Ensure that image files aren't left open

changes/3730.bugfix.rst

@@ -0,0 +1 @@
+When creating a ``toga.Image`` from a file path, it's now guaranteed that the file won't remain open after construction.

testbed/tests/test_images.py

@@ -3,6 +3,7 @@
 import re
 import shutil
 from importlib import import_module
+from pathlib import Path
 
 import pytest
 from PIL import Image as PIL_Image, ImageDraw as PIL_ImageDraw
@@ -16,14 +17,21 @@ def image_probe(app, image):
 
 
 async def test_local_image(app):
-    "An image can be specified by filename"
+    """An image can be specified by filename"""
     image = toga.Image("resources/sample.png")
     assert image.width == 144
     assert image.height == 72
 
 
+async def test_closed_file_handle(app):
+    """The local image file isn't left open once the Image is created."""
+    _ = toga.Image("resources/sample.png")
+    # If the file still has an open handle, this should raise an IOError.
+    _ = Path(app.paths.app / "resources/sample.png").read_bytes()
+
+
 async def test_raw_image(app):
-    "An image can be created from the platform's raw representation"
+    """An image can be created from the platform's raw representation"""
     original = toga.Image("resources/sample.png")
 
     image = toga.Image(original._impl.native)
@@ -33,7 +41,7 @@ async def test_raw_image(app):
 
 
 async def test_bad_image_file(app):
-    "If a file isn't a loadable image, an error is raised"
+    """If a file isn't a loadable image, an error is raised"""
     with pytest.raises(
         ValueError,
         match=rf"Unable to load image from {re.escape(__file__)}",
@@ -42,7 +50,7 @@ async def test_bad_image_file(app):
 
 
 async def test_buffer_image(app):
-    "An image can be constructed from buffer data"
+    """An image can be constructed from buffer data"""
     # Generate an image using pillow
     pil_image = PIL_Image.new("RGBA", size=(110, 30))
     draw_context = PIL_ImageDraw.Draw(pil_image)
@@ -86,7 +94,7 @@ async def test_pil_raw_and_data_image(app):
 
 
 async def test_bad_image_data(app):
-    "If data isn't a valid image, an error is raised"
+    """If data isn't a valid image, an error is raised"""
     with pytest.raises(
         ValueError,
         match=r"Unable to load image from data",