easier operator hooks

This adds the ability to pass optional custom operators to the import
hook context and have the operators installed for the duration of the
context.

enaml/__init__.py

@@ -33,9 +33,42 @@ def createLock(self):
 #------------------------------------------------------------------------------
 # Import Helper
 #------------------------------------------------------------------------------
-def imports():
+def imports(operators=None, union=True):
     """ Lazily imports and returns an enaml imports context.
 
+    Parameters
+    ----------
+    operators : dict, optional
+        An optional dictionary of operators to push onto the operator
+        stack for the duration of the import context. If this is not
+        provided, the default Enaml operators will be used. Unless a
+        custom model framework is being used (i.e. not Atom), custom
+        operators will typically not be needed.
+
+    union : bool, optional
+        Whether to union the operators with the operators on the top
+        of the operator stack. The default is True and is typically
+        the correct choice to allow overriding a subset of the default
+        Enaml operators.
+
+    Returns
+    -------
+    result : context manager
+        A context manager which will install the Enaml import hook
+        (and optional operators) for the duration of the context.
+
     """
     from enaml.core.import_hooks import imports
-    return imports()
+    if operators is None:
+        return imports()
+
+    from contextlib import contextmanager
+    from enaml.core.operators import operator_context
+
+    @contextmanager
+    def imports_context():
+        with imports():
+            with operator_context(operators, union):
+                yield
+
+    return imports_context()

enaml/core/operators.py

@@ -535,15 +535,23 @@ def op_delegate(klass, binding):
 
 
 @contextmanager
-def operator_context(ops):
+def operator_context(ops, union=False):
     """ Push operators onto the stack for the duration of the context.
 
     Parameters
     ----------
     ops : dict
         The dictionary of operators to push onto the stack.
 
+    union : bool, optional
+        Whether or to union the operators with the existing operators
+        on the top of the stack. The default is False.
+
     """
+    if union:
+        new = dict(__get_operators())
+        new.update(ops)
+        ops = new
     __operator_stack.append(ops)
     yield
     __operator_stack.pop()