Fix Python wrappers for function callbacks (#1202)

Need to include pybind11/functional.h for function callbacks (std::function refs) to be passable. Otherwise the types are considered to be incompatible between a Python function and C++ function pointer.

python/MaterialXTest/main.py

@@ -429,14 +429,18 @@ def test_TraverseGraph(self):
         self.assertFalse(output.hasUpstreamCycle())
         self.assertTrue(doc.validate()[0])
 
-    def test_ReadXml(self):
+    def test_Xmlio(self):
         # Read the standard library.
         libs = []
         for filename in _libraryFilenames:
             lib = mx.createDocument()
             mx.readFromXmlFile(lib, filename, _searchPath)
             libs.append(lib)
 
+        # Declare write predicate for write filter test
+        def skipLibraryElement(elem):
+            return not elem.hasSourceUri()
+
         # Read and validate each example document.
         for filename in _exampleFilenames:
             doc = mx.createDocument()
@@ -473,6 +477,14 @@ def test_ReadXml(self):
                 doc2.importLibrary(lib)
             self.assertTrue(doc2.validate()[0])
 
+            # Write without definitions
+            writeOptions.writeXIncludeEnable = False
+            writeOptions.elementPredicate = skipLibraryElement
+            result = mx.writeToXmlString(doc2, writeOptions)
+            doc3 = mx.createDocument()
+            mx.readFromXmlString(doc3, result)    
+            self.assertTrue(len(doc3.getNodeDefs()) == 0)   
+
         # Read the same document twice, and verify that duplicate elements
         # are skipped.
         doc = mx.createDocument()

source/PyMaterialX/PyMaterialX.h

@@ -12,6 +12,7 @@
 // include within any PyMaterialX source file.
 //
 
+#include <pybind11/functional.h>
 #include <pybind11/operators.h>
 #include <pybind11/stl.h>