Simplify kernel manager code

docs/source/whatsnew/pr/incompat-kernelmanager-start-args

@@ -0,0 +1,2 b''
+- :meth:`~.KernelManager.start_kernel` and :meth:`~.KernelManager.format_kernel_cmd`
+  no longer accept a ``executable`` parameter. Use the kernelspec machinery instead.

IPython/kernel/kernelspec.py

@@ -9,6 +9,7 b' pjoin = os.path.join'
 from IPython.utils.path import get_ipython_dir
 from IPython.utils.py3compat import PY3
 from IPython.utils.traitlets import HasTraits, List, Unicode, Dict, Any
+from .launcher import make_ipkernel_cmd
 
 if os.name == 'nt':
     programdata = os.environ.get('PROGRAMDATA', None)
@@ -107,9 +108,8 b' class KernelSpecManager(HasTraits):'
         The native kernel is the kernel using the same Python runtime as this
         process. This will put its informatino in the user kernels directory.
         """
-        return {'argv':[sys.executable, '-c',
-                       'from IPython.kernel.zmq.kernelapp import main; main()',
-                        '-f', '{connection_file}'],
+        return {'argv':make_ipkernel_cmd(
+                       'from IPython.kernel.zmq.kernelapp import main; main()'),
                'display_name': 'IPython (Python %d)' % (3 if PY3 else 2),
                'language': 'python',
                'codemirror_mode': {'name': 'ipython',

IPython/kernel/launcher.py

@@ -97,14 +97,11 b' def make_ipkernel_cmd(code, executable=None, extra_arguments=[], **kw):'
     
     A Popen command list
     """
-    
-    # Build the kernel launch command.
     if executable is None:
         executable = sys.executable
     arguments = [ executable, '-c', code, '-f', '{connection_file}' ]
     arguments.extend(extra_arguments)
 
-    # Spawn a kernel.
     if sys.platform == 'win32':
 
         # If the kernel is running on pythonw and stdout/stderr are not been

IPython/kernel/manager.py

@@ -159,18 +159,13 b' class KernelManager(ConnectionFileMixin):'
     # Kernel management
     #--------------------------------------------------------------------------
 
-    def format_kernel_cmd(self, **kw):
+    def format_kernel_cmd(self, extra_arguments=None):
         """replace templated args (e.g. {connection_file})"""
+        extra_arguments = extra_arguments or []
         if self.kernel_cmd:
-            cmd = self.kernel_cmd
-        elif self.kernel_name == kernelspec.NATIVE_KERNEL_NAME:
-            # The native kernel gets special handling
-            cmd = make_ipkernel_cmd(
-                'from IPython.kernel.zmq.kernelapp import main; main()',
-                **kw
-            )
+            cmd = self.kernel_cmd + extra_arguments
         else:
-            cmd = self.kernel_spec.argv
+            cmd = self.kernel_spec.argv + extra_arguments
 
         ns = dict(connection_file=self.connection_file)
         ns.update(self._launch_args)
@@ -227,7 +222,8 b' class KernelManager(ConnectionFileMixin):'
         # save kwargs for use in restart
         self._launch_args = kw.copy()
         # build the Popen cmd
-        kernel_cmd = self.format_kernel_cmd(**kw)
+        extra_arguments = kw.pop('extra_arguments', [])
+        kernel_cmd = self.format_kernel_cmd(extra_arguments=extra_arguments)
         if self.kernel_cmd:
             # If kernel_cmd has been set manually, don't refer to a kernel spec
             env = os.environ