Double output on Ctrl-enter-enter

[takluyver]

(Reported by Fujun Du on the mailing list)

When I run a cell in place with "Control-Enter" to make a plot, and if I press the key combination very quickly twice (namely, press "Control-Enter-Enter"), I will have two copies of the same plot shown below the cell.

This is nothing important but just for you to know the issue. To reproduce the behavior, I guess the computation in the cell must take quite some time to finish (to be slower than the key press). For example, put the following code into a new cell and press "Control-Enter-Enter" quickly you will get two output "xxx". In case your computer is very fast, add one or a few more zeros to the xrange:

import math
for i in xrange(1000000):
    a = math.sin(i)
print 'xxx'

[jsw-fnal]

This is not unique to Control-Enter. It works with Shift-Enter and the various "Run" options from the cell menu as well. You don't even have to use the same means of execution twice, for example Control-Enter followed by "Run all" will produce doubled output.

Finally, you don't need a spinning delay (like a long computation) to produce this effect. Making a delay with time.sleep() works just as well.

import time
time.sleep(5)
print 'xxx'

[minrk]

This is a bit more general than an order of operations bug. This should be a summary of the relevant bits:

• when you execute, cell.output_area.handle_output is registered as the callback
• this callback is never cleared
• when you submit a new execution, the output_area is cleared, but it remains the same object, so the old callback is still registered
• if output from an execution has not come by the time the cell is resubmitted, the earlier executions output will still be appended

I think we can get away with fixing this by simply clearing the callbacks for the last execution when we submit a new one.

[minrk]

should be fixed by #4224.