watcher.py

"""
Simple IPython namespace watcher

watches a variable in an IPython kernel's namespace,
and displays it any time it changes.

Requires IPython 1.0-dev

Usage:

    python watcher.py [variable_name] [connection-file-pattern]
"""
import sys
import pprint

from IPython.kernel.blocking import BlockingKernelClient
from IPython.kernel import find_connection_file

block = """
ip = get_ipython()
from IPython.display import display
from IPython.utils import io
parent = ip.display_pub.parent_header
def display_the_frame():
    if "{0}" not in ip.user_ns:
        return
    
    # detach display from triggering cell,
    # so they don't see our extra output
    ip.display_pub.set_parent(parent)
    
    display(ip.user_ns['{0}'], metadata={{'watcher': True}})

ip.register_post_execute(display_the_frame)
"""

def watcher(cf, key):
    kc = BlockingKernelClient(connection_file=cf)
    kc.load_connection_file()
    kc.start_channels()
    kc.execute(block.format(key))
    reply = kc.get_shell_msg()
    if reply['content']['status'] != 'ok':
        print("Watcher setup failed:")
        print('\n'.join(reply['content']['traceback']))
        return
    
    previous = ''
    while True:
        msg = kc.get_iopub_msg()
        if msg['msg_type'] != 'display_data':
            continue
        content = msg['content']
        if not content['metadata'].get('watcher'):
            # not the result of our watch display
            continue
        todisplay = content['data'].get('text/plain', '')
        if todisplay == previous:
            # no change
            continue
        print('-' * 30)
        print(todisplay)
        previous = todisplay

if __name__ == '__main__':
    if len(sys.argv) > 1:
        key = sys.argv[1]
    else:
        key = raw_input("key to watch: ")
    
    if len(sys.argv) > 2:
        pattern = sys.argv[2]
    else:
        pattern = '*'
    cf = find_connection_file(pattern)
    print("loading connection file: %r" % cf)
    print("watching key: %r" % key)
    watcher(cf, key)