+
 if __name__ == "__main__":
     logging.basicConfig(level=logging.DEBUG)
     wheel = _wheel_filename("python_dateutil")