code: def test_first_last_nth(df):
      # tests for first / last / nth
      grouped = df.groupby("A")
      first = grouped.first()
      expected = df.loc[[1, 0], ["B", "C", "D"]]
      expected.index = Index(["bar", "foo"], name="A")
      expected = expected.sort_index()
      tm.assert_frame_equal(first, expected)
  
      nth = grouped.nth(0)
      expected = df.loc[[0, 1]]
      tm.assert_frame_equal(nth, expected)
  
      last = grouped.last()
      expected = df.loc[[5, 7], ["B", "C", "D"]]
      expected.index = Index(["bar", "foo"], name="A")
      tm.assert_frame_equal(last, expected)
  
      nth = grouped.nth(-1)
      expected = df.iloc[[5, 7]]
      tm.assert_frame_equal(nth, expected)
  
      nth = grouped.nth(1)
      expected = df.iloc[[2, 3]]
      tm.assert_frame_equal(nth, expected)
  
      # it works!
      grouped["B"].first()
      grouped["B"].last()
      grouped["B"].nth(0)
  
      df = df.copy()
      df.loc[df["A"] == "foo", "B"] = np.nan
      grouped = df.groupby("A")
      assert isna(grouped["B"].first()["foo"])
      assert isna(grouped["B"].last()["foo"])
      assert isna(grouped["B"].nth(0).iloc[0])
  
      # v0.14.0 whatsnew
      df = DataFrame([[1, np.nan], [1, 4], [5, 6]], columns=["A", "B"])
      g = df.groupby("A")
      result = g.first()
      expected = df.iloc[[1, 2]].set_index("A")
      tm.assert_frame_equal(result, expected)
  
      expected = df.iloc[[1, 2]]
      result = g.nth(0, dropna="any")
      tm.assert_frame_equal(result, expected)
  
imports: import numpy
  import pytest
  import pandas
  from pandas import DataFrame, Index, MultiIndex, Series, Timestamp, isna
  import pandas._testing
label_desc: Use image node_id 1 for calling a global function with example usage: test_first_last_nth(df) without return types