fix example

and raise error when running the buggy version
instead of python mysteriously crashing

fixes #1711

Signed-off-by: Heshy Roskes <heshyr@gmail.com>

src/wrappers/python/Imath.py

@@ -164,12 +164,18 @@ class Channel:
        >>> import Imath
        >>> print Imath.Channel(Imath.PixelType(Imath.PixelType.FLOAT), 4, 4)
        FLOAT (4, 4)
+       >>> print Imath.Channel(Imath.PixelType.FLOAT, 4, 4)
+       Traceback (most recent call last):
+         ...
+       TypeError: type needs to be a PixelType.
     """
 
     def __init__(self, type = PixelType(PixelType.HALF), xSampling = 1, ySampling = 1):
         self.type = type
         self.xSampling = xSampling
         self.ySampling = ySampling
+        if not isinstance(self.type, PixelType):
+          raise TypeError("type needs to be a PixelType.")
     def __repr__(self):
         return repr(self.type) + " " + repr((self.xSampling, self.ySampling))
     def __eq__(self, other):

src/wrappers/python/README.md

@@ -71,17 +71,18 @@ for more information.
 
 The "hello, world" image writer:
 
-    import OpenEXR
+    import OpenEXR, Imath
+    from array import array
 
     width = 10
     height = 10
     size = width * height
 
     h = OpenEXR.Header(width,height)
-    h['channels'] = {'R' : Imath.Channel(FLOAT),
-                     'G' : Imath.Channel(FLOAT),
-                     'B' : Imath.Channel(FLOAT),
-                     'A' : Imath.Channel(FLOAT)} 
+    h['channels'] = {'R' : Imath.Channel(Imath.PixelType(Imath.PixelType.FLOAT)),
+                     'G' : Imath.Channel(Imath.PixelType(Imath.PixelType.FLOAT)),
+                     'B' : Imath.Channel(Imath.PixelType(Imath.PixelType.FLOAT)),
+                     'A' : Imath.Channel(Imath.PixelType(Imath.PixelType.FLOAT))} 
     o = OpenEXR.OutputFile("hello.exr", h)
     r = array('f', [n for n in range(size*0,size*1)]).tobytes()
     g = array('f', [n for n in range(size*1,size*2)]).tobytes()

src/wrappers/python/tests/test_unittest.py

@@ -197,6 +197,13 @@ def test_invalid_pixeltype():
     else:
         assert 0
 
+    try:
+        Imath.Channel(FLOAT)
+    except:
+        pass
+    else:
+        assert 0
+
     print("invalid pixeltype ok")
 
 testList.append(("test_invalid_pixeltype", test_invalid_pixeltype))