feat: Add {Expr,Series}.is_close

docs/api-reference/expr.md

@@ -23,6 +23,7 @@
         - filter
         - clip
         - is_between
+        - is_close
         - is_duplicated
         - is_finite
         - is_first_distinct

docs/api-reference/series.md

@@ -35,6 +35,7 @@
         - hist
         - implementation
         - is_between
+        - is_close
         - is_duplicated
         - is_empty
         - is_finite

narwhals/_compliant/expr.py

@@ -27,6 +27,7 @@
     LazyExprT,
     NativeExprT,
 )
+from narwhals._compliant.utils import IsClose
 from narwhals._utils import _StoresCompliant
 from narwhals.dependencies import get_numpy, is_numpy_array
 
@@ -75,7 +76,7 @@ def __eq__(self, value: Any, /) -> Self: ...  # type: ignore[override]
     def __ne__(self, value: Any, /) -> Self: ...  # type: ignore[override]
 
 
-class CompliantExpr(Protocol[CompliantFrameT, CompliantSeriesOrNativeExprT_co]):
+class CompliantExpr(IsClose, Protocol[CompliantFrameT, CompliantSeriesOrNativeExprT_co]):
     _implementation: Implementation
     _version: Version
     _evaluate_output_names: EvalNames[CompliantFrameT]
@@ -902,6 +903,22 @@ def exp(self) -> Self:
     def sqrt(self) -> Self:
         return self._reuse_series("sqrt")
 
+    def is_close(
+        self,
+        other: Self | NumericLiteral,
+        *,
+        abs_tol: float,
+        rel_tol: float,
+        nans_equal: bool,
+    ) -> Self:
+        return self._reuse_series(
+            "is_close",
+            other=other,
+            abs_tol=abs_tol,
+            rel_tol=rel_tol,
+            nans_equal=nans_equal,
+        )
+
     @property
     def cat(self) -> EagerExprCatNamespace[Self]:
         return EagerExprCatNamespace(self)

narwhals/_compliant/series.py

@@ -16,6 +16,7 @@
     NativeSeriesT,
     NativeSeriesT_co,
 )
+from narwhals._compliant.utils import IsClose
 from narwhals._translate import FromIterable, FromNative, NumpyConvertible, ToNarwhals
 from narwhals._typing_compat import TypeVar, assert_never
 from narwhals._utils import (
@@ -78,6 +79,7 @@ class HistData(TypedDict, Generic[NativeSeriesT, "_CountsT_co"]):
 
 
 class CompliantSeries(
+    IsClose,
     NumpyConvertible["_1DArray", "Into1DArray"],
     FromIterable,
     FromNative[NativeSeriesT],

narwhals/_compliant/utils.py

@@ -0,0 +1,88 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING, Any, Protocol
+
+if TYPE_CHECKING:
+    from typing_extensions import Self
+
+    from narwhals.typing import NumericLiteral, TemporalLiteral
+
+
+class IsClose(Protocol):
+    """Every member defined is a dependency of `is_close` method."""
+
+    def __and__(self, other: Any) -> Self: ...
+    def __or__(self, other: Any) -> Self: ...
+    def __invert__(self) -> Self: ...
+    def __sub__(self, other: Any) -> Self: ...
+    def __mul__(self, other: Any) -> Self: ...
+    def __eq__(self, other: Self | Any) -> Self: ...  # type: ignore[override]
+    def __gt__(self, other: Any) -> Self: ...
+    def __le__(self, other: Any) -> Self: ...
+    def abs(self) -> Self: ...
+    def is_nan(self) -> Self: ...
+    def is_finite(self) -> Self: ...
+    def clip(
+        self,
+        lower_bound: Self | NumericLiteral | TemporalLiteral | None,
+        upper_bound: Self | NumericLiteral | TemporalLiteral | None,
+    ) -> Self: ...
+    def is_close(
+        self,
+        other: Self | NumericLiteral,
+        *,
+        abs_tol: float,
+        rel_tol: float,
+        nans_equal: bool,
+    ) -> Self:
+        from decimal import Decimal
+
+        other_abs: Self | NumericLiteral
+        other_is_nan: Self | bool
+        other_is_inf: Self | bool
+        other_is_not_inf: Self | bool
+
+        if isinstance(other, (float, int, Decimal)):
+            from math import isinf, isnan
+
+            # NOTE: See https://discuss.python.org/t/inferred-type-of-function-that-calls-dunder-abs-abs/101447
+            other_abs = other.__abs__()
+            other_is_nan = isnan(other)
+            other_is_inf = isinf(other)
+
+            # Define the other_is_not_inf variable to prevent triggering the following warning:
+            # > DeprecationWarning: Bitwise inversion '~' on bool is deprecated and will be
+            # >     removed in Python 3.16.
+            other_is_not_inf = not other_is_inf
+
+        else:
+            other_abs, other_is_nan = other.abs(), other.is_nan()
+            other_is_not_inf = other.is_finite() | other_is_nan
+            other_is_inf = ~other_is_not_inf
+
+        rel_threshold = self.abs().clip(lower_bound=other_abs, upper_bound=None) * rel_tol
+        tolerance = rel_threshold.clip(lower_bound=abs_tol, upper_bound=None)
+
+        self_is_nan = self.is_nan()
+        self_is_not_inf = self.is_finite() | self_is_nan
+
+        # Values are close if abs_diff <= tolerance, and both finite
+        is_close = (
+            ((self - other).abs() <= tolerance) & self_is_not_inf & other_is_not_inf
+        )
+
+        # Handle infinity cases: infinities are close/equal if they have the same sign
+        self_sign, other_sign = self > 0, other > 0
+        is_same_inf = (~self_is_not_inf) & other_is_inf & (self_sign == other_sign)
+
+        # Handle nan cases:
+        #   * If any value is NaN, then False (via `& ~either_nan`)
+        #   * However, if `nans_equals = True` and if _both_ values are NaN, then True
+        either_nan = self_is_nan | other_is_nan
+        result = (is_close | is_same_inf) & ~either_nan
+
+        if nans_equal:
+            both_nan = self_is_nan & other_is_nan
+            result = result | both_nan
+
+        return result

