Nice error messages on import errors

TODO

@@ -122,7 +122,6 @@ Minor features
 - Hiding scrollbars
 - Ctrl+A/X to increase/decrease last number in URL
 - logging contexts
-- catch import errors for PyQt and QtWebKit
 - Add more element-selection-detection code (with options?) based on:
       -> javascript: http://stackoverflow.com/a/2848120/2085149
       -> microFocusChanged and check active element via:

qutebrowser/app.py

@@ -17,6 +17,26 @@
 
 """Initialization of qutebrowser and application-wide things."""
 
+### Things we want to do before normal imports ###
+
+# Print a nice traceback on segfault -- only available on Python 3.3+, but if
+# it's unavailable, it doesn't matter much.
+try:
+    import faulthandler  # pylint: disable=import-error
+except ImportError:
+    pass
+else:
+    faulthandler.enable()
+
+# See https://bugreports.qt-project.org/browse/QTBUG-36099
+# We need to do this before importing PyQt
+import qutebrowser.utils.harfbuzz as harfbuzz
+harfbuzz.fix()
+
+# Check if everything we should be able to import is there
+import qutebrowser.utils.dependencies as dependencies
+dependencies.check()
+
 import os
 import sys
 import logging
@@ -27,20 +47,6 @@ from functools import partial
 from signal import signal, SIGINT
 from argparse import ArgumentParser
 from base64 import b64encode
-# Print a nice traceback on segfault -- only available on Python 3.3+, but if
-# it's unavailable, it doesn't matter much.
-try:
-    import faulthandler  # pylint: disable=import-error
-except ImportError:
-    pass
-else:
-    faulthandler.enable()
-
-# This is a really odd place to do this, but we have to do this before
-# importing PyQt or it won't work.
-# See https://bugreports.qt-project.org/browse/QTBUG-36099
-import qutebrowser.utils.harfbuzz as harfbuzz
-harfbuzz.fix()
 
 from PyQt5.QtWidgets import QApplication, QDialog, QMessageBox
 from PyQt5.QtCore import pyqtSlot, QTimer, QEventLoop

qutebrowser/utils/dependencies.py

@@ -0,0 +1,90 @@
+# Copyright 2014 Florian Bruhin (The Compiler) <mail@qutebrowser.org>
+#
+# This file is part of qutebrowser.
+#
+# qutebrowser is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# qutebrowser is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with qutebrowser.  If not, see <http://www.gnu.org/licenses/>.
+
+"""Check if everything needed for qutebrowser is there.
+
+In a separate file because this needs to be done early before any imports.
+"""
+
+import sys
+
+
+def check():
+    """Check if all dependencies are met."""
+    if sys.hexversion < 0x03030000:
+        print("Fatal error: At least Python 3.3 is required to run "
+              "qutebrowser, but {} is installed!".format(
+                  '.'.join(map(str, sys.version_info[:3]))))
+        sys.exit(1)
+    import textwrap
+    try:
+        import PyQt5.QtCore  # pylint: disable=unused-variable
+    except ImportError:
+        print(textwrap.dedent("""
+            Fatal error: PyQt5 is required to run qutebrowser but could not
+            be imported! Maybe it's not installed?
+
+            On Debian:
+                apt-get install python3-pyqt5 python3-pyqt5.qtwebkit
+
+            On Archlinux:
+                pacman -S python-pyqt5 qt5-webkit
+                or install the qutebrowser package from AUR
+
+            On Windows:
+                Use the installer by Riverbank computing or the standalone
+                qutebrowser exe.
+
+                http://www.riverbankcomputing.co.uk/software/pyqt/download5
+
+            For other distributions:
+                Check your package manager for similiarly named packages.
+            """).strip())
+        if '--debug' in sys.argv:
+            import traceback
+            print()
+            traceback.print_exc()
+        sys.exit(1)
+    # At this point we can rely on QtCore being available, so we can display an
+    # error dialog
+    try:
+        import PyQt5.QtWebKit
+    except ImportError:
+        from PyQt5.QtWidgets import QApplication, QMessageBox
+        app = QApplication(sys.argv)
+        msgbox = QMessageBox(QMessageBox.Critical, "qutebrowser: Fatal error!",
+                             textwrap.dedent("""
+            Fatal error: QtWebKit is required to run qutebrowser but could not
+            be imported! Maybe it's not installed?
+
+            On Debian:
+                apt-get install python3-pyqt5.qtwebkit
+
+            On Archlinux:
+                pacman -S qt5-webkit
+
+            For other distributions:
+                Check your package manager for similiarly named packages.
+            """).strip())
+        if '--debug' in sys.argv:
+            import traceback
+            print()
+            traceback.print_exc()
+        msgbox.resize(msgbox.sizeHint())
+        msgbox.exec_()
+        app.quit()
+    # At this point we have everything we need.