Refactor autogui selection to take imported backends into account bet…

…ter (#384)

wgpu/gui/auto.py

@@ -24,32 +24,80 @@ def is_jupyter():
         return False
 
 
+def _load_backend(backend_name):
+    """Load a gui backend by name."""
+    if backend_name == "glfw":
+        from . import glfw as module  # noqa
+    elif backend_name == "qt":
+        from . import qt as module  # noqa
+    elif backend_name == "jupyter":
+        from . import jupyter as module  # noqa
+    elif backend_name == "wx":
+        from . import wx as module  # noqa
+    elif backend_name == "offscreen":
+        from . import offscreen as module  # noqa
+    else:  # no-cover
+        raise ImportError("Unknown wgpu gui backend: '{backend_name}'")
+    return module
+
+
+def _auto_load_backend():
+    # Backends to auto load, ordered by preference. Maps libname -> backend_name
+    gui_backends = {
+        "glfw": "glfw",
+        "PySide6": "qt",
+        "PyQt6": "qt",
+        "PySide2": "qt",
+        "PyQt5": "qt",
+    }
+
+    # The module that we try to find
+    module = None
+
+    # Any errors we come accross as we try to import the gui backends
+    errors = []
+
+    # Prefer a backend for which the lib is already imported
+    imported = [libname for libname in gui_backends if libname in sys.modules]
+    for libname in imported:
+        try:
+            module = _load_backend(gui_backends[libname])
+            break
+        except Exception as err:
+            errors.append(err)
+
+    # If no module found yet, try importing the lib, then import the backend
+    if not module:
+        for libname in gui_backends:
+            try:
+                importlib.import_module(libname)
+            except ModuleNotFoundError:
+                continue
+            try:
+                module = _load_backend(gui_backends[libname])
+            except Exception as err:
+                errors.append(err)
+
+    # If still nothing found, raise a useful error
+    if not module:
+        msg = "\n".join(str(err) for err in errors)
+        msg += "\n\n  Could not find either glfw or Qt framework."
+        msg += "\n  Install glfw using e.g. ``pip install -U glfw``,"
+        msg += "\n  or install a qt framework using e.g. ``pip install -U pyside6``."
+        if sys.platform.startswith("linux"):
+            msg += "\n  You may also need to run the equivalent of ``apt install libglfw3``."
+        raise ImportError(msg) from None
+
+    return module
+
+
+# Triage
 if os.environ.get("WGPU_FORCE_OFFSCREEN") == "true":
-    from .offscreen import WgpuCanvas, run, call_later  # noqa
+    module = _load_backend("offscreen")
 elif is_jupyter():
-    from .jupyter import WgpuCanvas, run, call_later  # noqa
+    module = _load_backend("jupyter")
 else:
-    try:
-        from .glfw import WgpuCanvas, run, call_later  # noqa
-    except ImportError as glfw_err:
-        qt_backends = ("PySide6", "PyQt6", "PySide2", "PyQt5")
-        for backend in qt_backends:
-            if backend in sys.modules:
-                break
-        else:
-            for libname in qt_backends:
-                try:
-                    importlib.import_module(libname)
-                    break
-                except ModuleNotFoundError:
-                    pass
-            else:
-                msg = str(glfw_err)
-                msg += "\n\n  Could not find either glfw or Qt framework."
-                msg += "\n  Install glfw using e.g. ``pip install -U glfw``,"
-                msg += "\n  or install a qt framework using e.g. ``pip install -U pyside6``."
-                if sys.platform.startswith("linux"):
-                    msg += "\n  You may also need to run the equivalent of ``apt install libglfw3``."
-                raise ImportError(msg) from None
-
-        from .qt import WgpuCanvas, run, call_later
+    module = _auto_load_backend()
+
+
+WgpuCanvas, run, call_later = module.WgpuCanvas, module.run, module.call_later