BUG: Fix nosetest and autodoc errors when PyQt4 not available

nose and autodoc imports the viewer modules so all PyQt4 imports must be wrapped in a try-except block. In addition, any classes derived from PyQt4 must be proxied since the class definition are run on import. This is really hacky.

skimage/viewer/plugins/base.py

@@ -1,14 +1,23 @@
 """
 Base class for Plugins that interact with ImageViewer.
 """
-from PyQt4 import QtGui
-from PyQt4.QtCore import Qt
-import matplotlib as mpl
+try:
+    from PyQt4 import QtGui
+    from PyQt4.QtCore import Qt
+    from PyQt4.QtGui import QDialog
+except ImportError:
+    QDialog = object # hack to prevent nosetest and autodoc errors
+    print("Could not import PyQt4 -- skimage.viewer not available.")
+
+try:
+    import matplotlib as mpl
+except ImportError:
+    print("Could not import matplotlib -- skimage.viewer not available.")
 
 from ..utils import RequiredAttr
 
 
-class Plugin(QtGui.QDialog):
+class Plugin(QDialog):
     """Base class for plugins that interact with an ImageViewer.
 
     A plugin connects an image filter (or another function) to an image viewer.

skimage/viewer/utils/core.py

@@ -1,8 +1,18 @@
 import numpy as np
-import matplotlib.pyplot as plt
-from matplotlib.colors import LinearSegmentedColormap
-from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg
-from PyQt4 import QtGui
+
+try:
+    import matplotlib.pyplot as plt
+    from matplotlib.colors import LinearSegmentedColormap
+    from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg
+except ImportError:
+    FigureCanvasQTAgg = object # hack to prevent nosetest and autodoc errors
+    LinearSegmentedColormap = object
+    print("Could not import matplotlib -- skimage.viewer not available.")
+
+try:
+    from PyQt4 import QtGui
+except ImportError:
+    print("Could not import PyQt4 -- skimage.viewer not available.")
 
 
 __all__ = ['figimage', 'LinearColormap', 'ClearColormap', 'MatplotlibCanvas',

skimage/viewer/viewers/core.py

@@ -3,7 +3,12 @@ ImageViewer class for viewing and interacting with images.
 """
 import sys
 
-from PyQt4 import QtGui, QtCore
+try:
+    from PyQt4 import QtGui, QtCore
+    from PyQt4.QtGui import QMainWindow
+except ImportError:
+    QMainWindow = object # hack to prevent nosetest and autodoc errors
+    print("Could not import PyQt4 -- skimage.viewer not available.")
 
 from ..utils import figimage, MatplotlibCanvas
 
@@ -18,7 +23,7 @@ class ImageCanvas(MatplotlibCanvas):
         super(ImageCanvas, self).__init__(parent, self.fig, **kwargs)
 
 
-class ImageViewer(QtGui.QMainWindow):
+class ImageViewer(QMainWindow):
     """Viewer for displaying images.
 
     This viewer is a simple container object that holds a Matplotlib axes

skimage/viewer/widgets/core.py

@@ -15,9 +15,14 @@ parameter type specified by its `ptype` attribute, which can be:
         property of the same name that updates the display.
 
 """
-from PyQt4.QtCore import Qt
-from PyQt4 import QtGui
-from PyQt4 import QtCore
+try:
+    from PyQt4.QtCore import Qt
+    from PyQt4 import QtGui
+    from PyQt4 import QtCore
+    from PyQt4.QtGui import QWidget
+except ImportError:
+    QWidget = object # hack to prevent nosetest and autodoc errors
+    print("Could not import PyQt4 -- skimage.viewer not available.")
 
 from ..utils import RequiredAttr
 
@@ -25,7 +30,7 @@ from ..utils import RequiredAttr
 __all__ = ['BaseWidget', 'Slider', 'ComboBox']
 
 
-class BaseWidget(QtGui.QWidget):
+class BaseWidget(QWidget):
 
     plugin = RequiredAttr("Widget is not attached to a Plugin.")
 

skimage/viewer/widgets/history.py

@@ -1,7 +1,10 @@
 import os
 from textwrap import dedent
 
-from PyQt4 import QtGui
+try:
+    from PyQt4 import QtGui
+except ImportError:
+    print("Could not import PyQt4 -- skimage.viewer not available.")
 
 from skimage import io
 from .core import BaseWidget