Fixes #32313.

Root cause

FixedLocator.__init__ stores its tick locations via np.asarray(locs):

def __init__(self, locs, nbins=None):
    self.locs = np.asarray(locs)

np.asarray is a no-op when its input is already an ndarray — it returns the same array object, not a copy. Since Axis.set_ticks()/set_xticks() build a FixedLocator directly from whatever array the caller passed in, FixedLocator.locs ends up aliasing that array. Any later in-place mutation of it retroactively moves the axis' already-set tick locations:

import numpy as np
import matplotlib.pyplot as plt

fig, ax = plt.subplots()
theta = np.linspace(0, 2 * np.pi, 5, endpoint=False)
ax.set_xticks(theta)
theta += 2 * np.pi  # unrelated in-place mutation elsewhere in the program
# ax's ticks silently moved too

A plain list input doesn't show this, since converting a list to an array always produces a fresh one — which is presumably why this went unnoticed for a while.

Fix

Use np.array(locs) instead, which copies by default, so FixedLocator owns its storage regardless of whether the input was a list or an existing ndarray.

Testing

• Added TestFixedLocator.test_locs_is_a_copy_of_ndarray_input, which constructs a FixedLocator from an ndarray, mutates the array in place afterward, and asserts .locs is unaffected. Verified it fails on unpatched ticker.py (fixed.locs is not locs → AssertionError) and passes with the fix.
• Manually reproduced the original report's polar-plot example end-to-end (prebuilt matplotlib wheel with just ticker.py swapped in, since I couldn't build the C extensions from source in my sandbox — no network access for the bundled freetype/etc. fetch): confirmed ax.get_xticks() changes after mutating the input array on unpatched code, and stays stable with the fix. Also confirmed a plain-list set_xticks([1, 2, 3]) call is unaffected.
• ruff check on both changed files — clean (matplotlib's pyproject.toml only configures ruff check, not ruff format, so I didn't run the formatter).

AI disclosure

I used Claude Code to investigate and draft this fix; I reviewed the diagnosis (traced set_ticks → FixedLocator.__init__ → np.asarray), verified the reproduction and fix behavior myself as described above, and reviewed every changed line before opening this PR.

🤖 Generated with Claude Code