[Matplotlib-devel] Catching parsing errors in Mathtext

Antony Lee antony.lee at institutoptique.fr
Tue Oct 12 04:03:57 EDT 2021

Currently there is no "intermediate format" that would allow generic
caching: 1) when the text object is created, we don't know yet whether it
will ultimately be rendered on screen or to a bitmap or a vector file, and
2) when rendering to a vector format (pdf, ps, svg), we do cache the
individual glyphs, but when rendering to a bitmap (screen, png), we
directly convert the expression to a bitmap, and there is no code to
instead convert individual glyphs to a bitmap.  In fact, I do have some
vague project of having a list-of-glyphs intermediate format, but that'll
require quite a bit of refactoring.

On Tue, Oct 12, 2021 at 2:16 AM Raymond Osborn via Matplotlib-devel <
matplotlib-devel at python.org> wrote:

> When testing v3.5.0rc1, I had a fright when my plots suddenly disappeared
> while attempting to change their title with ax.set_title. It turned out
> that this occurred because of a typo in the title text; I had closed a
> brace with a parenthesis by mistake. However, I was puzzled why this hadn’t
> triggered an exception. In the debugger, it looks as if this is because the
> exception was caught when calling draw_idle, but was never reraised because
> context managers stopped it. For some reason, errors do get raised when I
> run Pyplot in IPython with the same typo, but I haven’t worked out yet why
> embedding Matplotlib stops them appearing. Instead, the plot just
> disappears when I click on the PyQt canvas.
> The point of this post is too ask whether it might not be better to call
> the pyparsing module earlier in this process so that the user is
> immediately alerted to the invalid text rather than waiting for an
> attempted redraw. I don’t know enough about the text rendering engine to
> know whether there are reasons this is not possible, so I thought I would
> ask the question. Shouldn’t any call to set_title, set_xlabel, etc,
> immediately warn the user that the text doesn’t parse? If this is an
> efficiency issue, couldn’t the parsing result be cached in the Text object?
> With regards,
> Ray Osborn
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> Matplotlib-devel at python.org
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