Is this a Python 3 regression in IPython Notebook? - python-3.x

Question

I am attempting to create some simple asynchronously-executing animations based on ipythonblocks and I am trying to update the cell output area using clear_output() followed by a grid.show().
For text output the basis of the technique is discussed in Per-cell output for threaded IPython Notebooks so my simplistic assumption was to use the same method to isolate HTML output. Since I want to repeatedly replace a grid with its updated HTML version I try to use clear_output() to ensure that only one copy of the grid is displayed.
I verified that this proposed technique works for textual output with the following cells. First the context manager.
import sys
from contextlib import contextmanager
import threading
stdout_lock = threading.Lock()
n = 0
#contextmanager
def set_stdout_parent(parent):
"""a context manager for setting a particular parent for sys.stdout
(i.e. redirecting output to a specific cell). The parent determines
the destination cell of output
"""
global n
save_parent = sys.stdout.parent_header
# we need a lock, so that other threads don't snatch control
# while we have set a temporary parent
with stdout_lock:
sys.stdout.parent_header = parent
try:
yield
finally:
# the flush is important, because that's when the parent_header actually has its effect
n += 1; print("Flushing", n)
sys.stdout.flush()
sys.stdout.parent_header = save_parent
Then the test code
import threading
import time
class timedThread(threading.Thread):
def run(self):
# record the parent (uncluding the stdout cell) when the thread starts
thread_parent = sys.stdout.parent_header
for i in range(3):
time.sleep(2)
# then ensure that the parent is the same as when the thread started
# every time we print
with set_stdout_parent(thread_parent):
print(i)
timedThread().start()
This provided the output
0
Flushing 1
1
Flushing 2
2
Flushing 3
So I modified the code to clear the cell between cycles.
import IPython.core.display
class clearingTimedThread(threading.Thread):
def run(self):
# record the parent (uncluding the stdout cell) when the thread starts
thread_parent = sys.stdout.parent_header
for i in range(3):
time.sleep(2)
# then ensure that the parent is the same as when the thread started
# every time we print
with set_stdout_parent(thread_parent):
IPython.core.display.clear_output()
print(i)
clearingTimedThread().start()
As expected the output area of the cell was repeatedly cleared, and ended up reading
2
Flushing 6
I therefore thought I was on safe ground in using the same technique to clear a cell's output area when using ipythonblocks. Alas no. This code
from ipythonblocks import BlockGrid
w = 10
h = 10
class clearingBlockThread(threading.Thread):
def run(self):
grid = BlockGrid(w, h)
# record the parent (uncluding the stdout cell) when the thread starts
thread_parent = sys.stdout.parent_header
for i in range(10):
# then ensure that the parent is the same as when the thread started
# every time we print
with set_stdout_parent(thread_parent):
block = grid[i, i]
block.green = 255
IPython.core.display.clear_output(other=True)
grid.show()
time.sleep(0.2)
clearingBlockThread().start()
does indeed produce the desired end state (a black matrix with a green diagonal) but the intermediate steps don't appear in the cell's output area. To complicate things slightly (?) this example is running on Python 3. In checking before posting here I discover that the expected behavior (a simple animation) does in fact occur under Python 2.7. Hence I though to ask whether this is an issue I need to report.