Add tests and whatsnew

Author: phofl

doc/source/whatsnew/v1.2.0.rst

@@ -235,6 +235,7 @@ Other enhancements
 - Improve error reporting for :meth:`DataFrame.merge()` when invalid merge column definitions were given (:issue:`16228`)
 - Improve numerical stability for :meth:`Rolling.skew()`, :meth:`Rolling.kurt()`, :meth:`Expanding.skew()` and :meth:`Expanding.kurt()` through implementation of Kahan summation (:issue:`6929`)
 - Improved error reporting for subsetting columns of a :class:`DataFrameGroupBy` with ``axis=1`` (:issue:`37725`)
+- Add support for ``min_count`` keyword for :meth:`DataFrame.groupby` and :meth:`DataFrame.resample` for functions ``min``, ``max``, ``first`` and ``last`` (:issue:`37821`, :issue:`37768`)
 
 .. ---------------------------------------------------------------------------
 

pandas/tests/groupby/aggregate/test_other.py

@@ -643,7 +643,7 @@ def weird_func(x):
 @pytest.mark.parametrize("func", ["first", "last", "max", "min"])
 def test_min_count_implementation_min_max_first_last(func):
     # GH#37821
-    df = DataFrame({"a": [1] * 3, "b": [np.nan] * 3})
+    df = DataFrame({"a": [1] * 3, "b": [1, np.nan, np.nan]})
     result = getattr(df.groupby("a"), func)(min_count=2)
     expected = DataFrame({"b": [np.nan]}, index=Index([1], name="a"))
     tm.assert_frame_equal(result, expected)

pandas/tests/resample/test_datetime_index.py

@@ -1785,3 +1785,16 @@ def test_resample_calendar_day_with_dst(
         1.0, pd.date_range(first, exp_last, freq=freq_out, tz="Europe/Amsterdam")
     )
     tm.assert_series_equal(result, expected)
+
+
+@pytest.mark.parametrize("func", ["min", "max", "first", "last"])
+def test_resample_aggregate_functions_min_count(func):
+    # GH#37768
+    index = date_range(start="2020", freq="M", periods=3)
+    ser = Series([1, np.nan, np.nan], index)
+    result = getattr(ser.resample("Q"), func)(min_count=2)
+    expected = Series(
+        [np.nan],
+        index=DatetimeIndex(["2020-03-31"], dtype="datetime64[ns]", freq="Q-DEC"),
+    )
+    tm.assert_series_equal(result, expected)