Add docstrings

.github/workflows/pytest.yml

@@ -32,3 +32,5 @@ jobs:
         run: python -m pip install --upgrade modin[dask]
       - name: Run pytest
         run: pytest tests --cov=narwhals --cov=tests --cov-fail-under=50
+      - name: Run doctests
+        run: pytest narwhals --doctest-modules

narwhals/dataframe.py

@@ -69,7 +69,7 @@ def __repr__(self) -> str:  # pragma: no cover
             + "| Use `narwhals.to_native()` to see native output |\n"
             + "└"
             + "─" * length
-            + "┘\n"
+            + "┘"
         )
 
     @property
@@ -212,25 +212,268 @@ def to_dict(self, *, as_series: bool = True) -> dict[str, Any]:
     # inherited
     @property
     def schema(self) -> dict[str, DType]:
+        r"""
+        Get a dict[column name, DataType].
+
+        Examples:
+            >>> import polars as pl
+            >>> import narwhals as nw
+            >>> df_pl = pl.DataFrame(
+            ...     {
+            ...         "foo": [1, 2, 3],
+            ...         "bar": [6.0, 7.0, 8.0],
+            ...         "ham": ["a", "b", "c"],
+            ...     }
+            ... )
+            >>> df = nw.DataFrame(df_pl)
+            >>> df.schema  # doctest: +SKIP
+            OrderedDict({'foo': Int64, 'bar': Float64, 'ham': String})
+            ```
+        """
         return super().schema
 
     @property
     def columns(self) -> list[str]:
+        r"""
+        Get column names.
+
+        Examples:
+            Get column names.
+
+            >>> import polars as pl
+            >>> import narwhals as nw
+            >>> df_pl = pl.DataFrame(
+            ...     {
+            ...         "foo": [1, 2, 3],
+            ...         "bar": [6, 7, 8],
+            ...         "ham": ["a", "b", "c"],
+            ...     }
+            ... )
+            >>> df = nw.DataFrame(df_pl)
+            >>> df.columns
+            ['foo', 'bar', 'ham']
+        """
         return super().columns
 
     def with_columns(
         self, *exprs: IntoExpr | Iterable[IntoExpr], **named_exprs: IntoExpr
     ) -> Self:
+        r"""
+        Add columns to this DataFrame.
+
+        Added columns will replace existing columns with the same name.
+
+        Arguments:
+            *exprs: Column(s) to add, specified as positional arguments.
+                     Accepts expression input. Strings are parsed as column names, other
+                     non-expression inputs are parsed as literals.
+
+            **named_exprs: Additional columns to add, specified as keyword arguments.
+                            The columns will be renamed to the keyword used.
+
+        Returns:
+            DataFrame: A new DataFrame with the columns added.
+
+        Note:
+            Creating a new DataFrame using this method does not create a new copy of
+            existing data.
+
+        Examples:
+            Pass an expression to add it as a new column.
+
+            >>> import polars as pl
+            >>> import narwhals as nw
+            >>> df_pl = pl.DataFrame(
+            ...     {
+            ...         "a": [1, 2, 3, 4],
+            ...         "b": [0.5, 4, 10, 13],
+            ...         "c": [True, True, False, True],
+            ...     }
+            ... )
+            >>> df = nw.DataFrame(df_pl)
+            >>> dframe = df.with_columns((nw.col("a") * 2).alias("a*2"))
+            >>> dframe
+            ┌─────────────────────────────────────────────────┐
+            | Narwhals DataFrame                              |
+            | Use `narwhals.to_native()` to see native output |
+            └─────────────────────────────────────────────────┘
+
+            >>> nw.to_native(dframe)
+            shape: (4, 4)
+            ┌─────┬──────┬───────┬─────┐
+            │ a   ┆ b    ┆ c     ┆ a*2 │
+            │ --- ┆ ---  ┆ ---   ┆ --- │
+            │ i64 ┆ f64  ┆ bool  ┆ i64 │
+            ╞═════╪══════╪═══════╪═════╡
