Execute

This message type is used by frontends to ask the kernel to execute code on behalf of the user, in a namespace reserved to the user's variables (and thus separate from the kernel's own internal code and variables).

Message type: execute_request:

content = {
    # Source code to be executed by the kernel, one or more lines.
'code' : str,

# A boolean flag which, if True, signals the kernel to execute
# this code as quietly as possible.
# silent=True forces store_history to be False,
# and will *not*:
#   - broadcast output on the IOPUB channel
#   - have an execute_result
# The default is False.
'silent' : bool,

# A boolean flag which, if True, signals the kernel to populate history
# The default is True if silent is False.  If silent is True, store_history
# is forced to be False.
'store_history' : bool,

# A dict mapping names to expressions to be evaluated in the
# user's dict. The rich display-data representation of each will be evaluated after execution.
# See the display_data content for the structure of the representation data.
'user_expressions' : dict,

# Some frontends do not support stdin requests.
# If raw_input is called from code executed from such a frontend,
# a StdinNotImplementedError will be raised.
'allow_stdin' : True,

# A boolean flag, which, if True, does not abort the execution queue, if an exception is encountered.
# This allows the queued execution of multiple execute_requests, even if they generate exceptions.
'stop_on_error' : False,
}

The code field contains a single string (possibly multiline) to be executed.

The user_expressions field deserves a detailed explanation. In the past, IPython had the notion of a prompt string that allowed arbitrary code to be evaluated, and this was put to good use by many in creating prompts that displayed system status, path information, and even more esoteric uses like remote instrument status acquired over the network. But now that IPython has a clean separation between the kernel and the clients, the kernel has no prompt knowledge; prompts are a frontend feature, and it should be even possible for different frontends to display different prompts while interacting with the same kernel. user_expressions can be used to retrieve this information.

Any error in evaluating any expression in user_expressions will result in only that key containing a standard error message, of the form:

{
    'status' : 'error',
    'ename' : 'NameError',
    'evalue' : 'foo',
    'traceback' : ...
}

Upon completion of the execution request, the kernel always sends a reply, with a status code indicating what happened and additional data depending on the outcome. See :ref:`below <execution_results>` for the possible return codes and associated data.

content = {
  # One of: 'ok' OR 'error' OR 'abort'
  'status' : str,

  # The global kernel counter that increases by one with each request that
  # stores history.  This will typically be used by clients to display
  # prompt numbers to the user.  If the request did not store history, this will
  # be the current value of the counter in the kernel.
  'execution_count' : int,
}

{
  # 'payload' will be a list of payload dicts, and is optional.
  # payloads are considered deprecated.
  # The only requirement of each payload dict is that it have a 'source' key,
  # which is a string classifying the payload (e.g. 'page').

  'payload' : list(dict),

  # Results for the user_expressions.
  'user_expressions' : dict,
}

{
  'ename' : str,   # Exception name, as a string
  'evalue' : str,  # Exception value, as a string

  # The traceback will contain a list of frames, represented each as a
  # string.  For now we'll stick to the existing design of ultraTB, which
  # controls exception level of detail statefully.  But eventually we'll
  # want to grow into a model where more information is collected and
  # packed into the traceback object, with clients deciding how little or
  # how much of it to unpack.  But for now, let's start with a simple list
  # of strings, since that requires only minimal changes to ultratb as
  # written.
  'traceback' : list,
}

{
  "source": "page",
  # mime-bundle of data to display in the pager.
  # Must include text/plain.
  "data": mimebundle,
  # line offset to start from
  "start": int,
}

{
  "source": "set_next_input",
  # the text contents of the cell to create
  "text": "some cell content",
  # If true, replace the current cell in document UIs instead of inserting
  # a cell. Ignored in console UIs.
  "replace": bool,
}

{
  "source": "edit",
  "filename": "/path/to/file.py", # the file to edit
  "line_number": int, # the line number to start with
}

{
  "source": "ask_exit",
  # whether the kernel should be left running, only closing the client
  "keepkernel": bool,
}

Introspection

Code can be inspected to show useful information to the user. It is up to the Kernel to decide what information should be displayed, and its formatting.

