Use color to highlight error locations

[pablogsal]

This has several advantages:

• Will help a lot with readability as parsing the error lines is easier if color highlights the error ranges.
• In the future we can optionally (via config) drop the ranges and only use color, recovering back the extra lines that the carets are taking.
• All the cool kids are doing it: This feature has already been successfully implemented in various tools. It has proven to be an effective aid for developers in quickly identifying the source and location of errors.

Control features:

• The feature will use 16 ANSI color scapes so the actual color is configured via the terminal emulator.
• Users can set the following env vars to control it (in the following order):
  • PY_COLORS=1 activates the feature (used by pytest: https://github.com/pytest-dev/pytest/blob/022f1b4de546c8b3529e071965555888ecf01cb4/src/_pytest/_io/terminalwriter.py#L28)
  • PY_COLORS=0 deactivates the feature
  • NO_COLOR=1 deactivates the feature
  • FORCE_COLOR=1 activates the feature
  • Feature is deactivated if terminal is not a tty or if TERM is set to dumb.

Linked PRs

• gh-112730: Use color to highlight error locations #112732
• gh-112730: Update docs for colour env vars #112837
• gh-112730: Make the test suite resilient to color-activation environment variables #117672
• gh-112730: use keyword only argument for colorize #118086
• gh-112730: Respect tests that require environment variables with no-colorize fixes #118288

[hugovk]

This looks really good, thanks!

Perhaps something for a followup, can we customise the colouring a bit?

PR #112732 right now:

GitHub / MagicPython

GitHub markdown (with pytb after the triple backticks):

Traceback (most recent call last):
  File "/Users/hugo/github/python/cpython/main/1.py", line 1, in <module>
    1/0
    ~^~
ZeroDivisionError: division by zero

(GitHub screenshot)

(Uses https://github.com/MagicStack/MagicPython for Python tracebacks: https://github.com/github-linguist/linguist/blob/master/vendor/README.md)

Sphinx / pygments

Sphinx with .. code-block:: pytb, for example on the PEPs repo:

(Uses Pygments' Native style)

[pablogsal]

Perhaps something for a followup, can we customise the colouring a bit?

Yup, see #112732 (comment).

I want to explore customization options AFTER we land the basic version first. EDIT Or are you referring to customizing the coloring as "adding more colors"?

On the other hand, are you suggesting to also colorize the name of the exception, the file, the number and the Traceback text?

I have been told that yellow and cian are really bad for light mode, so we should do something else there.

[pablogsal]

@hugovk What about like this?

[hugovk]

I wasn't thinking about allowing the user to customise them (although that's something to consider) but about having different colours for the different elements (exception name, file, etc).

Yes, we should check the contrast is good for both light and dark modes.

The GitHub colours (ignoring the plain black/white text) look the same for each mode and display well for both:

edit: and the blues are different, but you get the idea :)