+            │ 1   ┆ 0.5  ┆ true  ┆ 2   │
+            │ 2   ┆ 4.0  ┆ true  ┆ 4   │
+            │ 3   ┆ 10.0 ┆ false ┆ 6   │
+            │ 4   ┆ 13.0 ┆ true  ┆ 8   │
+            └─────┴──────┴───────┴─────┘
+        """
         return super().with_columns(*exprs, **named_exprs)
 
     def select(
         self,
         *exprs: IntoExpr | Iterable[IntoExpr],
         **named_exprs: IntoExpr,
     ) -> Self:
+        r"""
+        Select columns from this DataFrame.
+
+        Arguments:
+            *exprs: Column(s) to select, specified as positional arguments.
+                     Accepts expression input. Strings are parsed as column names,
+                     other non-expression inputs are parsed as literals.
+
+            **named_exprs: Additional columns to select, specified as keyword arguments.
+                            The columns will be renamed to the keyword used.
+
+        Examples:
+            Pass the name of a column to select that column.
+
+            >>> import polars as pl
+            >>> import narwhals as nw
+            >>> df_pl = pl.DataFrame(
+            ...     {
+            ...         "foo": [1, 2, 3],
+            ...         "bar": [6, 7, 8],
+            ...         "ham": ["a", "b", "c"],
+            ...     }
+            ... )
+            >>> df = nw.DataFrame(df_pl)
+            >>> dframe = df.select("foo")
+            >>> dframe
+            ┌─────────────────────────────────────────────────┐
+            | Narwhals DataFrame                              |
+            | Use `narwhals.to_native()` to see native output |
+            └─────────────────────────────────────────────────┘
+            >>> nw.to_native(dframe)
+            shape: (3, 1)
+            ┌─────┐
+            │ foo │
+            │ --- │
+            │ i64 │
+            ╞═════╡
+            │ 1   │
+            │ 2   │
+            │ 3   │
+            └─────┘
+
+            Multiple columns can be selected by passing a list of column names.
+
+            >>> dframe = df.select(["foo", "bar"])
+            >>> dframe
+            ┌─────────────────────────────────────────────────┐
+            | Narwhals DataFrame                              |
+            | Use `narwhals.to_native()` to see native output |
+            └─────────────────────────────────────────────────┘
+            >>> nw.to_native(dframe)
+            shape: (3, 2)
+            ┌─────┬─────┐
+            │ foo ┆ bar │
+            │ --- ┆ --- │
+            │ i64 ┆ i64 │
+            ╞═════╪═════╡
+            │ 1   ┆ 6   │
+            │ 2   ┆ 7   │
+            │ 3   ┆ 8   │
+            └─────┴─────┘
+
+            Multiple columns can also be selected using positional arguments instead of a
+            list. Expressions are also accepted.
+
+            >>> dframe = df.select(nw.col("foo"), nw.col("bar") + 1)
+            >>> dframe
+            ┌─────────────────────────────────────────────────┐
+            | Narwhals DataFrame                              |
+            | Use `narwhals.to_native()` to see native output |
+            └─────────────────────────────────────────────────┘
+            >>> nw.to_native(dframe)
+            shape: (3, 2)
+            ┌─────┬─────┐
+            │ foo ┆ bar │
+            │ --- ┆ --- │
+            │ i64 ┆ i64 │
+            ╞═════╪═════╡
+            │ 1   ┆ 7   │
+            │ 2   ┆ 8   │
+            │ 3   ┆ 9   │
+            └─────┴─────┘
+
+            Use keyword arguments to easily name your expression inputs.
+
+            >>> dframe = df.select(threshold=nw.col('foo')*2)
+            >>> dframe
+            ┌─────────────────────────────────────────────────┐
+            | Narwhals DataFrame                              |
+            | Use `narwhals.to_native()` to see native output |
+            └─────────────────────────────────────────────────┘
+            >>> nw.to_native(dframe)
