python - How to redirect print statements to Tkinter text widget

Question

I have a Python program which performs a set of operations and prints the response on STDOUT. Now I am writing a GUI which will call that already existing code and I want to print the same contents in the GUI instead of STDOUT. I will be using the Text widget for this purpose. I do not want to modify my existing code which does the task (This code is used by some other programs as well).

Can someone please point me to how I can use this existing task definition and use its STDOUT result and insert it into a text widget? In the main GUI program I want to call this task definition and print its results to STDOUT. Is there a way to use this information?

Answer 1

You can probably solve this by replacing sys.stdout with your own file-like object that writes to the text widget.

For example:

import Tkinter as tk
import sys

class ExampleApp(tk.Tk):
    def __init__(self):
        tk.Tk.__init__(self)
        toolbar = tk.Frame(self)
        toolbar.pack(side="top", fill="x")
        b1 = tk.Button(self, text="print to stdout", command=self.print_stdout)
        b2 = tk.Button(self, text="print to stderr", command=self.print_stderr)
        b1.pack(in_=toolbar, side="left")
        b2.pack(in_=toolbar, side="left")
        self.text = tk.Text(self, wrap="word")
        self.text.pack(side="top", fill="both", expand=True)
        self.text.tag_configure("stderr", foreground="#b22222")

        sys.stdout = TextRedirector(self.text, "stdout")
        sys.stderr = TextRedirector(self.text, "stderr")

    def print_stdout(self):
        '''Illustrate that using 'print' writes to stdout'''
        print "this is stdout"

    def print_stderr(self):
        '''Illustrate that we can write directly to stderr'''
        sys.stderr.write("this is stderr\n")

class TextRedirector(object):
    def __init__(self, widget, tag="stdout"):
        self.widget = widget
        self.tag = tag

    def write(self, str):
        self.widget.configure(state="normal")
        self.widget.insert("end", str, (self.tag,))
        self.widget.configure(state="disabled")

app = ExampleApp()
app.mainloop()

Answer 2

In python, whenever you call print('examplestring'), you're indirectly calling sys.stdout.write('examplestring') :

from tkinter import *
root=Tk()
textbox=Text(root)
textbox.pack()
button1=Button(root, text='output', command=lambda : print('printing to GUI'))
button1.pack()

Method 1: Print out on GUI

def redirector(inputStr):
    textbox.insert(INSERT, inputStr)

sys.stdout.write = redirector #whenever sys.stdout.write is called, redirector is called.

root.mainloop()

Infact we're calling print -(callsfor)-> sys.stdout.write -(callsfor)-> redirector

Method 2: Writing a decorator - print out on both CLI and GUI

def decorator(func):
    def inner(inputStr):
        try:
            textbox.insert(INSERT, inputStr)
            return func(inputStr)
        except:
            return func(inputStr)
    return inner

sys.stdout.write=decorator(sys.stdout.write)
#print=decorator(print)  #you can actually write this but not recommended

root.mainloop()

What a decorator does is it actually assign the func sys.stdout.write to func inner

sys.stdout.write=inner

and func inner adds an extra line of code before calling back the actual sys.stdout.write

This is in a way updating the older func sys.stdout.write to have new feature. You will notice that I used a try-except such that if there's any error in printing to the textbox, I would at least retain the original func of sys.stdout.write to the CLI

Method 3: Bryan Oakley's example

...
    sys.stdout = TextRedirector(self.text, "stdout")
...
class TextRedirector(object):
    def __init__(self, widget, tag="stdout"):
        self.widget = widget
        self.tag = tag

    def write(self, str):
        self.widget.configure(state="normal")
        self.widget.insert("end", str, (self.tag,))
        self.widget.configure(state="disabled")

What he did was that he assigned sys.stdout to Class TextRedirector with a Method .write(str)

so calling print('string') -calls for-> sys.stdout.write('string') -callsfor-> TextRedirector.write('string')

Answer 3

You can call the CLI program using subprocess.Popen, grab the stdout it produces, and display it in the text widget.

Something along the lines of (untested):

import subprocess

with subprocess.Popen(your_CLI_program, stdout=subprocess.PIPE) as cli
    line = cli.stdout.readline()

    #process the output of your_CLI_program
    print (line)

Note that this will block until the CLI program finishes executing, freezing your GUI. To get around the blocking, you can put this code in a threading.Thread and let the GUI update while waiting for the thread to finish.

Answer 4

In fact, I think this problem is not limited to tkinter. Any framework can be applied because it is actually redirecting sys.stdout.

