Closes #31400

PR summary

This PR updates figsize handling based on the discussion in #31400.

Partial None is supported only when creating a figure:

plt.figure(figsize=(None, h)) uses the default width from rcParams["figure.figsize"]
plt.figure(figsize=(w, None)) uses the default height from rcParams["figure.figsize"]
If both values are None (figsize=(None, None)), a clear ValueError is raised.

Figure.set_size_inches now requires explicit width and height and rejects None to avoid ambiguous behavior.

changes

Updated figure creation logic to resolve a single missing figsize dimension from rcParams before numeric validation.

Updated Figure.set_size_inches to reject None for either width or height (no fallback/guessing).
Ensured None never reaches finite/positivity checks.

updated tests for:
valid partial-None figure creation
invalid figsize=(None, None)
invalid Figure().set_size_inches(None, 3) and Figure().set_size_inches(6, None)

Examples

plt.figure(figsize=(None, 4)) # Uses default width, height = 4

plt.figure(figsize=(6, None)) # Uses default height, width = 6

Testing

Ran
python -m pytest lib/matplotlib/tests/test_figure.py
All tests passed ✅

AI Disclosure

I used AI assistance to help draft parts of the implementation and tests, as well as to refine the PR description. All changes were reviewed and validated manually.

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines