Your analysis and proposal look fine to me.

Hi @timhoffm ,

While testing the behavior of patch.edgecolor and patch.force_edgecolor, I encountered an problem where the artist becomes completely invisible when facecolor="None", which should not happen according to the case you mentioned above

If facecolor is "none", Patch and Collection take rcParams["patch.edgecolor"] as edgecolor.
I assume, this is done to prevent the artist being completely invisible (no facecolor and no edgecolor).

What I Did:
I created a simple Rectangle patch with facecolor="None", expecting the default patch.edgecolor to apply:

import matplotlib.pyplot as plt
import matplotlib.patches as patches

fig, ax = plt.subplots()
rect = patches.Rectangle((0.1, 0.1), 0.4, 0.4, facecolor="None")
ax.add_patch(rect)
plt.show()

Is this the expected behavior? Or did I miss something in my setup? Further, I’d like to contribute to this issue.

I can’t really tell on the expectation. I’ve just tried to deduce the current behavior, but I may have done a mistake.

I don’t have time to dig into this deeper right now. Anyway the way forward on this issue is

1. Find out in which cases patch.edgecolor is used, and what patch.force_edgecolor exactly does.
2. When we have this, we can come up with a migration strategy.

Okay, I am working on it, If I encounter any specific questions or uncertainties during this process, I’ll reach out to you.

Hi @timhoffm,
I analyzed how patch.edgecolor and patch.force_edgecolor are used in _set_edgecolor() and here’s what I found:

If color is explicitly provided, edgecolor uses that color directly.
If color is None, the function decides which edge color to use:

• If patch.force_edgecolor=True, patch.edgecolor is applied .
• If the patch is not filled (not self._fill) or the subclass enables _edge_default, patch.edgecolor is applied.
• Otherwise, edgecolor is set to "none" (meaning no visible edge).

if we want change default patch.edgecolor we should set patch.force_edgecolor=True

the problem occurs when facecolor="None", and patch.force_edgecolor=False, we get an invisible artist, to fix this we can add a condition that if facecolor="None", then edgecolor will be default patch.edgecolor

Hi @timhoffm, just checking in on my previous comment. Let me know if you have any thoughts on it or if I should clarify anything. Thanks!

@timhoffm can you please take a look at the above comments.

Sorry, I don't have enough quiet time right now to really think this through. It's tying a knot in my brain when I try to have a quick look.

From what it seems, patch.force_edgecolor is quite used, so we need to be extra careful if we change this. I'm unclear what would be better: just explicitly documenting the behavior (and finding a good way of describing it) or coming up with a more natural logic.

Hi @timhoffm currently, if facecolor="None" and patch.force_edgecolor=False, the artist becomes invisible. For now, we can fix this.

I believe the best way forward is to codify the current (intended) behavior in tests. Then refactor and make sure the resulting edgecolor stays the same.

@timhoffm I have made the PR, containing tests cases for patch.force_edgecolor when facecolor="none", please review and suggest required changes.

As noted in #29690, it only codifies current behaviour in tests, and does not fully fix this issue.