feat: add replace and replace_strict (#1327)

---------

Co-authored-by: Edoardo Abati <[email protected]>

Author: MarcoGorelli

docs/api-reference/expr.md

@@ -36,6 +36,7 @@
         - over
         - pipe
         - quantile
+        - replace_strict
         - round
         - sample
         - shift

docs/api-reference/series.md

@@ -44,6 +44,7 @@
         - pipe
         - quantile
         - rename
+        - replace_strict
         - round
         - sample
         - scatter

narwhals/_arrow/expr.py

@@ -4,6 +4,7 @@
 from typing import Any
 from typing import Callable
 from typing import Literal
+from typing import Sequence
 
 from narwhals._expression_parsing import reuse_series_implementation
 from narwhals._expression_parsing import reuse_series_namespace_implementation
@@ -320,6 +321,13 @@ def is_last_distinct(self: Self) -> Self:
     def unique(self: Self, *, maintain_order: bool = False) -> Self:
         return reuse_series_implementation(self, "unique", maintain_order=maintain_order)
 
+    def replace_strict(
+        self: Self, old: Sequence[Any], new: Sequence[Any], *, return_dtype: DType
+    ) -> Self:
+        return reuse_series_implementation(
+            self, "replace_strict", old, new, return_dtype=return_dtype
+        )
+
     def sort(self: Self, *, descending: bool = False, nulls_last: bool = False) -> Self:
         return reuse_series_implementation(
             self, "sort", descending=descending, nulls_last=nulls_last

narwhals/_arrow/namespace.py

@@ -65,7 +65,7 @@ def _create_expr_from_series(self, series: ArrowSeries) -> ArrowExpr:
     def _create_series_from_scalar(self, value: Any, series: ArrowSeries) -> ArrowSeries:
         from narwhals._arrow.series import ArrowSeries
 
-        if self._backend_version < (13,) and hasattr(value, "as_py"):  # pragma: no cover
+        if self._backend_version < (13,) and hasattr(value, "as_py"):
             value = value.as_py()
         return ArrowSeries._from_iterable(
             [value],

narwhals/_arrow/series.py

@@ -655,6 +655,26 @@ def unique(self: Self, *, maintain_order: bool = False) -> ArrowSeries:
 
         return self._from_native_series(pc.unique(self._native_series))
 
+    def replace_strict(
+        self, old: Sequence[Any], new: Sequence[Any], *, return_dtype: DType
+    ) -> ArrowSeries:
+        import pyarrow as pa  # ignore-banned-import
+        import pyarrow.compute as pc  # ignore-banned-import
+
+        # https://stackoverflow.com/a/79111029/4451315
+        idxs = pc.index_in(self._native_series, pa.array(old))
+        result_native = pc.take(pa.array(new), idxs).cast(
+            narwhals_to_native_dtype(return_dtype, self._dtypes)
+        )
+        result = self._from_native_series(result_native)
+        if result.is_null().sum() != self.is_null().sum():
+            msg = (
+                "replace_strict did not replace all non-null values.\n\n"
+                f"The following did not get replaced: {self.filter(~self.is_null() & result.is_null()).unique().to_list()}"
+            )
+            raise ValueError(msg)
+        return result
+
     def sort(
         self: Self, *, descending: bool = False, nulls_last: bool = False
     ) -> ArrowSeries:

narwhals/_dask/expr.py

@@ -6,6 +6,7 @@
 from typing import Callable
 from typing import Literal
 from typing import NoReturn
+from typing import Sequence
 
 from narwhals._dask.utils import add_row_index
 from narwhals._dask.utils import maybe_evaluate
@@ -477,6 +478,12 @@ def head(self) -> NoReturn:
         msg = "`Expr.head` is not supported for the Dask backend. Please use `LazyFrame.head` instead."
         raise NotImplementedError(msg)
 
+    def replace_strict(
+        self, old: Sequence[Any], new: Sequence[Any], *, return_dtype: DType
+    ) -> Self:
+        msg = "`replace_strict` is not yet supported for Dask expressions"
+        raise NotImplementedError(msg)
+
     def sort(self, *, descending: bool = False, nulls_last: bool = False) -> NoReturn:
         # We can't (yet?) allow methods which modify the index
         msg = "`Expr.sort` is not supported for the Dask backend. Please use `LazyFrame.sort` instead."