I created a class (RedirectStdMsg) to do this.

tl;dr

original = sys.stdout
sys.stdout = everything_you_like
...
sys.stdout = original  # restore

import sys
from typing import TextIO
from typing import Callable
# import tkinter as tk

class RedirectStdMsg:
    __slots__ = ('original', 'output_device',)

    def __init__(self, sys_std: TextIO):
        self.output_device = None
        self.original = sys_std

    def __call__(self, output_device=Callable[[str], None]):
        self.output_device = output_device
        return self

    def __enter__(self):
        if self.output_device is None:
            raise AttributeError('output_device is empty')
        self.start(self.output_device)

    def __exit__(self, exc_type, exc_val, exc_tb):
        if exc_val:
            self.write(str(exc_val))
        self.stop()

    def start(self, output_device):
        self.output_device = output_device
        std_name = self.original.name.translate(str.maketrans({'<': '', '>': ''}))
        exec(f'sys.{std_name} = self')  # just like: ``sys.stderr = self``

    def stop(self):
        std_name = self.original.name.translate(str.maketrans({'<': '', '>': ''}))
        exec(f'sys.{std_name} = self.original')
        self.output_device = None

    def write(self, message: str):
        """ When sys.{stderr, stdout ...}.write is called, it will redirected here"""
        if self.output_device is None:
            self.original.write(message)
            self.original.flush()
            return
        self.output_device(message)

Test

test tk class

modified from @Bryan Oakley

class ExampleApp(tk.Tk):
    def __init__(self, **options):
        tk.Tk.__init__(self)
        toolbar = tk.Frame(self)
        toolbar.pack(side="top", fill="x")
        b1 = tk.Button(self, text="print to stdout", command=self.print_stdout)
        b2 = tk.Button(self, text="print to stderr", command=self.print_stderr)
        b1.pack(in_=toolbar, side="left")
        b2.pack(in_=toolbar, side="left")
        self.text = tk.Text(self, wrap="word")
        self.text.pack(side="top", fill="both", expand=True)
        self.text.tag_configure("stderr", foreground="#b22222")

        self.re_stdout = options.get('stdout')
        self.re_stderr = options.get('stderr')

        if self.re_stderr or self.re_stderr:
            tk.Button(self, text='Start redirect', command=self.start_redirect).pack(in_=toolbar, side="left")
            tk.Button(self, text='Stop redirect', command=self.stop_redirect).pack(in_=toolbar, side="left")

    def start_redirect(self):
        self.re_stdout.start(TextRedirector(self.text, "stdout").write) if self.re_stdout else ...
        self.re_stderr.start(TextRedirector(self.text, "stderr").write) if self.re_stderr else ...

    def stop_redirect(self):
        self.re_stdout.stop() if self.re_stdout else ...
        self.re_stderr.stop() if self.re_stderr else ...

    @staticmethod
    def print_stdout():
        """Illustrate that using 'print' writes to stdout"""
        print("this is stdout")

    @staticmethod
    def print_stderr():
        """Illustrate that we can write directly to stderr"""
        sys.stderr.write("this is stderr\n")


class TextRedirector(object):
    def __init__(self, widget, tag="stdout"):
        self.widget = widget
        self.tag = tag

    def write(self, msg):
        self.widget.configure(state="normal")
        self.widget.insert("end", msg, (self.tag,))
        self.widget.configure(state="disabled")

test cases

def test_tk_without_stop_btn():
    app = ExampleApp()
    with RedirectStdMsg(sys.stdout)(TextRedirector(app.text, "stdout").write), \
            RedirectStdMsg(sys.stderr)(TextRedirector(app.text, "stderr").write):
        app.mainloop()


def test_tk_have_stop_btn():
    director_out = RedirectStdMsg(sys.stdout)
    director_err = RedirectStdMsg(sys.stderr)
    app = ExampleApp(stdout=director_out, stderr=director_err)
    app.mainloop()


def test_to_file():

    # stdout test
    with open('temp.stdout.log', 'w') as file_obj:
        with RedirectStdMsg(sys.stdout)(file_obj.write):
            print('stdout to file')
    print('stdout to console')


    # stderr test
    with open('temp.stderr.log', 'w') as file_obj:
        with RedirectStdMsg(sys.stderr)(file_obj.write):
            sys.stderr.write('stderr to file')
    sys.stderr.write('stderr to console')

    # another way
    cs_stdout = RedirectStdMsg(sys.stdout)
    cs_stdout.start(open('temp.stdout.log', 'a').write)
    print('stdout to file 2')
    ...
    cs_stdout.stop()
    print('stdout to console 2')


if __name__ == '__main__':
    test_to_file()
    test_tk_without_stop_btn()
    test_tk_have_stop_btn()

this is test_tk_have_stop_btn():

Answer 5

The function which normally prints to stdout should instead put the text into text widget.