YTEP-0039: Rich Terminal User Interface


Created: March 3, 2021 Author: Clément Robert

Use rich to prettify our TUI (Terminal User Interface). Most notably logs, and progress bars (as a replacement for tqdm).



Detailed Description

rich is a library to build colorful and styled terminal user interfaces.


In particular it offers a rich.logging.RichHandler class that can be used to replace standard logging.Handler instances, such as the one currently used by yt.

It supersedes our custom code to turn on colors in log entries and overall produces much prettier (as well as more useful) logs at a marginal maintainance cost, arguably cheaper than our existing facility.

Let’s illustrate our existing logger outputs and what rich turns them into, using the following script, and a minimalist configuration

import yt

yt.mylog.debug("2 + 2 = 5")"Oh, people can come up with statistics to prove anything, Kent. 14% of people know that.")
yt.mylog.warning("Don't eat that yellow snow.")
yt.mylog.error("I am Bender, please insert girder.")
yt.mylog.critical("You have 24h left to live.")

data = {
    "Nonetype": None,
    "a number": 1.657,
    "a boolean": True,
    "a list": [1, 2, 3, "spam", "bacon"],
}"Logging some data:\n %s", data)
../_images/ytep-0039_color_shell_log_legacy.png ../_images/ytep-0039_color_shell_log_rich.png ../_images/ytep-0039_color_notebook_log_legacy.png ../_images/ytep-0039_color_notebook_log_rich.png

Note that rich adds a clickable path to the source file where each entry was emmited from. Only the filename + line number are displayed but those are actually absolute links. Advanced terminal apps like iterm support link integration to make the best out of rich logging.

rich is flexible, supports a handful of color systems, and adapts to the system it runs against, which makes it more robust than our existing on/off switch for colored logs.

Currently, colored logs are turned off by default and activated from yt’s config file as

colored_logs = true

With rich, colored logs could be turned on by default at no cost, and with no risk of crashing a shell lacking color support. Because rich offers a lot of configuration options, we could choose to expose some of them in yt’s config file within a new, dedicated section, which I’m drafting here with proposed default values. This should be aligned with the current state of the documentation in #3106


# replaces yt.log_level
# logging level can be specifies as case (insensitive) string
# and passed down to yt.utilities.logger.set_log_level
level = "INFO"

# replaces yt.colored_logs
use_color = false

# replaces yt.stdout_stream_logging as well as yt.suppress_stream_logging
# accepted values are "stderr", "stdout" and "none" (completely disable logging)
# this is case insensitive to avoid breakage if a user was to write e.g., "None"
stream = "stderr"

# this is passed to a logging.Formatter instance
format = "%(message)s"

# this is arguably a more sensible default that the legacy format (unspecified)
# where miliseconds are displayed.
# This default value mimicks rich's, but exposing it makes it more obvious how it
# can be customized.
date_format = "[%m/%d/%Y %H:%M:%S]"

# the other option would be "legacy", see Backwards Compatibility section below
handler = "rich"

# the following options are silently ignored when `handler = "legacy"`

# width <=0 leaves rich's default Console width (auto-adjusted if the window changes
# size)
# otherwise must be >0 and is the total size (in columns) of a log entry
width = -1

# path to a custom rich config file, either absolute or relative to the cwd
# this parameter should be a (non-empty) string when it's used.
custom_theme = ""

Note that it is pretty hard to come up with satisfying and intuitive solution to interpret a relative path for the custom_theme option. It could be interpreted by humans as relative to any of the following - the global config file - the local config file - the current config file - the current working directory - the python script being run - yt’s install dir (less likely)

For this reason, I am not convinced it’s worth supporting relative paths at all, or exposing this option, but I’m willingly leaving to it in this state as the most experimental part of the (logging) project. Feedback will be collected to decide how it should or shouldn’t work according to early addopters if any.

Progress bars and status

The rich.progress module offers progress bars that are arguably much cooler-looking than the leading concurent (and current yt dependency) tqdm. More importantly, they are also much more flexible in a multi-tasking context (threading). Typically, rich can display more than one progress bar at once without interrupting the logging stream. For a demo of this, run

python -m rich.progress

Coexisting progress bars open the possibility for mutli-tasking with long-running tasks in yt without sacrifying expressivity in logs and other outputs.

Note that rich also borrows so-called “spinners” from cli-spinners, which offer a nice alternative to progress bars to express on-going progress, in particular in tasks where completion time may be difficult to estimate. Try them for yourself with

python -m rich.spinner

Known caveats

because it only knows tqdm. Replacing tqdm or more realistically adding support for rich.progress, or even arbitrary progress bar classes in pooch clearly requires a change upstream and is not a high priority, but eventually, this looks feasible.

yt CLI

A marginal side effect is that interactive command line applications could be writen in simpler ways than with vanilla Python using rich.

For instance, let’s look at a snippet that was proposed for inclusion our config migration script yt config migrate (see #3044)

prompt = "Perform the migration now [yn]? "
user_input = input(prompt).lower()
while user_input not in ("y", "yes", "n", "no"):
    print(f"Did not understand your input '{user_input}'. Please enter 'y' or 'n'.")
    user_input = input(prompt).lower()
if user_input in ("y", "yes"):
    raise SystemExit("Migration not performed: exiting.")

This can be expressed much more efficiently using rich.prompt

from rich.prompt import Confirm
if not Confirm.ask("Perform the migration now ?"):
    raise SystemExit("Migration not performed: exiting.")

At the time of writing however there is no clear spot where this functionality would shine in yt.


YTEP-0035 (pytest) is making progress and closing final implementation. pytest has builtin fixtures to capture logs (caplog) and standard outputs (capsys) to inspect them, which makes testing of logging format much easier.

I have started a branch to test and fix existing and new problems with the migration CLI yt config migrate in #3112, which relies on pytest.


  • Release notes.
  • config file migration/conversion facility: produce warnings when deprecated log-related parameters are found and offer to auto-convert them in place. In case a conflict is detected at runtime between old and new parameters, use the new ones, but advise the user to manually remove old ones (list them).

Backwards Compatibility

Downstream projects may rely on yt’s existing logging format. Some users may also simply prefer this style over what rich offers. Even if we make rich’s logging handler the default, we could offer a option to restore the “legacy” logging format in yt’s configuration file.

handler = "legacy"
use_color = false

Note that by construction, switching back to the legacy format would be an opt-in, which should be ok as long as it is properly documented in release notes. Keeping support for old-style progress bars would be relatively straightforward but it would create friction on the side of dependency specifications: if we support both styles at any point, then we have no way to formally specify tqdm OR rich is required but not both. Considering this, I suggest to simply drop tqdm and make rich a hard dependency immediately (#3114).

Functions yt.utilities.logging.colorize_logging and yt.utilities.logging.uncolorize_logging won’t be necessary anymore except if we want to maintain full backwards compatibility (legacy handler + color). What should be done with them is up for discussion but here’s my personnal opinion. They live in a pretty nested part of the yt namespace, but they may be used downstream since they are not explicitly (or implicitly) documented as private. I think it’s unlikely that anyone would want to use them at runtime instead of configuring yt, so this backwards compatibility breakage is likely acceptable. I also can’t think of a reasonable workflow for which users would care about pretty logs and wish to keep the legacy format at the same time. I propose to mark them as deprecated until the next release (following the acceptance of this YTEP), then remove them.


rich would be added to yt’s requirements.


Keep simple logs + tqdm.