GH730 allow str for float format to string

pandas-stubs/_typing.pyi

@@ -626,7 +626,7 @@ CompressionOptions: TypeAlias = (
 FormattersType: TypeAlias = (
     list[Callable] | tuple[Callable, ...] | Mapping[str | int, Callable]
 )
-FloatFormatType: TypeAlias = str | Callable | EngFormatter
+FloatFormatType: TypeAlias = str | Callable[[float], str] | EngFormatter
 # converters
 ConvertersArg: TypeAlias = dict[Hashable, Callable[[Dtype], Dtype]]
 

pandas-stubs/core/frame.pyi

@@ -18,7 +18,10 @@ from typing import (
     overload,
 )
 
-from _typing import TimeZones
+from _typing import (
+    FloatFormatType,
+    TimeZones,
+)
 from matplotlib.axes import Axes as PlotAxes
 import numpy as np
 from pandas import (
@@ -2311,7 +2314,7 @@ class DataFrame(NDFrame, OpsMixin, _GetItemHack):
         index: _bool = ...,
         na_rep: _str = ...,
         formatters: FormattersType | None = ...,
-        float_format: Callable[[float], str] | None = ...,
+        float_format: FloatFormatType | None = ...,
         sparsify: _bool | None = ...,
         index_names: _bool = ...,
         justify: _str | None = ...,
@@ -2334,7 +2337,7 @@ class DataFrame(NDFrame, OpsMixin, _GetItemHack):
         index: _bool = ...,
         na_rep: _str = ...,
         formatters: FormattersType | None = ...,
-        float_format: Callable[[float], str] | None = ...,
+        float_format: FloatFormatType | None = ...,
         sparsify: _bool | None = ...,
         index_names: _bool = ...,
         justify: _str | None = ...,

pandas-stubs/core/series.pyi

@@ -25,6 +25,7 @@ from typing import (
 )
 
 from _typing import (
+    FloatFormatType,
     Label,
     ReplaceValue,
     TimeZones,
@@ -530,7 +531,7 @@ class Series(IndexOpsMixin[S1], NDFrame):
         self,
         buf: FilePath | WriteBuffer[str],
         na_rep: _str = ...,
-        float_format: Callable[[float], str] = ...,
+        float_format: FloatFormatType = ...,
         header: _bool = ...,
         index: _bool = ...,
         length: _bool = ...,
@@ -544,7 +545,7 @@ class Series(IndexOpsMixin[S1], NDFrame):
         self,
         buf: None = ...,
         na_rep: _str = ...,
-        float_format: Callable[[float], str] = ...,
+        float_format: FloatFormatType = ...,
         header: _bool = ...,
         index: _bool = ...,
         length: _bool = ...,

tests/test_frame.py

@@ -55,6 +55,7 @@
     pytest_warns_bounded,
 )
 
+from pandas.io.formats.format import EngFormatter
 from pandas.io.formats.style import Styler
 from pandas.io.parsers import TextFileReader
 
@@ -1698,6 +1699,27 @@ def test_types_to_string() -> None:
     df.to_string(col_space={"col1": 1, "col2": 3})
 
 
+def test_dataframe_to_string_float_fmt() -> None:
+    """Test the different argument types for float_format."""
+    df = pd.DataFrame(
+        {"values": [2.304, 1.1, 3487392, 13.4732894237, 14.3, 18.0, 17.434, 19.3]}
+    )
+    check(assert_type(df.to_string(), str), str)
+
+    def _formatter(x) -> str:
+        return f"{x:.2f}"
+
+    check(assert_type(df.to_string(float_format=_formatter), str), str)
+    check(
+        assert_type(
+            df.to_string(float_format=EngFormatter(accuracy=2, use_eng_prefix=False)),
+            str,
+        ),
+        str,
+    )
+    check(assert_type(df.to_string(float_format="%.2f"), str), str)
+
+
 def test_types_to_html() -> None:
     df = pd.DataFrame(
         data={

tests/test_series.py

@@ -60,6 +60,8 @@
 )
 from tests.extension.decimal.array import DecimalDtype
 
+from pandas.io.formats.format import EngFormatter
+
 if TYPE_CHECKING:
     from pandas.core.series import (
         OffsetSeries,
@@ -2932,6 +2934,27 @@ def test_to_string() -> None:
     )
 
 
+def test_series_to_string_float_fmt() -> None:
+    """Test the different argument types for float_format."""
+    sr = pd.Series(
+        [2.304, 1.1, 3487392, 13.4732894237, 14.3, 18.0, 17.434, 19.3], name="values"
+    )
+    check(assert_type(sr.to_string(), str), str)
+
+    def _formatter(x) -> str:
+        return f"{x:.2f}"
+
+    check(assert_type(sr.to_string(float_format=_formatter), str), str)
+    check(
+        assert_type(
+            sr.to_string(float_format=EngFormatter(accuracy=2, use_eng_prefix=False)),
+            str,
+        ),
+        str,
+    )
+    check(assert_type(sr.to_string(float_format="%.2f"), str), str)
+
+
 def test_types_mask() -> None:
     s = pd.Series([1, 2, 3, 4, 5])