gh-118626: Use less red-ish colour for non-error traceback

[hugovk]

Fixes #118626.

The magenta color used by the new REPL and within the non-error text in the new colorized tracebacks in 3.13 looks quite a bit like an error-related color in some popular color schemes.

In this PR, I'm trying to mirror something like GitHub's pytb colour groups to avoid having too many colours:

>>> 4 / 0
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    4 / 0
    ~~^~~
ZeroDivisionError: division by zero
>>>
>>> 3.to_bytes(1, 'little')"
  File "<string>", line 1
    3.to_bytes(1, 'little')
     ^
SyntaxError: invalid decimal literal

In the before and after below, I'm using macOS and have iTerm with a dark theme, and Terminal with light theme (Novel), to make it easy to compare without having to dig in the settings each time.

Before
After
GitHub pytb

And a longer traceback using this script based on a test:

2.py

def foo(*args):
    x = {'a': {'b': None}}
    y = x['a']['b']['c']


def baz2(*args):
    return (lambda *args: foo(*args))(1, 2, 3, 4)


def baz1(*args):
    return baz2(1, 2, 3, 4)


def bar():
    return baz1(1,
                2, 3
                , 4)


bar()

pytb

❯ ./python.exe 2.py
Traceback (most recent call last):
  File "/Users/hugo/github/python/cpython/main/2.py", line 20, in <module>
    bar()
    ~~~^^
  File "/Users/hugo/github/python/cpython/main/2.py", line 15, in bar
    return baz1(1,
                2, 3
                , 4)
  File "/Users/hugo/github/python/cpython/main/2.py", line 11, in baz1
    return baz2(1, 2, 3, 4)
  File "/Users/hugo/github/python/cpython/main/2.py", line 7, in baz2
    return (lambda *args: foo(*args))(1, 2, 3, 4)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
  File "/Users/hugo/github/python/cpython/main/2.py", line 7, in <lambda>
    return (lambda *args: foo(*args))(1, 2, 3, 4)
                          ~~~^^^^^^^
  File "/Users/hugo/github/python/cpython/main/2.py", line 3, in foo
    y = x['a']['b']['c']
        ~~~~~~~~~~~^^^^^
TypeError: 'NoneType' object is not subscriptable

Before
After
GitHub pytb

• Issue: Use a less red-ish color for the new prompt and non-error colorized traceback text #118626

[nineteendo]

Hugo, could you change the color of ">>>" as well? It's not magenta for pytb. Or does that need its own PR?

cpython/Lib/asyncio/__main__.py

Line 101 in 70934fb

ps1 = f"{ANSIColors.BOLD_MAGENTA}{ps1}{ANSIColors.RESET}"

cpython/Lib/_pyrepl/reader.py

Line 466 in 70934fb

prompt = f"{ANSIColors.BOLD_MAGENTA}{prompt}{ANSIColors.RESET}"

[hugovk]

How keen is @ambv on keeping the magenta prompt?

main
PR now
PR + blue prompt

[ambv]

Let's keep the prompt color. In the end we want to make this configurable anyway since there's no way we will keep everybody happy.

[serban]

I find the red and magenta combination in the stack trace jarring, and I endorse this pull request with the following suggestion:

Given the vast number of environments and terminals that Python runs in, it should stick to a conservative ANSI color palette and avoid the use of “bold” colors since the result of setting the “bold” attribute varies between terminal emulators and user configuration [1] [2]. Using the six basic ANSI colors without any attributes set would be the safest option:

№	Color	Foreground
1	Red	31
2	Green	32
3	Yellow	33
4	Blue	34
5	Magenta	35
6	Cyan	36

[1] https://invisible-island.net/xterm/xterm.faq.html#bold_vs_16colors
[2] https://unix.stackexchange.com/a/696593

[hugovk]

Last week I made some slides to show the improved error messages coming in 3.13, and used the example from What's New.

These have quite big error messages, and I find the blue of this PR (lower example) much better than the original magenta (upper example):

@ambv @pablogsal You've spoken of adding config for all these things, but I think this improves the defaults. What do you think?

And cc @treyhunner as issue reporter, what do you think?

[pablogsal]

@ambv @pablogsal You've spoken of adding config for all these things, but I think this improves the defaults. What do you think?

I'm unfortunately very colourblind so I fear my answer here it's going to be very biased and maybe not representative of the average user when selecting the defaults 😕

[nineteendo]

Could you remove the quoted text from your comment? Or only include the question?

[treyhunner]

And cc @treyhunner as issue reporter, what do you think?

I like the blue that is currently in this PR better (on my color scheme at least). I like the blue prompt in the bottom row of #119966 (comment) even better.

I believe @ambv mentioned trying to keep the changes to just 2 colors, so as not to cause visual overwhelm.

A note: depending on whether "bold as bright" is enabled on the terminal emulator, the blue may show up as a different color instead (it does in the Solarized Light color scheme).

Without "bold text as bright colors" enabled:

With "bold text as bright colors" enabled:

With the default Ubuntu Terminal settings, the blue looks a bit low contrast:

"Bold as bright" fixes that, though this setting does not seem to be the default on Gnome Terminal in Ubuntu 22.04. I believe it is the default on many Mac terminal emulators, but I have not researched this.

[hugovk]

Closing in favour of theming support: #133346