narwhals/_pandas_like/dataframe.py

@@ -429,7 +429,7 @@ def with_columns(
             )
         else:
             # This is the logic in pandas' DataFrame.assign
-            if self._backend_version < (2,):  # pragma: no cover
+            if self._backend_version < (2,):
                 df = self._native_frame.copy(deep=True)
             else:
                 df = self._native_frame.copy(deep=False)

narwhals/_pandas_like/expr.py

@@ -4,6 +4,7 @@
 from typing import Any
 from typing import Callable
 from typing import Literal
+from typing import Sequence
 
 from narwhals._expression_parsing import reuse_series_implementation
 from narwhals._expression_parsing import reuse_series_namespace_implementation
@@ -14,6 +15,7 @@
 
     from narwhals._pandas_like.dataframe import PandasLikeDataFrame
     from narwhals._pandas_like.namespace import PandasLikeNamespace
+    from narwhals.dtypes import DType
     from narwhals.typing import DTypes
     from narwhals.utils import Implementation
 
@@ -271,6 +273,13 @@ def filter(self, *predicates: Any) -> Self:
     def drop_nulls(self) -> Self:
         return reuse_series_implementation(self, "drop_nulls")
 
+    def replace_strict(
+        self, old: Sequence[Any], new: Sequence[Any], *, return_dtype: DType
+    ) -> Self:
+        return reuse_series_implementation(
+            self, "replace_strict", old, new, return_dtype=return_dtype
+        )
+
     def sort(self, *, descending: bool = False, nulls_last: bool = False) -> Self:
         return reuse_series_implementation(
             self, "sort", descending=descending, nulls_last=nulls_last

narwhals/_pandas_like/series.py

@@ -491,6 +491,36 @@ def diff(self) -> PandasLikeSeries:
     def shift(self, n: int) -> PandasLikeSeries:
         return self._from_native_series(self._native_series.shift(n))
 
+    def replace_strict(
+        self, old: Sequence[Any], new: Sequence[Any], *, return_dtype: DType
+    ) -> PandasLikeSeries:
+        tmp_name = f"{self.name}_tmp"
+        dtype = narwhals_to_native_dtype(
+            return_dtype,
+            self._native_series.dtype,
+            self._implementation,
+            self._backend_version,
+            self._dtypes,
+        )
+        other = self.__native_namespace__().DataFrame(
+            {
+                self.name: old,
+                tmp_name: self.__native_namespace__().Series(new, dtype=dtype),
+            }
+        )
+        result = self._from_native_series(
+            self._native_series.to_frame()
+            .merge(other, on=self.name, how="left")[tmp_name]
+            .rename(self.name)
+        )
+        if result.is_null().sum() != self.is_null().sum():
+            msg = (
+                "replace_strict did not replace all non-null values.\n\n"
+                f"The following did not get replaced: {self.filter(~self.is_null() & result.is_null()).unique().to_list()}"
+            )
+            raise ValueError(msg)
+        return result
+
     def sort(
         self, *, descending: bool = False, nulls_last: bool = False
     ) -> PandasLikeSeries:

narwhals/_polars/dataframe.py

@@ -84,15 +84,15 @@ def func(*args: Any, **kwargs: Any) -> Any:
         return func
 
     def __array__(self, dtype: Any | None = None, copy: bool | None = None) -> np.ndarray:
-        if self._backend_version < (0, 20, 28) and copy is not None:  # pragma: no cover
+        if self._backend_version < (0, 20, 28) and copy is not None:
             msg = "`copy` in `__array__` is only supported for Polars>=0.20.28"
             raise NotImplementedError(msg)
-        if self._backend_version < (0, 20, 28):  # pragma: no cover
+        if self._backend_version < (0, 20, 28):
             return self._native_frame.__array__(dtype)
         return self._native_frame.__array__(dtype)
 
     def collect_schema(self) -> dict[str, Any]:
-        if self._backend_version < (1,):  # pragma: no cover
+        if self._backend_version < (1,):
             schema = self._native_frame.schema
         else:
             schema = dict(self._native_frame.collect_schema())
@@ -209,12 +209,12 @@ def group_by(self, *by: str, drop_null_keys: bool) -> Any:
         return PolarsGroupBy(self, list(by), drop_null_keys=drop_null_keys)
 
     def with_row_index(self, name: str) -> Any:
-        if self._backend_version < (0, 20, 4):  # pragma: no cover
+        if self._backend_version < (0, 20, 4):
             return self._from_native_frame(self._native_frame.with_row_count(name))
         return self._from_native_frame(self._native_frame.with_row_index(name))
 
