Devtools¶

Note

If you don't have the textual command on your path, you may have forgotten to install the textual-dev package.

Textual comes with a command line application of the same name. The textual command is a super useful tool that will help you to build apps.

Take a moment to look through the available subcommands. There will be even more helpful tools here in the future.

textual --help

Run¶

The run sub-command runs Textual apps. If you supply a path to a Python file it will load and run the app.

textual run my_app.py

This is equivalent to running python my_app.py from the command prompt, but will allow you to set various switches which can help you debug, such as --dev which enable the Console.

See the run subcommand's help for details:

textual run --help

You can also run Textual apps from a python import. The following command would import music.play and run a Textual app in that module:

textual run music.play

This assumes you have a Textual app instance called app in music.play. If your app has a different name, you can append it after a colon:

textual run music.play:MusicPlayerApp

Note

This works for both Textual app instances and classes.

Running from commands¶

If your app is installed as a command line script, you can use the -c switch to run it. For instance, the following will run the textual colors command:

textual run -c textual colors

Live editing¶

If you combine the run command with the --dev switch your app will run in development mode.

textual run --dev my_app.py

One of the features of dev mode is live editing of CSS files: any changes to your CSS will be reflected in the terminal a few milliseconds later.

This is a great feature for iterating on your app's look and feel. Open the CSS in your editor and have your app running in a terminal. Edits to your CSS will appear almost immediately after you save.

Console¶

When building a typical terminal application you are generally unable to use print when debugging (or log to the console). This is because anything you write to standard output will overwrite application content. Textual has a solution to this in the form of a debug console which restores print and adds a few additional features to help you debug.

To use the console, open up two terminal emulators. Run the following in one of the terminals:

textual console

You should see the Textual devtools welcome message:

In the other console, run your application with textual run and the --dev switch:

textual run --dev my_app.py

Anything you print from your application will be displayed in the console window. Textual will also write log messages to this window which may be helpful when debugging your application.

Increasing verbosity¶

Textual writes log messages to inform you about certain events, such as when the user presses a key or clicks on the terminal. To avoid swamping you with too much information, some events are marked as "verbose" and will be excluded from the logs. If you want to see these log messages, you can add the -v switch.

textual console -v

Decreasing verbosity¶

Log messages are classififed in to groups, and the -x flag can be used to exclude all message from a group. The groups are: EVENT, DEBUG, INFO, WARNING, ERROR, PRINT, SYSTEM, and LOGGING. The group a message belongs to is printed after its timestamp.

Multiple groups may be excluded, for example to exclude everything except warning, errors, and print statements:

textual console -x SYSTEM -x EVENT -x DEBUG -x INFO

Custom port¶

You can use the option --port to specify a custom port to run the console on, which comes in handy if you have other software running on the port that Textual uses by default:

textual console --port 7342

Then, use the command run with the same --port option:

textual run --dev --port 7342 my_app.py

Textual log¶

Use the log function to pretty-print data structures and anything that Rich can display.

You can import the log function as follows:

from textual import log

Here's a few examples of writing to the console, with log:

def on_mount(self) -> None:
    log("Hello, World")  # simple string
    log(locals())  # Log local variables
    log(children=self.children, pi=3.141592)  # key/values
    log(self.tree)  # Rich renderables

Log method¶

There's a convenient shortcut to log on the App and Widget objects. This is useful in event handlers. Here's an example:

from textual.app import App

class LogApp(App):

    def on_load(self):
        self.log("In the log handler!", pi=3.141529)

    def on_mount(self):
        self.log(self.tree)

if __name__ == "__main__":
    LogApp().run()

Logging handler¶

Textual has a logging handler which will write anything logged via the builtin logging library to the devtools. This may be useful if you have a third-party library that uses the logging module, and you want to see those logs with Textual logs.

Note

The logging library works with strings only, so you won't be able to log Rich renderables such as self.tree with the logging handler.

Here's an example of configuring logging to use the TextualHandler.

import logging
from textual.app import App
from textual.logging import TextualHandler

logging.basicConfig(
    level="NOTSET",
    handlers=[TextualHandler()],
)


class LogApp(App):
    """Using logging with Textual."""

    def on_mount(self) -> None:
        logging.debug("Logged via TextualHandler")


if __name__ == "__main__":
    LogApp().run()