Resizing problems when importing readline

[imneedle]

Textual application having problems resizing when importing readline

I have developed a Textual-based application that I want to integrate into my main application. I am facing some problems because my main application is calling import readline, which is breaking the Textual application's resizing capabilities.

Some context

• I have built a TUI using Textual, and now I would like to attach it to my main application.
• The main application is going to be running some code, such as printing information to stdout and writing to files.
• What I would like to do is to launch the main application, which itself launches the Textual application, and does so without blocking the main application's code.
• Additionally, it would be ideal if once I quit the textual application, it restores the stdout to be what it would have been if the textual application was never launched.
• Basically I want something like this:

import time
from datetime import datetime
from pathlib import Path

from textual.app import App, ComposeResult
from textual.widgets import Static


class DummyApp(App):
    def compose(self) -> ComposeResult:
        for _ in range(15):
            yield Static("This is a dummy Test GUI")


file_path = Path("some-file.txt")
launch_app = True
while True:
    # This is the main application's code doing some work
    print("Hello, doing work from the main application")
    file_path.write("Hello, main application writing to file")
 
    if launch_app:
        launch_app = False  # don't re launch app after
        # HERE start textual application
        app = App()
        app.run()

    time.sleep(5)  # simulate expensive workload

Solution I came up with and it's problem

• In order to launch a textual app without block the rest of the application's code, the main application launches a Thread. That thread will make a call to subprocess.Popen to run the textual application.
• The code roughly looks like this:

  import subprocess
  import threading
  import time
  from datetime import datetime
  from pathlib import Path
  
  from textual.app import App, ComposeResult
  from textual.widgets import Static
  
  
  class DummyApp(App):
     def compose(self) -> ComposeResult:
         for _ in range(15):
             yield Static("This is a dummy Test GUI")
  
  
  
  def run_textual_app():
      process = subprocess.Popen(["python3", "app.py"])  # where app.py launches the DummyApp via app.run()
      process.wait()
  
  file_path = Path("some-file.txt")
  launch_app = True
  while True:
      print("Hello, world!")  # simulating doing work in the main application
      file_path.write("Hello, world!")  # writing to files too
  
      if launch_app:
          launch_app = False  # don't re launch app after
          threading.Thread(target=run_textual_app).start()
   
      time.sleep(5)  # simulate expensive workload

• This partially works, as it does allow to launch the textual application without blocking the main application, however it causes problems because the main application is writing to stdout:
  

Fix for the stdout problem

• My idea for this problem was to use os.dup2 to switch the file descriptors of the main application to write to a temporary file instead of writing to stdout.
• I did this via :

import os
import sys
import subprocess

def run_textual_app():
    original_stdout_fd = sys.stdout.fileno()  # keep reference to original file descriptor of stdout
    temp_stdout_fd = os.dup(original_stdout_fd)  # store that inside temp_stdout_fd so we can restore stdout later
    
    with open("./tmp-stdout.log", "w") as f:
        os.dup2(f.fileno(), original_stdout_fd)  # make stdout of main application write the tmp-stdout.log file
        
        process = subprocess.Popen(["python", "app.py"])
        process.wait()
       
        # Here, the textual application is done running, we restore stdout
        os.dup2(temp_stdout_fd, original_stdout_fd)
        os.close(temp_stdout_fd)

    # Restore stdout to be as if we never launched the application
    with open("./tmp-stdout.log", "r") as f:
        content = f.read()
        sys.stdout.write(content)

# Same code for while True loop:

Despite solving the stdout issue, a new problem arose related to screen resizing after importing the readline module, which seems to affect the application's ability to adjust to terminal size changes

The resizing problem

My main application makes an import to readline (without even using it). This somehow, breaks the application's resizing capabilities.

If I add the following import statement to the previous code (without using the imported module):

import readline

• Before resizing:
  
• After rezising down (making screen smaller): I lose the first line
  
• After resizing up (making the screen bigger): The application doesn't resize:
  

All I did was add an import readline att the start of the previous code, and it breaks the rezising. I don't even use the module, I simply import it
If I remove the import readline, the resizing works:

I can't figure out why this is happening. Please help.

Note: If you have a better idea than launching a threading.Thread and calling subprocess, please do share.

[github-actions[bot]]

We found the following entry in the FAQ which you may find helpful:

• How do I fix WorkerDeclarationError?

Feel free to close this issue if you found an answer in the FAQ. Otherwise, please give us a little time to review.

This is an automated reply, generated by FAQtory

[willmcgugan]

You might want to consider running your application from the Textual app with a worker. Alternatively you could use a subprocess for your main application. Allow the Textual app to respond to stdin and stdout. Consider using the asyncio subprocess API for that.

I'm guessing the readline issue is caused by readline handling the terminal resize signal. The readline module doesn't really make sense in the same context as a Textual app.

[domdfcoding]

I have noticed readline only breaks resizing when imported in a libVTE terminal (e.g. gnome-terminal), not in VSCode's built in terminal. In my case the import is from a click extension library (I use a click CLI to launch the app either as the Textual interface or a plainer one). I can patch the library to work around this, but I wonder if there is a better solution.

Is Textual using SIGWINCH to detect resizes? If so I think this is being caused by python/cpython#46927. There is a patch available that should fix it.