BUG: to_csv not respecting float_format for Float64 (#46007)

Author: phofl

doc/source/whatsnew/v1.5.0.rst

@@ -353,6 +353,7 @@ I/O
 - Bug in :func:`read_excel` results in an infinite loop with certain ``skiprows`` callables (:issue:`45585`)
 - Bug in :meth:`DataFrame.info` where a new line at the end of the output is omitted when called on an empty :class:`DataFrame` (:issue:`45494`)
 - Bug in :func:`read_csv` not recognizing line break for ``on_bad_lines="warn"`` for ``engine="c"`` (:issue:`41710`)
+- Bug in :meth:`DataFrame.to_csv` not respecting ``float_format`` for ``Float64`` dtype (:issue:`45991`)
 - Bug in :func:`read_parquet` when ``engine="pyarrow"`` which caused partial write to disk when column of unsupported datatype was passed (:issue:`44914`)
 - Bug in :func:`DataFrame.to_excel` and :class:`ExcelWriter` would raise when writing an empty DataFrame to a ``.ods`` file (:issue:`45793`)
 

pandas/core/internals/blocks.py

@@ -51,6 +51,7 @@
     is_dtype_equal,
     is_interval_dtype,
     is_list_like,
+    is_sparse,
     is_string_dtype,
 )
 from pandas.core.dtypes.dtypes import (
@@ -2257,14 +2258,7 @@ def to_native_types(
             results_converted.append(result.astype(object, copy=False))
         return np.vstack(results_converted)
 
-    elif isinstance(values, ExtensionArray):
-        mask = isna(values)
-
-        new_values = np.asarray(values.astype(object))
-        new_values[mask] = na_rep
-        return new_values
-
-    elif values.dtype.kind == "f":
+    elif values.dtype.kind == "f" and not is_sparse(values):
         # see GH#13418: no special formatting is desired at the
         # output (important for appropriate 'quoting' behaviour),
         # so do not pass it through the FloatArrayFormatter
@@ -2294,6 +2288,13 @@ def to_native_types(
         res = res.astype(object, copy=False)
         return res
 
+    elif isinstance(values, ExtensionArray):
+        mask = isna(values)
+
+        new_values = np.asarray(values.astype(object))
+        new_values[mask] = na_rep
+        return new_values
+
     else:
 
         mask = isna(values)

pandas/tests/io/formats/test_to_csv.py

@@ -315,6 +315,32 @@ def test_to_csv_date_format_in_categorical(self):
         ser = ser.astype("category")
         assert ser.to_csv(index=False, date_format="%Y-%m-%d") == expected
 
+    def test_to_csv_float_ea_float_format(self):
+        # GH#45991
+        df = DataFrame({"a": [1.1, 2.02, pd.NA, 6.000006], "b": "c"})
+        df["a"] = df["a"].astype("Float64")
+        result = df.to_csv(index=False, float_format="%.5f")
+        expected = """a,b
+1.10000,c
+2.02000,c
+,c
+6.00001,c
+"""
+        assert result == expected
+
+    def test_to_csv_float_ea_no_float_format(self):
+        # GH#45991
+        df = DataFrame({"a": [1.1, 2.02, pd.NA, 6.000006], "b": "c"})
+        df["a"] = df["a"].astype("Float64")
+        result = df.to_csv(index=False)
+        expected = """a,b
+1.1,c
+2.02,c
+,c
+6.000006,c
+"""
+        assert result == expected
+
     def test_to_csv_multi_index(self):
         # see gh-6618
         df = DataFrame([1], columns=pd.MultiIndex.from_arrays([[1], [2]]))