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I'm trying to get output from a python multiprocessing Process displayed in a Tkinter gui.

I can send output from Processes via a gui to a command shell, for example by running the fllowing tiny script at a shell prompt:

from multiprocessing import Process import sys def myfunc(text): print text sys.stdout.flush() def f1(): p1 = Process(target = myfunc, args = ("Surprise",)) p1.start() def f2(): p2 = Process(target = myfunc, args = ("Fear",)) p2.start() def fp(): myfunc("... and an almost fanatical devotion to the Pope") a = Tk() b1 = Button(a, text="Process 1", command=f1) b1.grid(row=0, column=0, pady=10, padx=10, sticky=SE) b2 = Button(a, text="Process 2", command=f2) b2.grid(row=0, column=1, pady=10, padx=10, sticky=SE) b3 = Button(a, text="Parent", command=fp) b3.grid(row=0, column=2, pady=10, padx=10, sticky=SE) if __name__ == "__main__": a.mainloop()

I can also send output from the parent to a Text box, for example by modifying the above by commenting out the flushing of stdout in myfunc

# sys.stdout.flush()

and adding immediately after the "b3.grid..." line the following:

class STDText(Text): def __init__(self, parent, cnf={}, **kw): Text.__init__(self, parent, cnf, **kw) def write(self, stuff): self.config(state=NORMAL) self.insert(END, stuff) self.yview_pickplace("end") self.config(state=DISABLED) messages = STDText(a, height=2.5, width=30, bg="light cyan", state=DISABLED) messages.grid(row=1, column=0, columnspan=3) sys.stdout = messages

However I can't figure out how to send output from the Processes to the text box. Am I missing something simple?

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3 Answers 3

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You could redirect stdout/stderr to a StringIO in myfunc(), then send whatever gets written into that StringIO back to the parent (as suggested by unutbu). See my answer to this question for one way of doing this redirection.

Since that example does a bit more than you need, here's a version that's more aligned with your stated goals:

#!/usr/bin/env python import sys from cStringIO import StringIO from code import InteractiveConsole from contextlib import contextmanager from multiprocessing import Process, Pipe @contextmanager def std_redirector(stdin=sys.stdin, stdout=sys.stdin, stderr=sys.stderr): tmp_fds = stdin, stdout, stderr orig_fds = sys.stdin, sys.stdout, sys.stderr sys.stdin, sys.stdout, sys.stderr = tmp_fds yield sys.stdin, sys.stdout, sys.stderr = orig_fds class Interpreter(InteractiveConsole): def __init__(self, locals=None): InteractiveConsole.__init__(self, locals=locals) self.output = StringIO() self.output = StringIO() def push(self, command): self.output.reset() self.output.truncate() with std_redirector(stdout=self.output, stderr=self.output): try: more = InteractiveConsole.push(self, command) result = self.output.getvalue() except (SyntaxError, OverflowError): pass return more, result def myfunc(conn, commands): output = StringIO() py = Interpreter() results = "" for line in commands.split('\n'): if line and len(line) > 0: more, result = py.push(line + '\n') if result and len(result) > 0: results += result conn.send(results) conn.close() if __name__ == '__main__': parent_conn, child_conn = Pipe() commands = """ print "[42, None, 'hello']" def greet(name, count): for i in range(count): print "Hello, " + name + "!" greet("Beth Cooper", 5) fugazi print "Still going..." """ p = Process(target=myfunc, args=(child_conn, commands)) p.start() print parent_conn.recv() p.join()

The usual caveats about security apply here (i.e., don't do this unless you can trust the sender of these code snippets to not do anything stupid/malicious).

Also note that you can simplify this a lot if you don't need to interpret an arbitrary mix of python expressions and statements. If you only need to call a top-level function that generates some outputs, something like this may be more appropriate:

def dosomething(): print "Doing something..." def myfunc(conn, command): output = StringIO() result = "" with std_redirector(stdout=output, stderr=output): try: eval(command) result = output.getvalue() except Exception, err: result = repr(err) conn.send(result) conn.close() if __name__ == '__main__': parent_conn, child_conn = Pipe() command = "dosomething()" p = Process(target=myfunc, args=(child_conn, command)) p.start() print parent_conn.recv() p.join()
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You can pass (picklable) data between processes using a multiprocessing.Pipe. For example:

import Tkinter import multiprocessing as mp class STDText(Tkinter.Text): def __init__(self, parent, cnf={}, **kw): Tkinter.Text.__init__(self, parent, cnf, **kw) def write(self, stuff): self.config(state=Tkinter.NORMAL) self.insert(Tkinter.END, stuff) self.yview_pickplace("end") self.config(state=Tkinter.DISABLED) def myfunc(conn,text): conn.send(text) conn.close() class Gui(object): def __init__(self): self.a=Tkinter.Tk() b1=Tkinter.Button(self.a, text="Process 1", command=self.foo) b1.grid(row=0, column=0, pady=10, padx=10, sticky=Tkinter.SE) b2=Tkinter.Button(self.a, text="Process 2", command=self.bar) b2.grid(row=0, column=1, pady=10, padx=10, sticky=Tkinter.SE) b3=Tkinter.Button(self.a, text="Parent", command=self.baz) b3.grid(row=0, column=2, pady=10, padx=10, sticky=Tkinter.SE) self.messages=STDText( self.a, height=2.5, width=30, bg="light cyan", state=Tkinter.DISABLED) self.messages.grid(row=1, column=0, columnspan=3) self.a.mainloop() def call_myfunc(self,text): parent_conn, child_conn=mp.Pipe() proc=mp.Process(target=myfunc, args=(child_conn,text,)) proc.start() self.messages.write(parent_conn.recv()) proc.join() def foo(self): self.call_myfunc('Foo\n') def bar(self): self.call_myfunc('Bar\n') def baz(self): parent_conn, child_conn=mp.Pipe() myfunc(child_conn,'Baz\n') self.messages.write(parent_conn.recv()) if __name__ == "__main__": Gui()

See Doug Hellman's tutorial on multiprocessing for more information.

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1 Comment

Unutbu - Thanks for the very helpful answer. It certainly solves my toy example. (In the real application the processes will run different functions and produce output messages spontaneously (ie not printing some specific text given to them as a parameter). I had wondered whether it was possible to find a 'cheapo' to avoid getting into pipes, but I guess that probably is the way to go.
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Assuming that myfunc is called with the output of the process, just write myfunc like:

def myfunc(text): textwidget.insert("end", text)

where textwidget is a handle to the text widget

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Thanks for the suggestion. It would certainly help with my toy example. However I maybe oversimplified what I need a little - in the real application the processes will run different functions and produce output messages spontaneously (ie not printing some specific text given to them as a parameter).

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