Finetune behavior for bare canvas usage (#464)

* Finetune behavior for bare canvas usage

* warn

tests/test_gui_base.py

@@ -3,6 +3,8 @@
 """
 
 import gc
+import sys
+import subprocess
 
 import numpy as np
 import wgpu.gui  # noqa
@@ -118,6 +120,39 @@ def present(self, texture):
         self.array = np.frombuffer(data, np.uint8).reshape(size[1], size[0], 4)
 
 
+@mark.skipif(not can_use_wgpu_lib, reason="Needs wgpu lib")
+def test_run_bare_canvas():
+    """Test that a bare canvas does not error."""
+
+    # This is (more or less) the equivalent of:
+    #
+    #     from wgpu.gui.auto import WgpuCanvas, run
+    #     canvas = WgpuCanvas()
+    #     run()
+    #
+    # Note: run() calls _draw_frame_and_present() in event loop.
+
+    canvas = MyOffscreenCanvas()
+    canvas._draw_frame_and_present()
+
+
+def test_canvas_context_not_base():
+    """Check that it is prevented that canvas context is instance of base context class."""
+    code = "from wgpu.gui import WgpuCanvasBase; canvas = WgpuCanvasBase(); canvas.get_context()"
+
+    result = subprocess.run(
+        [sys.executable, "-c", code],
+        stdout=subprocess.PIPE,
+        stderr=subprocess.STDOUT,
+        universal_newlines=True,
+    )
+    out = result.stdout.rstrip()
+
+    assert "RuntimeError" in out
+    assert "backend must be selected" in out.lower()
+    assert "canvas.get_context" in out.lower()
+
+
 @mark.skipif(not can_use_wgpu_lib, reason="Needs wgpu lib")
 def test_offscreen_canvas():
     canvas = MyOffscreenCanvas()

wgpu/backends/wgpu_native/_api.py

@@ -650,10 +650,9 @@ def _create_python_texture(self, texture_id):
 
     def present(self):
         if not self._texture:
-            msg = "present() is called without a preceeding call to "
-            msg += "get_current_texture(). Note that present() is usually "
-            msg += "called automatically after the draw function returns."
-            raise RuntimeError(msg)
+            # Log warning but don't raise exception
+            msg = "No texture to present, missing call to get_current_texture()?"
+            logger.warning(msg)
         else:
             # Present the texture, then destroy it
             # H: void f(WGPUSurface surface)

wgpu/gui/base.py

@@ -126,9 +126,13 @@ def get_context(self, kind="webgpu"):
         if self._canvas_context is None:
             # Get the active wgpu backend module
             backend_module = sys.modules["wgpu"].gpu.__module__
+            if backend_module == "wgpu._classes":
+                raise RuntimeError(
+                    "A backend must be selected (e.g. with request_adapter()) before canvas.get_context() can be called."
+                )
             # Instantiate the context
-            PC = sys.modules[backend_module].GPUCanvasContext  # noqa: N806
-            self._canvas_context = PC(self)
+            CC = sys.modules[backend_module].GPUCanvasContext  # noqa: N806
+            self._canvas_context = CC(self)
         return self._canvas_context