DOC: clarify to_csv float format docstring (#47436)

* DOC: clarify to_csv float format docstring

* Clarify doc

* Clarify doc

Author: phofl

pandas/core/generic.py

@@ -3302,7 +3302,7 @@ def to_csv(
         path_or_buf: FilePath | WriteBuffer[bytes] | WriteBuffer[str] | None = None,
         sep: str = ",",
         na_rep: str = "",
-        float_format: str | None = None,
+        float_format: str | Callable | None = None,
         columns: Sequence[Hashable] | None = None,
         header: bool_t | list[str] = True,
         index: bool_t = True,
@@ -3341,8 +3341,9 @@ def to_csv(
             String of length 1. Field delimiter for the output file.
         na_rep : str, default ''
             Missing data representation.
-        float_format : str, default None
-            Format string for floating point numbers.
+        float_format : str, Callable, default None
+            Format string for floating point numbers. If a Callable is given, it takes
+            precedence over other numeric formatting parameters, like decimal.
         columns : sequence, optional
             Columns to write.
         header : bool or list of str, default True

pandas/tests/frame/methods/test_to_csv.py

@@ -833,6 +833,18 @@ def test_to_csv_float_format(self):
             )
             tm.assert_frame_equal(rs, xp)
 
+    def test_to_csv_float_format_over_decimal(self):
+        # GH#47436
+        df = DataFrame({"a": [0.5, 1.0]})
+        result = df.to_csv(
+            decimal=",",
+            float_format=lambda x: np.format_float_positional(x, trim="-"),
+            index=False,
+        )
+        expected_rows = ["a", "0.5", "1"]
+        expected = tm.convert_rows_list_to_csv_str(expected_rows)
+        assert result == expected
+
     def test_to_csv_unicodewriter_quoting(self):
         df = DataFrame({"A": [1, 2, 3], "B": ["foo", "bar", "baz"]})