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Title: [BUG] Warn when legend() receives mismatched handles and labels in 2-argument positional form by EncryptedDoom · Pull Request #31315 · matplotlib/matplotlib · GitHub

Open Graph Title: [BUG] Warn when legend() receives mismatched handles and labels in 2-argument positional form by EncryptedDoom · Pull Request #31315 · matplotlib/matplotlib

X Title: [BUG] Warn when legend() receives mismatched handles and labels in 2-argument positional form by EncryptedDoom · Pull Request #31315 · matplotlib/matplotlib

Description: PR Summary Closes #24050 Why is this change necessary? When calling ax.legend() using the 2-argument positional form — ax.legend(handles, labels) — with mismatched list lengths, matplotlib silently truncates to the shorter list with no warning raised to the user. This is a silent data loss scenario: the user gets a legend that is missing entries with no indication that anything went wrong. What problem does it solve? A warning already exists in _parse_legend_args() for the keyword argument path (ax.legend(handles=..., labels=...)), implemented at lines 1329–1334: if (hasattr(handles, "__len__") and hasattr(labels, "__len__") and len(handles) != len(labels)): _api.warn_external(f"Mismatched number of handles and labels: ...") However, the 2-argument positional path (elif len(args) == 2) at line 1365 had no equivalent check: elif len(args) == 2: handles, labels = args[:2] # No mismatch check — silent truncation This PR brings the positional path to parity with the keyword path by adding the same mismatch check. What is the reasoning for this implementation? The fix uses _api.warn_external() — consistent with the existing warning convention in this function — which correctly sets the stacklevel so the warning points to the user's legend() call site rather than to internal matplotlib code. No API changes are introduced. The behaviour after the warning is unchanged — matplotlib still truncates to the shorter list, preserving backward compatibility. Minimum self-contained example Before this fix (no warning raised): import matplotlib.pyplot as plt fig, ax = plt.subplots() line1, = ax.plot([1, 2], label='A') line2, = ax.plot([3, 4], label='B') # 2 handles, 1 label — silently drops line2 from legend ax.legend([line1, line2], ['only_one_label']) plt.show() After this fix: UserWarning: Mismatched number of handles and labels: len(handles) = 2 len(labels) = 1 AI Disclosure This contribution was developed with assistance from Claude (Anthropic) for identifying the bug location within the codebase and structuring the fix. The actual code changes, testing, and verification were performed by me. Claude was used as a reference tool to navigate the codebase, consistent with how one might use documentation or a senior developer for guidance. PR Checklist Closes #24050 is in the body of the PR description New and changed code is tested — test_legend_mismatched_positional_args() added in lib/matplotlib/tests/test_legend.py [N/A] Plotting related features demonstrated in an example — this is a bug fix (warning addition), not a new plotting feature [N/A] New Features and API Changes noted with directive — no API change introduced; behaviour is unchanged, only a warning is added Documentation complies with general and docstring guidelines — no new public API or docstring added; existing docstring already states "If they are not, they are truncated to the length of the shorter list" which remains accurate

Open Graph Description: PR Summary Closes #24050 Why is this change necessary? When calling ax.legend() using the 2-argument positional form — ax.legend(handles, labels) — with mismatched list lengths, matplotlib silently...

X Description: PR Summary Closes #24050 Why is this change necessary? When calling ax.legend() using the 2-argument positional form — ax.legend(handles, labels) — with mismatched list lengths, matplotlib silently...

Opengraph URL: https://github.com/matplotlib/matplotlib/pull/31315

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