Problem

I mistakenly supplied "blue\n" as the argument c for matplotlib.axes.Axes.scatter , then matplitlib claimed for illegal color name like this:

ValueError: 'c' argument must be a color, a sequence of colors, or a sequence of numbers, not blue

I was not aware that the argument actually contained a trailing newline so I was very confused.

Proposed solution

The error message would be nicer if it outputs user's input via repr.
For example, in this case the error message here can be easily replced with:

                    raise ValueError(
                        f"'c' argument must be a color, a sequence of colors, "
                        f"or a sequence of numbers, not {c!r}") from 

so that we may now get an easy-to-troubleshoot error like this:

ValueError: 'c' argument must be a color, a sequence of colors, or a sequence of numbers, not "blue\n"

This kind of improvement can be applied to many other places.