Warn for float dtype

Author: miker985

pandas/core/arrays/categorical.py

@@ -27,6 +27,7 @@
     is_timedelta64_dtype,
     is_categorical,
     is_categorical_dtype,
+    is_float_dtype,
     is_integer_dtype,
     is_list_like, is_sequence,
     is_scalar, is_iterator,
@@ -632,7 +633,19 @@ def from_codes(cls, codes, categories, ordered=False):
         """
         codes = np.asarray(codes)
         if not is_integer_dtype(codes):
-            raise ValueError("codes need to be array-like integers")
+            err = True
+            if is_float_dtype(codes):
+                icodes = codes.astype('i8')
+                if (icodes == codes).all():
+                    err = False
+                    codes = icodes
+                    warn("float codes will be disallowed in the future",
+                         FutureWarning)
+                else:
+                    err = True
+            if err:
+                raise ValueError("codes need to be array-like integers")
+
         try:
             codes = coerce_indexer_dtype(codes, categories)
         except (ValueError, TypeError):

pandas/tests/arrays/categorical/test_constructors.py

@@ -2,6 +2,7 @@
 
 import pytest
 from datetime import datetime
+import warnings
 
 import numpy as np
 
@@ -475,6 +476,15 @@ def test_from_codes_with_nan_code(self):
         with pytest.raises(ValueError):
             Categorical.from_codes(codes, categories)
 
+    def test_from_codes_with_float(self):
+        # GH21767
+        codes = [1.0, 2.0, 0]
+        categories = ['a', 'b', 'c']
+        with warnings.catch_warnings(record=True) as w:
+            Categorical.from_codes(codes, categories)
+
+        assert len(w) == 1
+
     @pytest.mark.parametrize('dtype', [None, 'category'])
     def test_from_inferred_categories(self, dtype):
         cats = ['a', 'b']