Merge pull request #555 from timkpaine/callable

cherrypick #333 and #523 onto 4.3.x

docs/source/trait_types.rst

@@ -110,5 +110,7 @@ Miscellaneous
 .. autoclass:: Union
    :members: __init__
 
+.. autoclass:: Callable
+
 .. autoclass:: Any
 

traitlets/tests/test_traitlets.py

@@ -7,6 +7,7 @@
 # Adapted from enthought.traits, Copyright (c) Enthought, Inc.,
 # also under the terms of the Modified BSD License.
 
+from __future__ import print_function
 import pickle
 import re
 import sys
@@ -19,7 +20,7 @@
 from traitlets import (
     HasTraits, MetaHasTraits, TraitType, Any, Bool, CBytes, Dict, Enum,
     Int, CInt, Long, CLong, Integer, Float, CFloat, Complex, Bytes, Unicode,
-    TraitError, Union, All, Undefined, Type, This, Instance, TCPAddress,
+    TraitError, Union, Callable, All, Undefined, Type, This, Instance, TCPAddress,
     List, Tuple, ObjectName, DottedObjectName, CRegExp, link, directional_link,
     ForwardDeclaredType, ForwardDeclaredInstance, validate, observe, default,
     observe_compat, BaseDescriptor, HasDescriptors,
@@ -1143,6 +1144,16 @@ class UnionTraitTest(TraitTestBase):
     _good_values = [int, float, True]
     _bad_values = [[], (0,), 1j]
 
+class CallableTrait(HasTraits):
+
+    value = Callable()
+
+class CallableTraitTest(TraitTestBase):
+
+    obj = CallableTrait(value=lambda x: type(x))
+    _good_values = [int, sorted, lambda x: print(x)]
+    _bad_values = [[], 1, '']
+
 class OrTrait(HasTraits):
 
     value = Bool() | Unicode()

traitlets/traitlets.py

@@ -2688,3 +2688,19 @@ def info(self):
         if self.allow_none:
             return result + " or None"
         return result
+
+class Callable(TraitType):
+    """A trait which is callable.
+
+    Notes
+    -----
+    Classes are callable, as are instances
+    with a __call__() method."""
+
+    info_text = 'a callable'
+
+    def validate(self, obj, value):
+        if six.callable(value):
+            return value
+        else:
+            self.error(obj, value)