From 54af4176dd3de0adc2cdd1e819910e4897831598 Mon Sep 17 00:00:00 2001
From: Tony S Yu <tsyu80@gmail.com>
Date: Fri, 3 Aug 2012 20:45:01 -0400
Subject: [PATCH] ENH: Add RequiredAttr to raise warnings when attr not set.

---
 skimage/viewer/plugins/base.py | 16 ++++------------
 skimage/viewer/utils/core.py   | 19 ++++++++++++++++++-
 2 files changed, 22 insertions(+), 13 deletions(-)

diff --git a/skimage/viewer/plugins/base.py b/skimage/viewer/plugins/base.py
index ee2cbb5d..562237b2 100644
--- a/skimage/viewer/plugins/base.py
+++ b/skimage/viewer/plugins/base.py
@@ -3,9 +3,10 @@ Base class for Plugins that interact with ImageViewer.
 """
 from PyQt4 import QtGui
 from PyQt4.QtCore import Qt
-
 import matplotlib as mpl
 
+from ..utils import RequiredAttr
+
 
 class Plugin(QtGui.QDialog):
     """Base class for plugins that interact with an ImageViewer.
@@ -68,6 +69,7 @@ class Plugin(QtGui.QDialog):
 
     """
     name = 'Plugin'
+    image_viewer = RequiredAttr("%s is not attached to ImageViewer" % name)
     draws_on_image = False
 
     def __init__(self, image_filter=None, height=0, width=400, useblit=None):
@@ -92,18 +94,8 @@ class Plugin(QtGui.QDialog):
         self.cids = []
         self.artists = []
 
-    @property
-    def image_viewer(self):
-        if self._image_viewer is None:
-            raise RuntimeError("Plugin is not attached to ImageViewer")
-        return self._image_viewer
-
-    @image_viewer.setter
-    def image_viewer(self, image_viewer):
-        self._image_viewer = image_viewer
-
     def attach(self, image_viewer):
-        """Attach the plugin to an  ImageViewer.
+        """Attach the plugin to an ImageViewer.
 
         Note that the ImageViewer will automatically call this method when the
         plugin is added to the ImageViewer. For example:
diff --git a/skimage/viewer/utils/core.py b/skimage/viewer/utils/core.py
index 81d66183..36476df8 100644
--- a/skimage/viewer/utils/core.py
+++ b/skimage/viewer/utils/core.py
@@ -5,7 +5,24 @@ from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg
 from PyQt4 import QtGui
 
 
-__all__ = ['figimage', 'LinearColormap', 'ClearColormap', 'MatplotlibCanvas']
+__all__ = ['figimage', 'LinearColormap', 'ClearColormap', 'MatplotlibCanvas',
+           'RequiredAttr']
+
+
+class RequiredAttr(object):
+    """A class attribute that must be set before use."""
+
+    def __init__(self, msg):
+        self.msg = msg
+        self.val = None
+
+    def __get__(self, obj, objtype):
+        if self.val is None:
+            raise RuntimeError(self.msg)
+        return self.val
+
+    def __set__(self, obj, val):
+        self.val = val
 
 
 def figimage(image, scale=1, dpi=None, **kwargs):