     def drop(self: Self, columns: list[str], strict: bool) -> Self:  # noqa: FBT001
-        if self._backend_version < (1, 0, 0):  # pragma: no cover
+        if self._backend_version < (1, 0, 0):
             to_drop = parse_columns_to_drop(
                 compliant_frame=self, columns=columns, strict=strict
             )
@@ -228,7 +228,7 @@ def unpivot(
         variable_name: str | None,
         value_name: str | None,
     ) -> Self:
-        if self._backend_version < (1, 0, 0):  # pragma: no cover
+        if self._backend_version < (1, 0, 0):
             return self._from_native_frame(
                 self._native_frame.melt(
                     id_vars=index,
@@ -296,7 +296,7 @@ def schema(self) -> dict[str, Any]:
         }
 
     def collect_schema(self) -> dict[str, Any]:
-        if self._backend_version < (1,):  # pragma: no cover
+        if self._backend_version < (1,):
             schema = self._native_frame.schema
         else:
             schema = dict(self._native_frame.collect_schema())
@@ -318,12 +318,12 @@ def group_by(self, *by: str, drop_null_keys: bool) -> Any:
         return PolarsLazyGroupBy(self, list(by), drop_null_keys=drop_null_keys)
 
     def with_row_index(self, name: str) -> Any:
-        if self._backend_version < (0, 20, 4):  # pragma: no cover
+        if self._backend_version < (0, 20, 4):
             return self._from_native_frame(self._native_frame.with_row_count(name))
         return self._from_native_frame(self._native_frame.with_row_index(name))
 
     def drop(self: Self, columns: list[str], strict: bool) -> Self:  # noqa: FBT001
-        if self._backend_version < (1, 0, 0):  # pragma: no cover
+        if self._backend_version < (1, 0, 0):
             return self._from_native_frame(self._native_frame.drop(columns))
         return self._from_native_frame(self._native_frame.drop(columns, strict=strict))
 
@@ -334,7 +334,7 @@ def unpivot(
         variable_name: str | None,
         value_name: str | None,
     ) -> Self:
-        if self._backend_version < (1, 0, 0):  # pragma: no cover
+        if self._backend_version < (1, 0, 0):
             return self._from_native_frame(
                 self._native_frame.melt(
                     id_vars=index,

narwhals/_polars/expr.py

@@ -2,6 +2,7 @@
 
 from typing import TYPE_CHECKING
 from typing import Any
+from typing import Sequence
 
 from narwhals._polars.utils import extract_args_kwargs
 from narwhals._polars.utils import extract_native
@@ -16,16 +17,21 @@
 
 
 class PolarsExpr:
-    def __init__(self, expr: Any, dtypes: DTypes) -> None:
+    def __init__(
+        self, expr: Any, dtypes: DTypes, backend_version: tuple[int, ...]
+    ) -> None:
         self._native_expr = expr
         self._implementation = Implementation.POLARS
         self._dtypes = dtypes
+        self._backend_version = backend_version
 
     def __repr__(self) -> str:  # pragma: no cover
         return "PolarsExpr"
 
     def _from_native_expr(self, expr: Any) -> Self:
-        return self.__class__(expr, dtypes=self._dtypes)
+        return self.__class__(
+            expr, dtypes=self._dtypes, backend_version=self._backend_version
+        )
 
     def __getattr__(self, attr: str) -> Any:
         def func(*args: Any, **kwargs: Any) -> Any:
@@ -41,6 +47,18 @@ def cast(self, dtype: DType) -> Self:
         dtype = narwhals_to_native_dtype(dtype, self._dtypes)
         return self._from_native_expr(expr.cast(dtype))
 
+    def replace_strict(
+        self, old: Sequence[Any], new: Sequence[Any], *, return_dtype: DType
+    ) -> Self:
+        expr = self._native_expr
+        return_dtype = narwhals_to_native_dtype(return_dtype, self._dtypes)
+        if self._backend_version < (1,):
+            msg = f"`replace_strict` is only available in Polars>=1.0, found version {self._backend_version}"
+            raise NotImplementedError(msg)
+        return self._from_native_expr(
+            expr.replace_strict(old, new, return_dtype=return_dtype)
+        )
+
     def __eq__(self, other: object) -> Self:  # type: ignore[override]
         return self._from_native_expr(self._native_expr.__eq__(extract_native(other)))