narwhals/_polars/expr.py

@@ -27,7 +27,7 @@
     from narwhals._polars.dataframe import Method, PolarsDataFrame
     from narwhals._polars.namespace import PolarsNamespace
     from narwhals._utils import Version, _LimitedContext
-    from narwhals.typing import IntoDType
+    from narwhals.typing import IntoDType, NumericLiteral
 
 
 class PolarsExpr:
@@ -232,6 +232,46 @@ def __narwhals_namespace__(self) -> PolarsNamespace:  # pragma: no cover
 
         return PolarsNamespace(version=self._version)
 
+    def is_close(
+        self,
+        other: Self | NumericLiteral,
+        *,
+        abs_tol: float,
+        rel_tol: float,
+        nans_equal: bool,
+    ) -> Self:
+        left = self.native
+        right = other.native if isinstance(other, PolarsExpr) else pl.lit(other)
+
+        if self._backend_version < (1, 32, 0):
+            lower_bound = right.abs()
+            tolerance = (left.abs().clip(lower_bound) * rel_tol).clip(abs_tol)
+
+            # Values are close if abs_diff <= tolerance, and both finite
+            abs_diff = (left - right).abs()
+            all_ = pl.all_horizontal
+            is_close = all_((abs_diff <= tolerance), left.is_finite(), right.is_finite())
+
+            # Handle infinity cases: infinities are "close" only if they have the same sign
+            is_same_inf = all_(
+                left.is_infinite(), right.is_infinite(), (left.sign() == right.sign())
+            )
+
+            # Handle nan cases:
+            #   * nans_equals = True => if both values are NaN, then True
+            #   * nans_equals = False => if any value is NaN, then False
+            left_is_nan, right_is_nan = left.is_nan(), right.is_nan()
+            either_nan = left_is_nan | right_is_nan
+            result = (is_close | is_same_inf) & either_nan.not_()
+
+            if nans_equal:
+                result = result | (left_is_nan & right_is_nan)
+        else:
+            result = left.is_close(
+                right, abs_tol=abs_tol, rel_tol=rel_tol, nans_equal=nans_equal
+            )
+        return self._with_native(result)
+
     @property
     def dt(self) -> PolarsExprDateTimeNamespace:
         return PolarsExprDateTimeNamespace(self)

narwhals/_polars/series.py

@@ -41,6 +41,7 @@
         IntoDType,
         MultiIndexSelector,
         NonNestedLiteral,
+        NumericLiteral,
         _1DArray,
     )
 
@@ -90,6 +91,7 @@
         "gather_every",
         "head",
         "is_between",
+        "is_close",
         "is_finite",
         "is_first_distinct",
         "is_in",
@@ -131,6 +133,11 @@
 class PolarsSeries:
     _implementation = Implementation.POLARS
 
+    _HIST_EMPTY_SCHEMA: ClassVar[Mapping[IncludeBreakpoint, Sequence[str]]] = {
+        True: ["breakpoint", "count"],
+        False: ["count"],
+    }
+
     def __init__(self, series: pl.Series, *, version: Version) -> None:
         self._native_series: pl.Series = series
         self._version = version
@@ -478,10 +485,27 @@ def __contains__(self, other: Any) -> bool:
         except Exception as e:  # noqa: BLE001
             raise catch_polars_exception(e) from None
 
