[ENH]: ContourSet.set_paths

[rowan-stein]

Problem

To get contour labelling working with its special transforms, Cartopy has a workaround where it replaces all the paths on the ContourSet with transformed versions. This currently looks like

paths = cs.get_paths()
paths[:] = transformed_paths

which doesn’t smell very good.

Proposed solution

The above would smell better as

cs.set_paths(transformed_paths)

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Additional context

I'm actually a bit confused as to why Collection.set_paths raises NotImplementedError instead of setting self._paths (which is what Collection.get_paths already returns).

Thanks @anntzer. So would it be enough to just copy what PathCollection has?

https://github.com/matplotlib/matplotlib/blob/2a4d905ff2e6493264190f07113dc7f2115a8c1c/lib/matplotlib/collections.py#L1004-L1006

Untested, but possibly yes?

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Specification (from research)

• Add ContourSet.set_paths(paths) to replace the internal list of matplotlib.path.Path objects used for drawing and labeling.
• Validation:
  • paths must be a sequence of Path instances.
  • If self._paths exists, len(paths) must match len(self._paths) to preserve style/level mapping.
• Behavior:
  • Assign self._paths = list(paths).
  • If present, clear deprecated split-collections cache (e.g., del self._old_style_split_collections).
  • Mark self.stale = True.
  • No implicit transform application; caller supplies paths in the coordinate system of cs.get_transform().
• Placement:
  • Implement in lib/matplotlib/contour.py within class ContourSet.
  • Add typing stub in lib/matplotlib/contour.pyi.
• Tests:
  • Replacing paths should not crash and should mark .stale.
  • Labeling (clabel) works with transformed paths.
  • Existing behavior unchanged when not calling set_paths.

This Issue tracks task ID: 285.