use rc_context, thanks @timhoffm for the tip

kbrose · kbrose · commit 03654551a3a3 · 2018-11-17T10:24:09.000-06:00
diff --git a/lib/matplotlib/tests/test_legend.py b/lib/matplotlib/tests/test_legend.py
@@ -569,10 +569,8 @@ def test_warn_big_data_best_loc():
     fig, ax = plt.subplots()
     ax.plot(np.arange(200001), label='Is this big data?')
     with pytest.warns(UserWarning) as records:
-        l = ax.legend()
-        # We need to call an internal set method tricking it into thinking
-        # 'best' location was default.
-        l._set_loc(l.codes['best'], is_initial_setting=True)
+        with rc_context({'legend.loc': 'best'}):
+            l = ax.legend()
         fig.canvas.draw()
     # The _find_best_position method of Legend is called twice, duplicating
     # the warning message.
