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# Making simple Python wrapper kernels

New in version 3.0.

You can now re-use the kernel machinery in IPython to easily make new kernels. This is useful for languages that have Python bindings, such as Octave (via Oct2Py), or languages where the REPL can be controlled in a tty using pexpect, such as bash.

See also

bash_kernel
  A simple kernel for bash, written using this machinery

## Required steps

Subclass `IPython.kernel.zmq.kernelbase.Kernel`, and implement the following methods and attributes:

class `MyKernel`
  `implementation`
  `implementation_version`
  `language`
  `language_version`
  `banner`

    Information for msging_kernel_info replies. ‘Implementation’ refers to the kernel (e.g. IPython), and ‘language’ refers to the language it interprets (e.g. Python). The ‘banner’ is displayed to the user in console UIs before the first prompt. All of these values are strings.

  `language_info`

    Language information for msging_kernel_info replies, in a dictionary. This should contain the key `mimetype` with the mimetype of code in the target language (e.g. `'text/x-python'`), and `file_extension` (e.g. `'py'`). It may also contain keys `codemirror_mode` and `pygments_lexer` if they need to differ from `language`.

    Other keys may be added to this later.

  `do_execute`(code, silent, store_history=True, user_expressions=None, allow_stdin=False)

    Execute user code.

    Parameters:

    - code (str) – The code to be executed.
    - silent (bool) – Whether to display output.
    - store_history (bool) – Whether to record this code in history and increase the execution count. If silent is True, this is implicitly False.
    - user_expressions (dict) – Mapping of names to expressions to evaluate after the code has run. You can ignore this if you need to.
    - allow_stdin (bool) – Whether the frontend can provide input on request (e.g. for Python’s `raw_input()`).

    Your method should return a dict containing the fields described in execution_results. To display output, it can send messages using `send_response()`. See Messaging in IPython for details of the different message types.

To launch your kernel, add this at the end of your module:

    if __name__ == '__main__':
        from IPython.kernel.zmq.kernelapp import IPKernelApp
        IPKernelApp.launch_instance(kernel_class=MyKernel)

## Example

`echokernel.py` will simply echo any input it’s given to stdout:

    from IPython.kernel.zmq.kernelbase import Kernel

    class EchoKernel(Kernel):
        implementation = 'Echo'
        implementation_version = '1.0'
        language = 'no-op'
        language_version = '0.1'
        language_info = {'mimetype': 'text/plain'}
        banner = "Echo kernel - as useful as a parrot"

        def do_execute(self, code, silent, store_history=True, user_expressions=None,
                       allow_stdin=False):
            if not silent:
                stream_content = {'name': 'stdout', 'text': code}
                self.send_response(self.iopub_socket, 'stream', stream_content)

            return {'status': 'ok',
                    # The base class increments the execution count
                    'execution_count': self.execution_count,
                    'payload': [],
                    'user_expressions': {},
                   }

    if __name__ == '__main__':
        from IPython.kernel.zmq.kernelapp import IPKernelApp
        IPKernelApp.launch_instance(kernel_class=EchoKernel)

Here’s the Kernel spec `kernel.json` file for this:

    {"argv":["python","-m","echokernel", "-f", "{connection_file}"],
     "display_name":"Echo"
    }

## Optional steps

You can override a number of other methods to improve the functionality of your kernel. All of these methods should return a dictionary as described in the relevant section of the messaging spec.

class `MyKernel`
  `do_complete`(code, cusor_pos)

    Code completion

    Parameters:

    - code (str) – The code already present
    - cursor_pos (int) – The position in the code where completion is requested

    See also

    msging_completion messages

  `do_inspect`(code, cusor_pos, detail_level=0)

    Object introspection

    Parameters:

    - code (str) – The code
    - cursor_pos (int) – The position in the code where introspection is requested
    - detail_level (int) – 0 or 1 for more or less detail. In IPython, 1 gets the source code.

    See also

    msging_inspection messages

  `do_history`(hist_access_type, output, raw, session=None, start=None, stop=None, n=None, pattern=None, unique=False)

    History access. Only the relevant parameters for the type of history request concerned will be passed, so your method definition must have defaults for all the arguments shown with defaults here.

    See also

    msging_history messages

  `do_is_complete`(code)

    Is code entered in a console-like interface complete and ready to execute, or should a continuation prompt be shown?

    Parameters: | code (str) – The code entered so far - possibly multiple lines

    See also

    msging_is_complete messages

  `do_shutdown`(restart)

    Shutdown the kernel. You only need to handle your own clean up - the kernel machinery will take care of cleaning up its own things before stopping.

    Parameters: | restart (bool) – Whether the kernel will be started again afterwards

    See also

    msging_shutdown messages

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