add language key to kernelspec doc

docs/source/development/kernels.rst

@@ -1,137 +1,143 b''
 ==========================
 Making kernels for IPython
 ==========================
 
 A 'kernel' is a program that runs and introspects the user's code. IPython
 includes a kernel for Python code, and people have written kernels for
 `several other languages <https://github.com/ipython/ipython/wiki/Projects-using-IPython#list-of-some-ipython-compatible-kernels>`_.
 
 When IPython starts a kernel, it passes it a connection file. This specifies
 how to set up communications with the frontend.
 
 There are two options for writing a kernel:
 
 1. You can reuse the IPython kernel machinery to handle the communications, and
    just describe how to execute your code. This is much simpler if the target
    language can be driven from Python. See :doc:`wrapperkernels` for details.
 2. You can implement the kernel machinery in your target language. This is more
    work initially, but the people using your kernel might be more likely to
    contribute to it if it's in the language they know.
 
 Connection files
 ================
 
 Your kernel will be given the path to a connection file when it starts (see
 :ref:`kernelspecs` for how to specify the command line arguments for your kernel).
 This file, which is accessible only to the current user, will contain a JSON
 dictionary looking something like this::
 
     {
       "control_port": 50160,
       "shell_port": 57503,
       "transport": "tcp",
       "signature_scheme": "hmac-sha256",
       "stdin_port": 52597,
       "hb_port": 42540,
       "ip": "127.0.0.1",
       "iopub_port": 40885,
       "key": "a0436f6c-1916-498b-8eb9-e81ab9368e84"
     }
 
 The ``transport``, ``ip`` and five ``_port`` fields specify five ports which the
 kernel should bind to using `ZeroMQ <http://zeromq.org/>`_. For instance, the
 address of the shell socket in the example above would be::
 
     tcp://127.0.0.1:57503
 
 New ports are chosen at random for each kernel started.
 
 ``signature_scheme`` and ``key`` are used to cryptographically sign messages, so
 that other users on the system can't send code to run in this kernel. See
 :ref:`wire_protocol` for the details of how this signature is calculated.
 
 Handling messages
 =================
 
 After reading the connection file and binding to the necessary sockets, the
 kernel should go into an event loop, listening on the hb (heartbeat), control
 and shell sockets.
 
 :ref:`Heartbeat <kernel_heartbeat>` messages should be echoed back immediately
 on the same socket - the frontend uses this to check that the kernel is still
 alive.
 
 Messages on the control and shell sockets should be parsed, and their signature
 validated. See :ref:`wire_protocol` for how to do this.
 
 The kernel will send messages on the iopub socket to display output, and on the
 stdin socket to prompt the user for textual input.
 
 .. seealso::
 
    :doc:`messaging`
      Details of the different sockets and the messages that come over them
 
    `Creating Language Kernels for IPython <http://andrew.gibiansky.com/blog/ipython/ipython-kernels/>`_
      A blog post by the author of `IHaskell <https://github.com/gibiansky/IHaskell>`_,
      a Haskell kernel
 
    `simple_kernel <https://github.com/dsblank/simple_kernel>`_
      A simple example implementation of the kernel machinery in Python
 
 
 .. _kernelspecs:
 
 Kernel specs
 ============
 
 A kernel identifies itself to IPython by creating a directory, the name of which
 is used as an identifier for the kernel. These may be created in a number of
 locations:
 
 +--------+--------------------------------------+-----------------------------------+
 |        | Unix                                 | Windows                           |
 +========+======================================+===================================+
 | System | ``/usr/share/ipython/kernels``       | ``%PROGRAMDATA%\ipython\kernels`` |
 |        |                                      |                                   |
 |        | ``/usr/local/share/ipython/kernels`` |                                   |
 +--------+--------------------------------------+-----------------------------------+
 | User   |                     ``~/.ipython/kernels``                               |
 +--------+--------------------------------------+-----------------------------------+
 
 The user location takes priority over the system locations, and the case of the
 names is ignored, so selecting kernels works the same way whether or not the
 filesystem is case sensitive.
 
 Inside the directory, the most important file is *kernel.json*. This should be a
 JSON serialised dictionary containing the following keys and values:
 
 - **argv**: A list of command line arguments used to start the kernel. The text
   ``{connection_file}`` in any argument will be replaced with the path to the
   connection file.
 - **display_name**: The kernel's name as it should be displayed in the UI.
   Unlike the kernel name used in the API, this can contain arbitrary unicode
   characters.
+- **language**: The name of the language of the kernel.
+  When loading notebooks, if no matching kernelspec key (may differ across machines)
+  is found, a kernel with a matching `language` will be used.
+  This allows a notebook written on any Python or Julia kernel to be properly associated
+  with the user's Python or Julia kernel, even if they aren't listed under the same name as the author's.
 - **env** (optional): A dictionary of environment variables to set for the kernel.
   These will be added to the current environment variables before the kernel is
   started.
 
 For example, the kernel.json file for IPython looks like this::
 
     {
      "argv": ["python3", "-c", "from IPython.kernel.zmq.kernelapp import main; main()", 
               "-f", "{connection_file}"],
-     "display_name": "IPython (Python 3)"
+     "display_name": "IPython (Python 3)",
+     "language": "python"
     }
 
 To see the available kernel specs, run::
 
     ipython kernelspec list
 
 To start the terminal console or the Qt console with a specific kernel::
 
     ipython console --kernel bash
     ipython qtconsole --kernel bash
 
 To use different kernels in the notebook, select a different kernel from the
 dropdown menu in the top-right of the UI.