+            shape: (3, 1)
+            ┌───────────┐
+            │ threshold │
+            │ ---       │
+            │ i64       │
+            ╞═══════════╡
+            │ 2         │
+            │ 4         │
+            │ 6         │
+            └───────────┘
+        """
         return super().select(*exprs, **named_exprs)
 
     def rename(self, mapping: dict[str, str]) -> Self:
+        r"""
+        Rename column names.
+
+        Arguments:
+            mapping: Key value pairs that map from old name to new name, or a function
+                      that takes the old name as input and returns the new name.
+
+        Examples:
+            >>> import polars as pl
+            >>> import narwhals as nw
+            >>> df_pl = pl.DataFrame(
+            ...     {"foo": [1, 2, 3], "bar": [6, 7, 8], "ham": ["a", "b", "c"]}
+            ... )
+            >>> df = nw.DataFrame(df_pl)
+            >>> dframe = df.rename({"foo": "apple"})
+            >>> dframe
+            ┌─────────────────────────────────────────────────┐
+            | Narwhals DataFrame                              |
+            | Use `narwhals.to_native()` to see native output |
+            └─────────────────────────────────────────────────┘
+
+            >>> nw.to_native(dframe)
+            shape: (3, 3)
+            ┌───────┬─────┬─────┐
+            │ apple ┆ bar ┆ ham │
+            │ ---   ┆ --- ┆ --- │
+            │ i64   ┆ i64 ┆ str │
+            ╞═══════╪═════╪═════╡
+            │ 1     ┆ 6   ┆ a   │
+            │ 2     ┆ 7   ┆ b   │
+            │ 3     ┆ 8   ┆ c   │
+            └───────┴─────┴─────┘
+            >>> dframe = df.rename(lambda column_name: "f" + column_name[1:])
+            >>> dframe
+            ┌─────────────────────────────────────────────────┐
+            | Narwhals DataFrame                              |
+            | Use `narwhals.to_native()` to see native output |
+            └─────────────────────────────────────────────────┘
+            >>> nw.to_native(dframe)
+            shape: (3, 3)
+            ┌─────┬─────┬─────┐
+            │ foo ┆ far ┆ fam │
+            │ --- ┆ --- ┆ --- │
+            │ i64 ┆ i64 ┆ str │
+            ╞═════╪═════╪═════╡
+            │ 1   ┆ 6   ┆ a   │
+            │ 2   ┆ 7   ┆ b   │
+            │ 3   ┆ 8   ┆ c   │
+            └─────┴─────┴─────┘
+        """
         return super().rename(mapping)
 
     def head(self, n: int) -> Self:

narwhals/pandas_like/group_by.py

@@ -47,8 +47,8 @@ def agg(
             if expr._output_names is None:
                 msg = (
                     "Anonymous expressions are not supported in group_by.agg.\n"
-                    "Instead of `pl.all()`, try using a named expression, such as "
-                    "`pl.col('a', 'b')`\n"
+                    "Instead of `nw.all()`, try using a named expression, such as "
+                    "`nw.col('a', 'b')`\n"
                 )
                 raise ValueError(msg)
             output_names.extend(expr._output_names)

narwhals/pandas_like/typing.py

@@ -1,9 +1,10 @@
 from __future__ import annotations
 
 from typing import TYPE_CHECKING
-from typing import TypeAlias
 
 if TYPE_CHECKING:
+    from typing import TypeAlias
+
     from narwhals.pandas_like.expr import PandasExpr
     from narwhals.pandas_like.series import PandasSeries
 

narwhals/series.py

@@ -61,7 +61,7 @@ def __repr__(self) -> str:  # pragma: no cover
             + "| Use `narwhals.to_native()` to see native output |\n"
             + "└"
             + "─" * length
-            + "┘\n"
+            + "┘"
         )
 
     def alias(self, name: str) -> Self:

narwhals/typing.py

@@ -1,8 +1,9 @@
 from typing import TYPE_CHECKING
-from typing import TypeAlias
 from typing import TypeVar
 
 if TYPE_CHECKING:
+    from typing import TypeAlias
+
     from narwhals.expression import Expr
     from narwhals.series import Series