code: def test_constructor_mixed(self):
      idx = PeriodIndex(["2011-01", NaT, Period("2011-01", freq="M")])
      exp = PeriodIndex(["2011-01", "NaT", "2011-01"], freq="M")
      tm.assert_index_equal(idx, exp)
  
      idx = PeriodIndex(["NaT", NaT, Period("2011-01", freq="M")])
      exp = PeriodIndex(["NaT", "NaT", "2011-01"], freq="M")
      tm.assert_index_equal(idx, exp)
  
      idx = PeriodIndex(
          [Period("2011-01-01", freq="D"), NaT, "2012-01-01"]
      )
      exp = PeriodIndex(["2011-01-01", "NaT", "2012-01-01"], freq="D")
      tm.assert_index_equal(idx, exp)
  
imports: import numpy
  import pytest
  from pandas._libs.tslibs.period import IncompatibleFrequency
  from pandas.core.dtypes.dtypes import PeriodDtype
  from pandas import Index, NaT, Period, PeriodIndex, Series, date_range, offsets, period_range
  import pandas._testing
  from pandas.core.arrays import PeriodArray
label_desc: Use image node_id 20 for calling the TestPeriodIndex obj's underlying member method code with example usage: obj.test_constructor_mixed() without return types