[IPython-dev] variable information request

Jason Grout jason-sage at creativetrax.com
Sat Apr 9 07:53:18 EDT 2011

Here's a crazy idea that could lead to some nice interactively-updating 
components of an IPython frontend.  Right now, in the new messaging 
protocol, it appears that I can send a computation request and then ask 
for the output to contain the values of several variables (the 
"user_variables" field of a execute_reply message).  However, what if I 
want to check the value of a variable *during* a computation and get an 
immediate response?  I might, for example, have a box that prints out 
the current value of a root approximation, for example, or a slider that 
contains the current iteration number, and I want these to be updated 
fairly frequently while the computation is running.

Here's one way to do it, I think:  Run a separate thread that just 
answers these queries.  Since the GIL handles access to variables, I 
think it's okay for the separate thread to retrieve the value of a 
variable and return that, and it seamlessly does this while the main 
thread is carrying on a computation.  For example, the following code 
will print out the current iteration number every time enter is pressed:

from threading import Timer, Thread
import time

def printi():
     global i
     while True:
         raw_input('Press enter to see the iteration number')
         print i


for i in range(20):

I can see a problem if the value you are querying actually changes the 
object state in the middle of another computation using that object. But 
simple queries about an object's value should work fine, and maybe we 
could leave it up to the user to not mess up a currently-running 
computation by changing an object state.

What do people think?



Jason Grout

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