Windows: Dont launch calibre as an external viewer

Windows: When viewing books with an external viewer, check if windows
will open the file with calibre itself, and if so, popup an error
message telling the user to install some software capable of viewing the
file. Fixes #1249698 [Opening a PDF in calibre changes the default "Open with" app](https://bugs.launchpad.net/calibre/+bug/1249698)

src/calibre/gui2/actions/view.py

@@ -10,9 +10,9 @@ from functools import partial
 
 from PyQt4.Qt import Qt, QAction, pyqtSignal
 
-from calibre.constants import isosx
-from calibre.gui2 import error_dialog, Dispatcher, question_dialog, config, \
-        open_local_file, info_dialog
+from calibre.constants import isosx, iswindows
+from calibre.gui2 import (
+    error_dialog, Dispatcher, question_dialog, config, open_local_file, info_dialog)
 from calibre.gui2.dialogs.choose_format import ChooseFormatDialog
 from calibre.utils.config import prefs, tweaks
 from calibre.ptempfile import PersistentTemporaryFile
@@ -119,6 +119,24 @@ class ViewAction(InterfaceAction):
                 self.gui.job_manager.launch_gui_app(viewer,
                         kwargs=dict(args=args))
             else:
+                if iswindows:
+                    from calibre.utils.file_associations import file_assoc_windows
+                    ext = name.rpartition('.')[-1]
+                    if ext:
+                        try:
+                            prog = file_assoc_windows(ext)
+                        except Exception:
+                            prog = None
+                        if prog and prog.lower().endswith('calibre.exe'):
+                            name = os.path.basename(name)
+                            return error_dialog(
+                                self.gui, _('No associated program'), _(
+                                    'Windows will try to open %s with calibre itself'
+                                    ' resulting in a duplicate in your calibre library. You'
+                                    ' should install some program capable of viewing this'
+                                    ' file format and tell windows to use that program to open'
+                                    ' files of this type.') % name, show=True)
+
                 open_local_file(name)
                 time.sleep(2)  # User feedback
         finally:
@@ -145,7 +163,8 @@ class ViewAction(InterfaceAction):
         all_fmts = set([])
         for x in formats:
             if x:
-                for f in x: all_fmts.add(f)
+                for f in x:
+                    all_fmts.add(f)
         if not all_fmts:
             error_dialog(self.gui,  _('Format unavailable'),
                     _('Selected books have no formats'), show=True)
@@ -257,7 +276,7 @@ class ViewAction(InterfaceAction):
             self.build_menus(db)
 
     def view_device_book(self, path):
-        pt = PersistentTemporaryFile('_view_device_book'+\
+        pt = PersistentTemporaryFile('_view_device_book'+
                 os.path.splitext(path)[1])
         self.persistent_files.append(pt)
         pt.close()

src/calibre/utils/file_associations.py

@@ -0,0 +1,15 @@
+#!/usr/bin/env python
+# vim:fileencoding=utf-8
+from __future__ import (unicode_literals, division, absolute_import,
+                        print_function)
+
+__license__ = 'GPL v3'
+__copyright__ = '2013, Kovid Goyal <kovid at kovidgoyal.net>'
+
+def file_assoc_windows(ft):
+    # See the IQueryAssociations::GetString method documentation on MSDN
+    from win32com.shell import shell, shellcon
+    a = shell.AssocCreate()
+    a.Init(0, '.' + ft.lower())
+    return a.GetString(0, shellcon.ASSOCSTR_EXECUTABLE)
+