BUG/TST: Fillna limit Error Reporting

doc/source/whatsnew/v0.25.0.rst

@@ -685,6 +685,7 @@ Missing
 
 - Fixed misleading exception message in :meth:`Series.interpolate` if argument ``order`` is required, but omitted (:issue:`10633`, :issue:`24014`).
 - Fixed class type displayed in exception message in :meth:`DataFrame.dropna` if invalid ``axis`` parameter passed (:issue:`25555`)
+- Fixed not throwing ValueError in :meth:`DataFrame.fillna` when called with a negative ``limit`` parameter value (:issue:`27042`)
 -
 
 MultiIndex

pandas/core/arrays/base.py

@@ -445,7 +445,7 @@ def fillna(self, value=None, method=None, limit=None):
         from pandas.util._validators import validate_fillna_kwargs
         from pandas.core.missing import pad_1d, backfill_1d
 
-        value, method = validate_fillna_kwargs(value, method)
+        value, method, limit = validate_fillna_kwargs(value, method, limit)
 
         mask = self.isna()
 

pandas/core/arrays/categorical.py

@@ -1756,8 +1756,8 @@ def fillna(self, value=None, method=None, limit=None):
         -------
         filled : Categorical with NA/NaN filled
         """
-        value, method = validate_fillna_kwargs(
-            value, method, validate_scalar_dict_value=False
+        value, method, limit = validate_fillna_kwargs(
+            value, method, limit, validate_scalar_dict_value=False
         )
 
         if value is None:

pandas/core/arrays/datetimelike.py

@@ -767,7 +767,7 @@ def fillna(self, value=None, method=None, limit=None):
         if isinstance(value, ABCSeries):
             value = value.array
 
-        value, method = validate_fillna_kwargs(value, method)
+        value, method, limit = validate_fillna_kwargs(value, method, limit)
 
         mask = self.isna()
 

pandas/core/arrays/numpy_.py

@@ -254,7 +254,7 @@ def isna(self):
 
     def fillna(self, value=None, method=None, limit=None):
         # TODO(_values_for_fillna): remove this
-        value, method = validate_fillna_kwargs(value, method)
+        value, method, limit = validate_fillna_kwargs(value, method, limit)
 
         mask = self.isna()
 

pandas/core/generic.py

@@ -6025,7 +6025,7 @@ def fillna(self, value=None, method=None, axis=None, inplace=False,
         3   NaN 3.0 NaN 4
         """
         inplace = validate_bool_kwarg(inplace, 'inplace')
-        value, method = validate_fillna_kwargs(value, method)
+        value, method, limit = validate_fillna_kwargs(value, method, limit)
 
         self._consolidate_inplace()
 

pandas/core/internals/blocks.py

@@ -352,10 +352,6 @@ def fillna(self, value, limit=None, inplace=False, downcast=None):
 
         mask = isna(self.values)
         if limit is not None:
-            if not is_integer(limit):
-                raise ValueError('Limit must be an integer')
-            if limit < 1:
-                raise ValueError('Limit must be greater than 0')
             if self.ndim > 2:
                 raise NotImplementedError("number of dimensions for 'fillna' "
                                           "is currently limited to 2")

pandas/tests/frame/test_missing.py

@@ -516,6 +516,22 @@ def test_fillna_skip_certain_blocks(self):
         # it works!
         df.fillna(np.nan)
 
+    @pytest.mark.parametrize("type", [int, float])
+    def test_fillna_positive_limit(self, type):
+        df = DataFrame(np.random.randn(10, 4)).astype(type)
+
+        msg = "Limit must be greater than 0"
+        with pytest.raises(ValueError, match=msg):
+            df.fillna(0, limit=-5)
+
+    @pytest.mark.parametrize("type", [int, float])
+    def test_fillna_integer_limit(self, type):
+        df = DataFrame(np.random.randn(10, 4)).astype(type)
+
+        msg = "Limit must be an integer"
+        with pytest.raises(ValueError, match=msg):
+            df.fillna(0, limit=0.5)
+
     def test_fillna_inplace(self):
         df = DataFrame(np.random.randn(10, 4))
         df[1][:4] = np.nan

pandas/tests/util/test_validate_fillna_kwargs.py

@@ -0,0 +1,43 @@
+import pytest
+
+from pandas.util._validators import validate_fillna_kwargs
+
+
+def test_no_value_or_method():
+
+    msg = "Must specify a fill 'value' or 'method'."
+
+    with pytest.raises(ValueError, match=msg):
+        validate_fillna_kwargs(None, None, None)
+
+
+def test_value_and_method():
+
+    msg = "Cannot specify both 'value' and 'method'."
+
+    with pytest.raises(ValueError, match=msg):
+        validate_fillna_kwargs(0, 'ffill', None)
+
+
+@pytest.mark.parametrize("value", [(1, 2, 3), [1, 2, 3]])
+def test_valid_value(value):
+
+    msg = ('"value" parameter must be a scalar or dict, but '
+           'you passed a "{0}"'.format(type(value).__name__))
+
+    with pytest.raises(TypeError, match=msg):
+        validate_fillna_kwargs(value, None, None)
+
+
+def test_integer_limit():
+
+    msg = "Limit must be an integer"
+    with pytest.raises(ValueError, match=msg):
+        validate_fillna_kwargs(0, None, 0.5)
+
+
+def test_positive_limit():
+
+    msg = "Limit must be greater than 0"
+    with pytest.raises(ValueError, match=msg):
+        validate_fillna_kwargs(5, None, -5)

pandas/util/_validators.py

@@ -4,7 +4,7 @@
 """
 import warnings
 
-from pandas.core.dtypes.common import is_bool
+from pandas.core.dtypes.common import is_bool, is_integer
 
 
 def _check_arg_length(fname, args, max_fname_arg_count, compat_args):
@@ -322,7 +322,8 @@ def validate_axis_style_args(data, args, kwargs, arg_name, method_name):
     return out
 
 
-def validate_fillna_kwargs(value, method, validate_scalar_dict_value=True):
+def validate_fillna_kwargs(value, method, limit,
+                           validate_scalar_dict_value=True):
     """Validate the keyword arguments to 'fillna'.
 
     This checks that exactly one of 'value' and 'method' is specified.
@@ -355,4 +356,10 @@ def validate_fillna_kwargs(value, method, validate_scalar_dict_value=True):
     elif value is not None and method is not None:
         raise ValueError("Cannot specify both 'value' and 'method'.")
 
-    return value, method
+    if limit is not None:
+        if not is_integer(limit):
+            raise ValueError('Limit must be an integer')
+        if limit < 1:
+            raise ValueError('Limit must be greater than 0')
+
+    return value, method, limit