Fix importing of opencv when libraries are not installed / extensions are not compiled.

scikits/image/opencv/__init__.py

@@ -1,9 +1,24 @@
 import ctypes
 import sys
 
-# try to open the opencv libs
-# prints a warning if the libs are not found
-from _libimport import cv, cxcore
-
 from opencv_constants import *
-from opencv_cv import *
+
+# Note: users should be able to import this module even if
+# the extensions are uncompiled or the opencv libraries unavailable.
+# In that case, the opencv functionality is simply unavailable.
+
+cv = None
+cxcore = None
+try:
+    from opencv_cv import *
+except ImportError:
+    print """*** The opencv extension was not compiled.  Run
+
+python setup.py build_ext -i
+
+in the source directory to build in-place.  Please refer to INSTALL.txt
+for further detail."""
+
+except RuntimeError:
+    # Libraries could not be loaded
+    print "*** Skipping import of OpenCV functions."

scikits/image/opencv/_libimport.py

@@ -16,7 +16,7 @@ import ctypes
 import sys
 import warnings
 
-def _import_opencv_lib(which = "cv"):
+def _import_opencv_lib(which="cv"):
     """
     Try to import a shared library of OpenCV.
 
@@ -32,8 +32,7 @@ def _import_opencv_lib(which = "cv"):
     if shared_lib is None:
         warnings.warn(RuntimeWarning(
             'The opencv libraries were not found.  Please ensure that they '
-            'are installed and available on the system path. '
-            '*** Skipping import of OpenCV functions.'))
+            'are installed and available on the system path. '))
     return shared_lib
 
 def _tryload_macosx(which):

scikits/image/opencv/opencv_backend.pyx

@@ -4,7 +4,17 @@ cimport numpy as np
 from python cimport *
 from opencv_constants import *
 from opencv_type cimport *
+
+# Without the opencv libraries, this extension module cannot function,
+# so we raise an exception if loading fails.
+#
+# Note, however, that users should be able to import scikits.image.opencv
+# itself without having any of the libraries installed
+# (the opencv functionality is then simply not available)
+#
 from _libimport import cxcore
+if cxcore is None:
+    raise RuntimeError('Could not load OpenCV libraries.')
 
 
 np.import_array()

scikits/image/opencv/opencv_cv.pyx

@@ -11,7 +11,16 @@ from opencv_constants import *
 from opencv_constants import *
 from opencv_cv import *
 
+# Without the opencv libraries, this extension module cannot function,
+# so we raise an exception if loading fails.
+#
+# Note, however, that users should be able to import scikits.image.opencv
+# itself without having any of the libraries installed
+# (the opencv functionality is then simply not available)
+#
 from _libimport import cv
+if cv is None:
+    raise RuntimeError('Could not load OpenCV libraries.')
 
 ###################################
 # opencv function declarations