code: def test_nonunique_assignment_1750(self):
      df = DataFrame(
          [[1, 1, "x", "X"], [1, 1, "y", "Y"], [1, 2, "z", "Z"]],
          columns=list("ABCD"),
      )
  
      df = df.set_index(["A", "B"])
      mi = MultiIndex.from_tuples([(1, 1)])
  
      df.loc[mi, "C"] = "_"
  
      assert (df.xs((1, 1))["C"] == "_").all()
  
imports: import numpy
  import pytest
  from pandas.errors import SettingWithCopyError
  import pandas
  from pandas import DataFrame, MultiIndex, Series, date_range, isna, notna
  import pandas._testing
label_desc: Use image node_id 19 for calling the TestMultiIndexSetItem obj's underlying member method code with example usage: obj.test_nonunique_assignment_1750() without return types