Message type: inspect_request:

content = {
    # The code context in which introspection is requested
    # this may be up to an entire multiline cell.
    'code' : str,

    # The cursor position within 'code' (in unicode characters) where inspection is requested
    'cursor_pos' : int,

    # The level of detail desired.  In IPython, the default (0) is equivalent to typing
    # 'x?' at the prompt, 1 is equivalent to 'x??'.
    # The difference is up to kernels, but in IPython level 1 includes the source code
    # if available.
    'detail_level' : 0 or 1,
}

The reply is a mime-bundle, like a display_data message, which should be a formatted representation of information about the context. In the notebook, this is used to show tooltips over function calls, etc.

Message type: inspect_reply:

content = {
    # 'ok' if the request succeeded or 'error', with error information as in all other replies.
    'status' : 'ok',

    # data can be empty if nothing is found
    'data' : dict,
    'metadata' : dict,
}

Completion

Message type: complete_request:

content = {
    # The code context in which completion is requested
    # this may be up to an entire multiline cell, such as
    # 'foo = a.isal'
    'code' : str,

    # The cursor position within 'code' (in unicode characters) where completion is requested
    'cursor_pos' : int,
}

Message type: complete_reply:

content = {
# The list of all matches to the completion request, such as
# ['a.isalnum', 'a.isalpha'] for the above example.
'matches' : list,

# The range of text that should be replaced by the above matches when a completion is accepted.
# typically cursor_end is the same as cursor_pos in the request.
'cursor_start' : int,
'cursor_end' : int,

# Information that frontend plugins might use for extra display information about completions.
'metadata' : dict,

# status should be 'ok' unless an exception was raised during the request,
# in which case it should be 'error', along with the usual error message content
# in other messages.
'status' : 'ok'
}

History

For clients to explicitly request history from a kernel. The kernel has all the actual execution history stored in a single location, so clients can request it from the kernel when needed.

Message type: history_request:

content = {

  # If True, also return output history in the resulting dict.
  'output' : bool,

  # If True, return the raw input history, else the transformed input.
  'raw' : bool,

  # So far, this can be 'range', 'tail' or 'search'.
  'hist_access_type' : str,

  # If hist_access_type is 'range', get a range of input cells. session can
  # be a positive session number, or a negative number to count back from
  # the current session.
  'session' : int,
  # start and stop are line numbers within that session.
  'start' : int,
  'stop' : int,

  # If hist_access_type is 'tail' or 'search', get the last n cells.
  'n' : int,

  # If hist_access_type is 'search', get cells matching the specified glob
  # pattern (with * and ? as wildcards).
  'pattern' : str,

  # If hist_access_type is 'search' and unique is true, do not
  # include duplicated history.  Default is false.
  'unique' : bool,

}

Message type: history_reply:

content = {
  # A list of 3 tuples, either:
  # (session, line_number, input) or
  # (session, line_number, (input, output)),
  # depending on whether output was False or True, respectively.
  'history' : list,
}

Code completeness

When the user enters a line in a console style interface, the console must decide whether to immediately execute the current code, or whether to show a continuation prompt for further input. For instance, in Python a = 5 would be executed immediately, while for i in range(5): would expect further input.

There are four possible replies:

• complete code is ready to be executed
• incomplete code should prompt for another line
• invalid code will typically be sent for execution, so that the user sees the error soonest.
• unknown - if the kernel is not able to determine this. The frontend should also handle the kernel not replying promptly. It may default to sending the code for execution, or it may implement simple fallback heuristics for whether to execute the code (e.g. execute after a blank line).

Frontends may have ways to override this, forcing the code to be sent for execution or forcing a continuation prompt.

Message type: is_complete_request:

content = {
    # The code entered so far as a multiline string
    'code' : str,
}

Message type: is_complete_reply:

content = {
    # One of 'complete', 'incomplete', 'invalid', 'unknown'
    'status' : str,

    # If status is 'incomplete', indent should contain the characters to use
    # to indent the next line. This is only a hint: frontends may ignore it
    # and use their own autoindentation rules. For other statuses, this
    # field does not exist.
    'indent': str,
}

Connect

When a client connects to the request/reply socket of the kernel, it can issue a connect request to get basic information about the kernel, such as the ports the other ZeroMQ sockets are listening on. This allows clients to only have to know about a single port (the shell channel) to connect to a kernel.

