redirect sys.stdout to specific Jupyter Notebook cell

Question

Jupyter==4.1.0, Python==2.7.10, IPython==4.2.0

I'm writing a SQL UI for my Jupyter Notebooks and would like to incorporate multithreading so that I can run a query in one cell and continue to work in other cells while the query is running.

The problem I'm having is that if I execute a query in one cell, the output will be displayed in the last-executed cell's output prompt instead of in the output prompt of the cell that executed the query.

I scoured the interwebs and discovered this clever trick, but I think it's outdated and/or no longer works in my version of Jupyter. When I run it, I only get output for whatever cell was last executed. So if I run both, I only get the last-executed output, instead of the output printing to separate cells simultaneously.

So I have my context manager which sets the parent_header:

import sys import threading from contextlib import contextmanager  # we need a lock so that other threads don't snatch control # while we have set a temporary parent stdout_lock = threading.Lock()  @contextmanager def set_stdout_parent(parent):     """a context manager for setting a particular parent for sys.stdout      the parent determines the destination cell of the output     """     save_parent = sys.stdout.parent_header     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             sys.stdout.flush()             sys.stdout.parent_header = save_parent 

I essentially want to be able to get the parent_header of a cell In[1] and redirect the output of cell In[2] to the output of In[1].

Example:

Get parent_header of In[1]:

In[1]: t = sys.stdout.parent_header 

Then the following code will run, but the output should print to Out[1] (currently, I get no output when I run this code):

In [2]: with set_stdout_parent(t):             print 'FOO' 

Which should produce:

In[1]: t = sys.stdout.parent_header Out[1]:'FOO' 

Answer 1

The documentation for ipywidgets.Output has a section about interacting with output widgets from background threads. Using the Output.append_stdout method there is no need for locking. The final cell in this answer can then be replaced with:

def t1_main():     for i in range(10):         output1.append_stdout(f'thread1 {i}\n')         time.sleep(0.5)   def t2_main():     for i in range(10):         output2.append_stdout(f'thread2 {i}\n')         time.sleep(0.5)  output1.clear_output() output2.clear_output()          t1 = Thread(target=t1_main) t2 = Thread(target=t2_main) t1.start() t2.start() t1.join() t2.join()