-    _HIST_EMPTY_SCHEMA: ClassVar[Mapping[IncludeBreakpoint, Sequence[str]]] = {
-        True: ["breakpoint", "count"],
-        False: ["count"],
-    }
+    def is_close(
+        self,
+        other: Self | NumericLiteral,
+        *,
+        abs_tol: float,
+        rel_tol: float,
+        nans_equal: bool,
+    ) -> PolarsSeries:
+        if self._backend_version < (1, 32, 0):
+            name = self.name
+            ns = self.__narwhals_namespace__()
+            other_expr = other._to_expr() if isinstance(other, PolarsSeries) else other
+            expr = ns.col(name).is_close(
+                other_expr, abs_tol=abs_tol, rel_tol=rel_tol, nans_equal=nans_equal
+            )
+            return self.to_frame().select(expr).get_column(name)
+        other_series = other.native if isinstance(other, PolarsSeries) else other
+        result = self.native.is_close(
+            other_series, abs_tol=abs_tol, rel_tol=rel_tol, nans_equal=nans_equal
+        )
+        return self._with_native(result)
 
     def hist_from_bins(
         self, bins: list[float], *, include_breakpoint: bool

narwhals/expr.py

@@ -12,7 +12,7 @@
 )
 from narwhals._utils import _validate_rolling_arguments, ensure_type, flatten
 from narwhals.dtypes import _validate_dtype
-from narwhals.exceptions import InvalidOperationError
+from narwhals.exceptions import ComputeError, InvalidOperationError
 from narwhals.expr_cat import ExprCatNamespace
 from narwhals.expr_dt import ExprDateTimeNamespace
 from narwhals.expr_list import ExprListNamespace
@@ -2347,6 +2347,97 @@ def sqrt(self) -> Self:
         """
         return self._with_elementwise(lambda plx: self._to_compliant_expr(plx).sqrt())
 
+    def is_close(
+        self,
+        other: Self | NumericLiteral,
+        *,
+        abs_tol: float = 0.0,
+        rel_tol: float = 1e-09,
+        nans_equal: bool = False,
+    ) -> Self:
+        r"""Check if this expression is close, i.e. almost equal, to the other expression.
+
+        Two values `a` and `b` are considered close if the following condition holds:
+
+        $$
+        |a-b| \le max \{ \text{rel\_tol} \cdot max \{ |a|, |b| \}, \text{abs\_tol} \}
+        $$
+
+        Arguments:
+            other: Values to compare with.
+            abs_tol: Absolute tolerance. This is the maximum allowed absolute difference
+                between two values. Must be non-negative.
+            rel_tol: Relative tolerance. This is the maximum allowed difference between
+                two values, relative to the larger absolute value. Must be in the range
+                [0, 1).
+            nans_equal: Whether NaN values should be considered equal.
+
+        Returns:
+            Expression of Boolean data type.
+
+        Notes:
+            The implementation of this method is symmetric and mirrors the behavior of
+            `math.isclose`. Specifically note that this behavior is different to
+            `numpy.isclose`.
+
+        Examples:
+            >>> import duckdb
+            >>> import pyarrow as pa
+            >>> import narwhals as nw
+            >>>
+            >>> data = {
+            ...     "x": [1.0, float("inf"), 1.41, None, float("nan")],
+            ...     "y": [1.2, float("inf"), 1.40, None, float("nan")],
+            ... }
+            >>> _table = pa.table(data)
+            >>> df_native = duckdb.table("_table")
+            >>> df = nw.from_native(df_native)
+            >>> df.with_columns(
+            ...     is_close=nw.col("x").is_close(
+            ...         nw.col("y"), abs_tol=0.1, nans_equal=True
+            ...     )
+            ... )
+            ┌──────────────────────────────┐
+            |      Narwhals LazyFrame      |
+            |------------------------------|
+            |┌────────┬────────┬──────────┐|
+            |│   x    │   y    │ is_close │|
+            |│ double │ double │ boolean  │|
+            |├────────┼────────┼──────────┤|
+            |│    1.0 │    1.2 │ false    │|
+            |│    inf │    inf │ true     │|
+            |│   1.41 │    1.4 │ true     │|
+            |│   NULL │   NULL │ NULL     │|
+            |│    nan │    nan │ true     │|
+            |└────────┴────────┴──────────┘|
+            └──────────────────────────────┘
+        """
+        if abs_tol < 0:
+            msg = f"`abs_tol` must be non-negative but got {abs_tol}"
+            raise ComputeError(msg)
+
+        if not (0 <= rel_tol < 1):
+            msg = f"`rel_tol` must be in the range [0, 1) but got {rel_tol}"
+            raise ComputeError(msg)
+
+        kwargs = {"abs_tol": abs_tol, "rel_tol": rel_tol, "nans_equal": nans_equal}
+        return self.__class__(
+            lambda plx: apply_n_ary_operation(
+                plx,
+                lambda *exprs: exprs[0].is_close(exprs[1], **kwargs),
+                self,
+                other,
+                str_as_lit=False,
+            ),
+            combine_metadata(
+                self,
+                other,
+                str_as_lit=False,
+                allow_multi_output=False,
+                to_single_output=False,
+            ),
+        )
+
     @property
     def str(self) -> ExprStringNamespace[Self]:
         return ExprStringNamespace(self)