Message type: connect_request:

content = {
}

Message type: connect_reply:

content = {
    'shell_port' : int,   # The port the shell ROUTER socket is listening on.
    'iopub_port' : int,   # The port the PUB socket is listening on.
    'stdin_port' : int,   # The port the stdin ROUTER socket is listening on.
    'hb_port' : int,      # The port the heartbeat socket is listening on.
}

Kernel info

If a client needs to know information about the kernel, it can make a request of the kernel's information. This message can be used to fetch core information of the kernel, including language (e.g., Python), language version number and IPython version number, and the IPython message spec version number.

Message type: kernel_info_request:

content = {
}

Message type: kernel_info_reply:

content = {
    # Version of messaging protocol.
    # The first integer indicates major version.  It is incremented when
    # there is any backward incompatible change.
    # The second integer indicates minor version.  It is incremented when
    # there is any backward compatible change.
    'protocol_version': 'X.Y.Z',

    # The kernel implementation name
    # (e.g. 'ipython' for the IPython kernel)
    'implementation': str,

    # Implementation version number.
    # The version number of the kernel's implementation
    # (e.g. IPython.__version__ for the IPython kernel)
    'implementation_version': 'X.Y.Z',

    # Information about the language of code for the kernel
    'language_info': {
        # Name of the programming language in which kernel is implemented.
        # Kernel included in IPython returns 'python'.
        'name': str,

        # Language version number.
        # It is Python version number (e.g., '2.7.3') for the kernel
        # included in IPython.
        'version': 'X.Y.Z',

        # mimetype for script files in this language
        'mimetype': str,

        # Extension without the dot, e.g. 'py'
        'file_extension': str,

        # Pygments lexer, for highlighting
        # Only needed if it differs from the top level 'language' field.
        'pygments_lexer': str,

        # Codemirror mode, for for highlighting in the notebook.
        # Only needed if it differs from the top level 'language' field.
        'codemirror_mode': str or dict,

        # Nbconvert exporter, if notebooks written with this kernel should
        # be exported with something other than the general 'script'
        # exporter.
        'nbconvert_exporter': str,
    },

    # A banner of information about the kernel,
    # which may be desplayed in console environments.
    'banner' : str,

    # Optional: A list of dictionaries, each with keys 'text' and 'url'.
    # These will be displayed in the help menu in the notebook UI.
    'help_links': [
        {'text': str, 'url': str}
    ],
}

Refer to the lists of available Pygments lexers and codemirror modes for those fields.

Kernel shutdown

The clients can request the kernel to shut itself down; this is used in multiple cases:

• when the user chooses to close the client application via a menu or window control.
• when the user types 'exit' or 'quit' (or their uppercase magic equivalents).
• when the user chooses a GUI method (like the 'Ctrl-C' shortcut in the IPythonQt client) to force a kernel restart to get a clean kernel without losing client-side state like history or inlined figures.

The client sends a shutdown request to the kernel, and once it receives the reply message (which is otherwise empty), it can assume that the kernel has completed shutdown safely.

Upon their own shutdown, client applications will typically execute a last minute sanity check and forcefully terminate any kernel that is still alive, to avoid leaving stray processes in the user's machine.

Message type: shutdown_request:

content = {
    'restart' : bool # whether the shutdown is final, or precedes a restart
}

Message type: shutdown_reply:

content = {
    'restart' : bool # whether the shutdown is final, or precedes a restart
}

Streams (stdout, stderr, etc)

Message type: stream:

content = {
    # The name of the stream is one of 'stdout', 'stderr'
    'name' : str,

    # The text is an arbitrary string to be written to that stream
    'text' : str,
}

Display Data

This type of message is used to bring back data that should be displayed (text, html, svg, etc.) in the frontends. This data is published to all frontends. Each message can have multiple representations of the data; it is up to the frontend to decide which to use and how. A single message should contain all possible representations of the same information. Each representation should be a JSON'able data structure, and should be a valid MIME type.

Some questions remain about this design:

• Do we use this message type for execute_result/displayhook? Probably not, because the displayhook also has to handle the Out prompt display. On the other hand we could put that information into the metadata section.

Message type: display_data:

content = {

    # Who create the data
    'source' : str,

    # The data dict contains key/value pairs, where the keys are MIME
    # types and the values are the raw data of the representation in that
    # format.
    'data' : dict,

    # Any metadata that describes the data
    'metadata' : dict
}

The metadata contains any metadata that describes the output. Global keys are assumed to apply to the output as a whole. The metadata dict can also contain mime-type keys, which will be sub-dictionaries, which are interpreted as applying only to output of that type. Third parties should put any data they write into a single dict with a reasonably unique name to avoid conflicts.

