Unfortunately QCoreApplication cannot be promoted to QApplication. So for the moment we back out the change to have img.py work without QApplication. Instead, run ensure_app() in the test loader. Will need to investigate a proper fix at my leisure

setup/test.py

@@ -78,6 +78,8 @@ def add_options(self, parser):
 
     def run(self, opts):
         from calibre.utils.run_tests import run_cli, filter_tests_by_name
+        from calibre.gui2 import ensure_app
+        ensure_app()
         tests = find_tests(which_tests=frozenset(opts.test_module))
         if opts.test_name:
             tests = filter_tests_by_name(tests, *opts.test_name)

src/calibre/test_build.py

@@ -101,9 +101,6 @@ def test_apsw(self):
         conn.close()
 
     def test_qt(self):
-        # Must be run before QApplication is constructed
-        # Test that the image formats are available without a QApplication being
-        # constructed
         from PyQt5.Qt import QImageReader, QNetworkAccessManager, QFontDatabase
         from calibre.utils.img import image_from_data, image_to_data, test
         fmts = set(map(unicode, QImageReader.supportedImageFormats()))

src/calibre/utils/img.py

@@ -8,19 +8,12 @@
 from io import BytesIO
 from threading import Thread
 
-from PyQt5.Qt import QImage, QByteArray, QBuffer, Qt, QImageReader, QColor, QImageWriter, QTransform, QCoreApplication
-
+from PyQt5.Qt import QImage, QByteArray, QBuffer, Qt, QImageReader, QColor, QImageWriter, QTransform
 from calibre import fit_image, force_unicode
 from calibre.constants import iswindows, plugins, get_version
 from calibre.utils.config_base import tweaks
 from calibre.utils.filenames import atomic_rename
 
-# Ensure that Qt can load the imageformat plugins
-
-if QCoreApplication.instance() is None:
-    # We need a QCoreApplication() in order to load imageformat plugins
-    _core_app = QCoreApplication([])
-
 # Utilities {{{
 imageops, imageops_err = plugins['imageops']
 if imageops is None: