feat(python): add "calamine" support to read_excel, using fastexcel

py-polars/docs/source/reference/lazyframe/descriptive.rst

@@ -6,5 +6,6 @@ Descriptive
 .. autosummary::
    :toctree: api/
 
+    LazyFrame.describe
     LazyFrame.explain
     LazyFrame.show_graph

py-polars/polars/dataframe/frame.py

@@ -52,6 +52,7 @@
     _check_for_pyarrow,
     dataframe_api_compat,
     hvplot,
+    import_optional,
 )
 from polars.dependencies import numpy as np
 from polars.dependencies import pandas as pd
@@ -3073,15 +3074,8 @@ def write_excel(
         ...     sheet_zoom=125,
         ... )
         """  # noqa: W505
-        try:
-            import xlsxwriter
-            from xlsxwriter.utility import xl_cell_to_rowcol
-        except ImportError:
-            msg = (
-                "Excel export requires xlsxwriter"
-                "\n\nPlease run: pip install XlsxWriter"
-            )
-            raise ImportError(msg) from None
+        xlsxwriter = import_optional("xlsxwriter", err_prefix="Excel export requires")
+        from xlsxwriter.utility import xl_cell_to_rowcol
 
         # setup workbook/worksheet
         wb, ws, can_close = _xl_setup_workbook(workbook, worksheet)
@@ -6751,7 +6745,10 @@ def drop_in_place(self, name: str) -> Series:
 
     def cast(
         self,
-        dtypes: Mapping[ColumnNameOrSelector, PolarsDataType] | PolarsDataType,
+        dtypes: (
+            Mapping[ColumnNameOrSelector | PolarsDataType, PolarsDataType]
+            | PolarsDataType
+        ),
         *,
         strict: bool = True,
     ) -> DataFrame:
@@ -6792,12 +6789,19 @@ def cast(
         │ 3.0 ┆ 8   ┆ 2022-05-06 │
         └─────┴─────┴────────────┘
 
-        Cast all frame columns to the specified dtype:
+        Cast all frame columns matching one dtype (or dtype group) to another dtype:
 
-        >>> df.cast(pl.String).to_dict(as_series=False)
-        {'foo': ['1', '2', '3'],
-         'bar': ['6.0', '7.0', '8.0'],
-         'ham': ['2020-01-02', '2021-03-04', '2022-05-06']}
+        >>> df.cast({pl.Date: pl.Datetime})
+        shape: (3, 3)
+        ┌─────┬─────┬─────────────────────┐
+        │ foo ┆ bar ┆ ham                 │
+        │ --- ┆ --- ┆ ---                 │
+        │ i64 ┆ f64 ┆ datetime[μs]        │
+        ╞═════╪═════╪═════════════════════╡
+        │ 1   ┆ 6.0 ┆ 2020-01-02 00:00:00 │
+        │ 2   ┆ 7.0 ┆ 2021-03-04 00:00:00 │
+        │ 3   ┆ 8.0 ┆ 2022-05-06 00:00:00 │
+        └─────┴─────┴─────────────────────┘
 
         Use selectors to define the columns being cast:
 
@@ -6813,6 +6817,13 @@ def cast(
         │ 2   ┆ 7   ┆ 2021-03-04 │
         │ 3   ┆ 8   ┆ 2022-05-06 │
         └─────┴─────┴────────────┘
+
+        Cast all frame columns to the specified dtype:
+
+        >>> df.cast(pl.String).to_dict(as_series=False)
+        {'foo': ['1', '2', '3'],
+         'bar': ['6.0', '7.0', '8.0'],
+         'ham': ['2020-01-02', '2021-03-04', '2022-05-06']}
         """
         return self.lazy().cast(dtypes, strict=strict).collect(_eager=True)
 

py-polars/polars/dependencies.py

@@ -229,6 +229,35 @@ def _check_for_pydantic(obj: Any, *, check_type: bool = True) -> bool:
     )
 
 
+def import_optional(
+    module_name: str,
+    err_prefix: str = "Required package",
+    err_suffix: str = "not installed",
+) -> Any:
+    """
+    Import an optional dependency, returning the module.
+
+    Parameters
+    ----------
+    module_name : str
+        Name of the dependency to import.
+    err_prefix : str, optional
+        Error prefix to use in the raised exception (appears before the module name).
+    err_suffix: str, optional
+        Error suffix to use in the raised exception (follows the module name).
+    """
+    try:
+        return import_module(module_name)
+    except ImportError:
+        prefix = f"{err_prefix.strip(' ')} " if err_prefix else ""
+        suffix = f" {err_prefix.strip(' ')}" if err_suffix else ""
+        err_message = (
+            f"{prefix}'{module_name}'{suffix}.\n"
+            f"Please install it using the command `pip install {module_name}`."
+        )
+        raise ImportError(err_message) from None
+
+
 __all__ = [
     # lazy-load rarely-used/heavy builtins (for fast startup)
     "dataclasses",