The only metadata keys currently defined in IPython are the width and height of images:

metadata = {
  'image/png' : {
    'width': 640,
    'height': 480
  }
}

Raw Data Publication

display_data lets you publish representations of data, such as images and html. This data_pub message lets you publish actual raw data, sent via message buffers.

data_pub messages are constructed via the :func:`IPython.lib.datapub.publish_data` function:

from IPython.kernel.zmq.datapub import publish_data
ns = dict(x=my_array)
publish_data(ns)

Message type: data_pub:

content = {
    # the keys of the data dict, after it has been unserialized
    'keys' : ['a', 'b']
}
# the namespace dict will be serialized in the message buffers,
# which will have a length of at least one
buffers = [b'pdict', ...]

The interpretation of a sequence of data_pub messages for a given parent request should be to update a single namespace with subsequent results.

Code inputs

To let all frontends know what code is being executed at any given time, these messages contain a re-broadcast of the code portion of an :ref:`execute_request <execute>`, along with the :ref:`execution_count <execution_counter>`.

Message type: execute_input:

content = {
    'code' : str,  # Source code to be executed, one or more lines

    # The counter for this execution is also provided so that clients can
    # display it, since IPython automatically creates variables called _iN
    # (for input prompt In[N]).
    'execution_count' : int
}

Execution results

Results of an execution are published as an execute_result. These are identical to display_data messages, with the addition of an execution_count key.

Results can have multiple simultaneous formats depending on its configuration. A plain text representation should always be provided in the text/plain mime-type. Frontends are free to display any or all of these according to its capabilities. Frontends should ignore mime-types they do not understand. The data itself is any JSON object and depends on the format. It is often, but not always a string.

Message type: execute_result:

content = {

    # The counter for this execution is also provided so that clients can
    # display it, since IPython automatically creates variables called _N
    # (for prompt N).
    'execution_count' : int,

    # data and metadata are identical to a display_data message.
    # the object being displayed is that passed to the display hook,
    # i.e. the *result* of the execution.
    'data' : dict,
    'metadata' : dict,
}

Execution errors

When an error occurs during code execution

Message type: error:

content = {
   # Similar content to the execute_reply messages for the 'error' case,
   # except the 'status' field is omitted.
}

Kernel status

This message type is used by frontends to monitor the status of the kernel.

Message type: status:

content = {
    # When the kernel starts to handle a message, it will enter the 'busy'
    # state and when it finishes, it will enter the 'idle' state.
    # The kernel will publish state 'starting' exactly once at process startup.
    execution_state : ('busy', 'idle', 'starting')
}

Clear output

This message type is used to clear the output that is visible on the frontend.

Message type: clear_output:

content = {

    # Wait to clear the output until new output is available.  Clears the
    # existing output immediately before the new output is displayed.
    # Useful for creating simple animations with minimal flickering.
    'wait' : bool,
}

Opening a Comm

Opening a Comm produces a comm_open message, to be sent to the other side:

{
  'comm_id' : 'u-u-i-d',
  'target_name' : 'my_comm',
  'data' : {}
}

Every Comm has an ID and a target name. The code handling the message on the receiving side is responsible for maintaining a mapping of target_name keys to constructors. After a comm_open message has been sent, there should be a corresponding Comm instance on both sides. The data key is always a dict and can be any extra JSON information used in initialization of the comm.

If the target_name key is not found on the receiving side, then it should immediately reply with a comm_close message to avoid an inconsistent state.

Comm Messages

Comm messages are one-way communications to update comm state, used for synchronizing widget state, or simply requesting actions of a comm's counterpart.

Essentially, each comm pair defines their own message specification implemented inside the data dict.

There are no expected replies (of course, one side can send another comm_msg in reply).

Message type: comm_msg:

{
  'comm_id' : 'u-u-i-d',
  'data' : {}
}

Tearing Down Comms

Since comms live on both sides, when a comm is destroyed the other side must be notified. This is done with a comm_close message.

Message type: comm_close:

{
  'comm_id' : 'u-u-i-d',
  'data' : {}
}

Output Side Effects

Since comm messages can execute arbitrary user code, handlers should set the parent header and publish status busy / idle, just like an execute request.