feat(app): subcmds can be callables

docs/source/config.rst

@@ -376,7 +376,9 @@ Subcommands are specified as a dictionary on :class:`~traitlets.config.Applicati
 instances, mapping subcommand names to 2-tuples containing:
 
 1. The application class for the subcommand, or a string which can be imported
-   to give this.
+   to give this application class, or a callable with a single argument for
+   the parent application (i.e. to read configs from) returning a not yet *initialized*
+   application instance.
 2. A short description of the subcommand for use in help output.
 
 To see a list of the available aliases, flags, and subcommands for a configurable

traitlets/config/application.py

@@ -246,11 +246,13 @@ def _flags_changed(self, change):
             assert isinstance(value[1], six.string_types), "Bad flag: %r:%s" % (key, value)
 
 
-    # subcommands for launching other applications
-    # if this is not empty, this will be a parent Application
-    # this must be a dict of two-tuples,
-    # the first element being the application class/import string
-    # and the second being the help string for the subcommand
+    #: subcommands for launching other applications
+    #: if this is not empty, this will be a parent Application
+    #: this must be a dict of two-tuples,
+    #: the first element is the application class, or and import string
+    #: or a callable with a single argument for the parent returnng
+    #: instanciated application, and
+    #: the second being the help string for the subcommand
     subcommands = Dict()
     # parse_command_line will initialize a subapp, if requested
     subapp = Instance('traitlets.config.application.Application', allow_none=True)
@@ -464,13 +466,16 @@ def initialize_subcommand(self, subc, argv=None):
         """Initialize a subcommand with argv."""
         subapp,help = self.subcommands.get(subc)
 
-        if isinstance(subapp, six.string_types):
-            subapp = import_item(subapp)
+        if callable(subapp):
+            self.subapp = subapp(self)
+        else:
+            if isinstance(subapp, six.string_types):
+                subapp = import_item(subapp)
 
-        # clear existing instances
-        self.__class__.clear_instance()
-        # instantiate
-        self.subapp = subapp.instance(parent=self)
+            # clear existing instances
+            self.__class__.clear_instance()
+            # instantiate
+            self.subapp = subapp.instance(parent=self)
         # and initialize subapp
         self.subapp.initialize(argv)