BUG: DataFrame.dropna must not allow to set both how= and thresh= parameters

doc/source/whatsnew/v1.5.0.rst

@@ -570,7 +570,7 @@ Missing
 - Bug in :meth:`Series.fillna` and :meth:`DataFrame.fillna` with ``downcast`` keyword not being respected in some cases where there are no NA values present (:issue:`45423`)
 - Bug in :meth:`Series.fillna` and :meth:`DataFrame.fillna` with :class:`IntervalDtype` and incompatible value raising instead of casting to a common (usually object) dtype (:issue:`45796`)
 - Bug in :meth:`DataFrame.interpolate` with object-dtype column not returning a copy with ``inplace=False`` (:issue:`45791`)
--
+- Bug in :meth:`DataFrame.dropna` allows to set both ``how`` and ``thresh`` incompatible arguments (:issue:`46575`)
 
 MultiIndex
 ^^^^^^^^^^

pandas/core/frame.py

@@ -42,7 +42,10 @@
     properties,
 )
 from pandas._libs.hashtable import duplicated
-from pandas._libs.lib import no_default
+from pandas._libs.lib import (
+    NoDefault,
+    no_default,
+)
 from pandas._typing import (
     AggFuncType,
     AnyArrayLike,
@@ -6110,8 +6113,8 @@ def notnull(self) -> DataFrame:
     def dropna(
         self,
         axis: Axis = 0,
-        how: str = "any",
-        thresh=None,
+        how: str | NoDefault = no_default,
+        thresh: int | NoDefault = no_default,
         subset: IndexLabel = None,
         inplace: bool = False,
     ):
@@ -6143,7 +6146,7 @@ def dropna(
             * 'all' : If all values are NA, drop that row or column.
 
         thresh : int, optional
-            Require that many non-NA values.
+            Require that many non-NA values. Cannot be combined with how.
         subset : column label or sequence of labels, optional
             Labels along other axis to consider, e.g. if you are dropping rows
             these would be a list of columns to include.
@@ -6218,6 +6221,14 @@ def dropna(
              name        toy       born
         1  Batman  Batmobile 1940-04-25
         """
+        if (how is not no_default) and (thresh is not no_default):
+            raise TypeError(
+                "You cannot set both the how and thresh arguments at the same time."
+            )
+
+        if how is no_default:
+            how = "any"
+
         inplace = validate_bool_kwarg(inplace, "inplace")
         if isinstance(axis, (tuple, list)):
             # GH20987
@@ -6238,7 +6249,7 @@ def dropna(
                 raise KeyError(np.array(subset)[check].tolist())
             agg_obj = self.take(indices, axis=agg_axis)
 
-        if thresh is not None:
+        if thresh is not no_default:
             count = agg_obj.count(axis=agg_axis)
             mask = count >= thresh
         elif how == "any":
@@ -6248,10 +6259,8 @@ def dropna(
             # faster equivalent to 'agg_obj.count(agg_axis) > 0'
             mask = notna(agg_obj).any(axis=agg_axis, bool_only=False)
         else:
-            if how is not None:
+            if how is not no_default:
                 raise ValueError(f"invalid how option: {how}")
-            else:
-                raise TypeError("must specify how or thresh")
 
         if np.all(mask):
             result = self.copy()

pandas/tests/frame/methods/test_dropna.py

@@ -158,9 +158,6 @@ def test_dropna_corner(self, float_frame):
         msg = "invalid how option: foo"
         with pytest.raises(ValueError, match=msg):
             float_frame.dropna(how="foo")
-        msg = "must specify how or thresh"
-        with pytest.raises(TypeError, match=msg):
-            float_frame.dropna(how=None)
         # non-existent column - 8303
         with pytest.raises(KeyError, match=r"^\['X'\]$"):
             float_frame.dropna(subset=["A", "X"])
@@ -274,3 +271,16 @@ def test_no_nans_in_frame(self, axis):
         expected = df.copy()
         result = df.dropna(axis=axis)
         tm.assert_frame_equal(result, expected, check_index_type=True)
+
+    def test_how_thresh_param_incompatible(self):
+        # GH46575
+        df = DataFrame([1, 2, pd.NA])
+        msg = "You cannot set both the how and thresh arguments at the same time"
+        with pytest.raises(TypeError, match=msg):
+            df.dropna(how="all", thresh=2)
+
+        with pytest.raises(TypeError, match=msg):
+            df.dropna(how="any", thresh=2)
+
+        with pytest.raises(TypeError, match=msg):
+            df.dropna(how=None, thresh=None)