Aaaah, now I get it :)

.travis.yml

@@ -2,10 +2,9 @@ sudo: false
 language: generic
 
 env:
-    - SW=$HOME/sw PATH=$SW/bin:$PATH CFLAGS=-I$SW/include LDFLAGS=-L$SW/lib LD_LIBRARY_PATH=$SW/qt/lib:$SW/lib PKG_CONFIG_PATH=$SW/lib/pkgconfig QMAKE=$SW/qt/bin/qmake QT_PLUGIN_PATH=$sw/qt/plugins
+    - SW=$HOME/sw PATH=$SW/bin:$PATH CFLAGS=-I$SW/include LDFLAGS=-L$SW/lib LD_LIBRARY_PATH=$SW/qt/lib:$SW/lib PKG_CONFIG_PATH=$SW/lib/pkgconfig QMAKE=$SW/qt/bin/qmake QT_PLUGIN_PATH=$SW/qt/plugins
 
 before_install:
     - curl https://download.calibre-ebook.com/travis/sw-linux.tar.xz | tar xJ -C $HOME
-    - python -c "from PyQt5.QtCore import QCoreApplication; print(QCoreApplication.libraryPaths())"
     - python setup.py bootstrap --ephemeral
 script: python setup.py test

setup/test.py

@@ -79,9 +79,6 @@ class Test(Command):
 
     def run(self, opts):
         from calibre.utils.run_tests import run_cli, filter_tests_by_name
-        if is_travis:
-            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

@@ -103,6 +103,13 @@ class BuildTest(unittest.TestCase):
     def test_qt(self):
         from PyQt5.Qt import QImageReader, QNetworkAccessManager, QFontDatabase
         from calibre.utils.img import image_from_data, image_to_data, test
+        # Ensure that images can be read before QApplication is constructed.
+        # Note that this requires QCoreApplication.libraryPaths() to return the
+        # path to the Qt plugins which it always does in the frozen build,
+        # because the QT_PLUGIN_PATH env var is set. On non-frozen builds,
+        # it should just work because the hard-coded paths of the Qt
+        # installation should work. If they do not, then it is a distro
+        # problem.
         fmts = set(map(unicode, QImageReader.supportedImageFormats()))
         testf = {'jpg', 'png', 'svg', 'ico', 'gif'}
         self.assertEqual(testf.intersection(fmts), testf, "Qt doesn't seem to be able to load some of its image plugins. Available plugins: %s" % fmts)