Move qgsfunction to qgis.core.

Add new register_function method when not using decorator
Wrap function call in try block to catch and report errors

python/core/__init__.py

@@ -1 +1,89 @@
+import string
 from qgis._core import *
+
+
+def register_function(function, arg_count, group, usesgeometry=False, **kwargs):
+    """
+    Register a Python function to be used as a expression function.
+
+    Functions should take (values, feature, parent) as args:
+
+    Example:
+        def myfunc(values, feature, parent):
+            pass
+
+    They can also shortcut naming feature and parent args by using *args
+    if they are not needed in the function.
+
+    Example:
+        def myfunc(values, *args):
+            pass
+
+    Functions should return a value compatible with QVariant
+
+    Eval errors can be raised using parent.setEvalErrorString("Error message")
+
+    :param function:
+    :param arg_count:
+    :param group:
+    :param usesgeometry:
+    :return:
+    """
+    class QgsExpressionFunction(QgsExpression.Function):
+        def __init__(self, func, name, args, group, helptext='', usesgeometry=False):
+            QgsExpression.Function.__init__(self, name, args, group, helptext, usesgeometry)
+            self.function = func
+
+        def func(self, values, feature, parent):
+            try:
+                return self.function(values, feature, parent)
+            except Exception as ex:
+                parent.setEvalErrorString(ex.message)
+
+    helptemplate = string.Template("""<h3>$name function</h3><br>$doc""")
+    name = kwargs.get('name', function.__name__)
+    helptext = function.__doc__ or ''
+    helptext = helptext.strip()
+    if arg_count == 0 and not name[0] == '$':
+        name = '${0}'.format(name)
+
+    if QgsExpression.isFunctionName(name):
+        if not QgsExpression.unregisterFunction(name):
+            raise TypeError("Unable to unregister function")
+
+    function.__name__ = name
+    helptext = helptemplate.safe_substitute(name=name, doc=helptext)
+    f = QgsExpressionFunction(function, name, arg_count, group, helptext, usesgeometry)
+
+    # This doesn't really make any sense here but does when used from a decorator context
+    # so it can stay.
+    register = kwargs.get('register', True)
+    if register:
+        QgsExpression.registerFunction(f)
+    return f
+
+
+def qgsfunction(args, group, **kwargs):
+    """
+    Decorator function used to define a user expression function.
+
+    Example:
+      @qgsfunction(2, 'test'):
+      def add(values, feature, parent):
+        pass
+
+    Will create and register a function in QgsExpression called 'add' in the
+    'test' group that takes two arguments.
+
+    or not using feature and parent:
+
+    Example:
+      @qgsfunction(2, 'test'):
+      def add(values, *args):
+        pass
+    """
+
+    def wrapper(func):
+        usesgeometry = kwargs.get('usesgeometry', False)
+        return register_function(func, args, group, usesgeometry, **kwargs)
+    return wrapper