From fe4102337065a26b552cdaffea037e228ebf9824 Mon Sep 17 00:00:00 2001
From: Tony S Yu <tsyu80@gmail.com>
Date: Tue, 4 Sep 2012 22:07:25 -0400
Subject: [PATCH 1/2] STY: Refactor initialization of QApplication.

---
 skimage/viewer/utils/core.py   | 21 +++++++++++++++++++--
 skimage/viewer/viewers/core.py | 16 +++++-----------
 2 files changed, 24 insertions(+), 13 deletions(-)

diff --git a/skimage/viewer/utils/core.py b/skimage/viewer/utils/core.py
index 5c5f878c..74fd333f 100644
--- a/skimage/viewer/utils/core.py
+++ b/skimage/viewer/utils/core.py
@@ -17,8 +17,25 @@ except ImportError:
     print("Could not import PyQt4 -- skimage.viewer not available.")
 
 
-__all__ = ['figimage', 'LinearColormap', 'ClearColormap', 'MatplotlibCanvas',
-           'RequiredAttr']
+__all__ = ['init_qtapp', 'start_qtapp', 'RequiredAttr', 'figimage',
+           'LinearColormap', 'ClearColormap', 'MatplotlibCanvas']
+
+
+QApp = None
+
+
+def init_qtapp():
+    """Initialize QAppliction.
+
+    The QApplication needs to be initialized before creating any QWidgets
+    """
+    global QApp
+    if QApp is None:
+        QApp = QtGui.QApplication([])
+
+def start_qtapp():
+    """Start Qt mainloop"""
+    QApp.exec_()
 
 
 class RequiredAttr(object):
diff --git a/skimage/viewer/viewers/core.py b/skimage/viewer/viewers/core.py
index d29f950e..7c4b4753 100644
--- a/skimage/viewer/viewers/core.py
+++ b/skimage/viewer/viewers/core.py
@@ -1,8 +1,6 @@
 """
 ImageViewer class for viewing and interacting with images.
 """
-import sys
-
 try:
     from PyQt4 import QtGui, QtCore
     from PyQt4.QtGui import QMainWindow
@@ -11,20 +9,18 @@ except ImportError:
     print("Could not import PyQt4 -- skimage.viewer not available.")
 
 from skimage.util.dtype import dtype_range
-from ..utils import figimage, MatplotlibCanvas
+from .. import utils
 from ..widgets import Slider
 
 
 __all__ = ['ImageViewer', 'CollectionViewer']
 
 
-qApp = None
 
-
-class ImageCanvas(MatplotlibCanvas):
+class ImageCanvas(utils.MatplotlibCanvas):
     """Canvas for displaying images."""
     def __init__(self, parent, image, **kwargs):
-        self.fig, self.ax = figimage(image, **kwargs)
+        self.fig, self.ax = utils.figimage(image, **kwargs)
         super(ImageCanvas, self).__init__(parent, self.fig, **kwargs)
 
 
@@ -60,9 +56,7 @@ class ImageViewer(QMainWindow):
     """
     def __init__(self, image):
         # Start main loop
-        global qApp
-        if qApp is None:
-            qApp = QtGui.QApplication(sys.argv)
+        utils.init_qtapp()
         super(ImageViewer, self).__init__()
 
         #TODO: Add ImageViewer to skimage.io window manager
@@ -136,7 +130,7 @@ class ImageViewer(QMainWindow):
         for p in self.plugins:
             p.show()
         super(ImageViewer, self).show()
-        qApp.exec_()
+        utils.start_qtapp()
 
     def redraw(self):
         self.canvas.draw_idle()

From d1e012ea30798fe0fa636a18aca7deb9e0525420 Mon Sep 17 00:00:00 2001
From: Tony S Yu <tsyu80@gmail.com>
Date: Tue, 4 Sep 2012 22:07:59 -0400
Subject: [PATCH 2/2] BUG: Initialize QApplication when creating Plugin.

QWidgets cannot be initialized unless QApplication has been created. In cases where the Plugin is created before the ImageViewer, ensure that QApplication exists.
---
 skimage/viewer/plugins/base.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/skimage/viewer/plugins/base.py b/skimage/viewer/plugins/base.py
index ae240c20..074e18d4 100644
--- a/skimage/viewer/plugins/base.py
+++ b/skimage/viewer/plugins/base.py
@@ -14,7 +14,7 @@ try:
 except ImportError:
     print("Could not import matplotlib -- skimage.viewer not available.")
 
-from ..utils import RequiredAttr
+from ..utils import RequiredAttr, init_qtapp
 
 
 class Plugin(QDialog):
@@ -82,6 +82,7 @@ class Plugin(QDialog):
     draws_on_image = False
 
     def __init__(self, image_filter=None, height=0, width=400, useblit=None):
+        init_qtapp()
         super(Plugin, self).__init__()
 
         self.image_viewer = None