feat: add to_string method to SparkLikeExprDateTimeNamespace

narwhals/_spark_like/expr_dt.py

@@ -4,6 +4,8 @@
 
 from pyspark.sql import functions as F  # noqa: N812
 
+from narwhals._spark_like.utils import strptime_to_pyspark_format
+
 if TYPE_CHECKING:
     from pyspark.sql import Column
     from typing_extensions import Self
@@ -15,6 +17,49 @@ class SparkLikeExprDateTimeNamespace:
     def __init__(self: Self, expr: SparkLikeExpr) -> None:
         self._compliant_expr = expr
 
+    def to_string(self: Self, format: str) -> SparkLikeExpr:  # noqa: A002
+        def _format_iso_week_with_day(_input: Column) -> Column:
+            """Format datetime as ISO week string with day."""
+            year = F.date_format(_input, "yyyy")
+            week = F.lpad(F.weekofyear(_input).cast("string"), 2, "0")
+            day = F.dayofweek(_input)
+            # Adjust Sunday from 1 to 7
+            day = F.when(day == 1, 7).otherwise(day - 1)
+            return F.concat(year, F.lit("-W"), week, F.lit("-"), day.cast("string"))
+
+        def _format_iso_week(_input: Column) -> Column:
+            """Format datetime as ISO week string."""
+            year = F.date_format(_input, "yyyy")
+            week = F.lpad(F.weekofyear(_input).cast("string"), 2, "0")
+            return F.concat(year, F.lit("-W"), week)
+
+        def _format_iso_datetime(_input: Column) -> Column:
+            """Format datetime as ISO datetime with microseconds."""
+            date_part = F.date_format(_input, "yyyy-MM-dd")
+            time_part = F.date_format(_input, "HH:mm:ss")
+            micros = F.unix_micros(_input) % 1_000_000
+            micros_str = F.lpad(micros.cast("string"), 6, "0")
+            return F.concat(date_part, F.lit("T"), time_part, F.lit("."), micros_str)
+
+        def _to_string(_input: Column) -> Column:
+            # Handle special formats
+            if format == "%G-W%V":
+                return _format_iso_week(_input)
+            if format == "%G-W%V-%u":
+                return _format_iso_week_with_day(_input)
+            if format in ("%Y-%m-%dT%H:%M:%S.%f", "%Y-%m-%dT%H:%M:%S%.f"):
+                return _format_iso_datetime(_input)
+
+            # Convert Python format to PySpark format
+            pyspark_fmt = strptime_to_pyspark_format(format)
+            return F.date_format(_input, pyspark_fmt)
+
+        return self._compliant_expr._from_call(
+            _to_string,
+            "to_string",
+            returns_scalar=self._compliant_expr._returns_scalar,
+        )
+
     def date(self: Self) -> SparkLikeExpr:
         return self._compliant_expr._from_call(
             F.to_date,

narwhals/_spark_like/expr_str.py

@@ -1,10 +1,11 @@
 from __future__ import annotations
 
 from typing import TYPE_CHECKING
-from typing import overload
 
 from pyspark.sql import functions as F  # noqa: N812
 
+from narwhals._spark_like.utils import strptime_to_pyspark_format
+
 if TYPE_CHECKING:
     from pyspark.sql import Column
     from typing_extensions import Self
@@ -123,44 +124,3 @@ def to_datetime(self: Self, format: str | None) -> SparkLikeExpr:  # noqa: A002
             "to_datetime",
             returns_scalar=self._compliant_expr._returns_scalar,
         )
-
-
-@overload
-def strptime_to_pyspark_format(format: None) -> None: ...
-
-
-@overload
-def strptime_to_pyspark_format(format: str) -> str: ...
-
-
-def strptime_to_pyspark_format(format: str | None) -> str | None:  # noqa: A002
-    """Converts a Python strptime datetime format string to a PySpark datetime format string."""
-    # Mapping from Python strptime format to PySpark format
-    if format is None:
-        return None
-
-    # see https://spark.apache.org/docs/latest/sql-ref-datetime-pattern.html
-    # and https://docs.python.org/3/library/datetime.html#strftime-strptime-behavior
-    format_mapping = {
-        "%Y": "y",  # Year with century
-        "%y": "y",  # Year without century
-        "%m": "M",  # Month
-        "%d": "d",  # Day of the month
-        "%H": "H",  # Hour (24-hour clock) 0-23
-        "%I": "h",  # Hour (12-hour clock) 1-12
-        "%M": "m",  # Minute
-        "%S": "s",  # Second
-        "%f": "S",  # Microseconds -> Milliseconds
-        "%p": "a",  # AM/PM
-        "%a": "E",  # Abbreviated weekday name
-        "%A": "E",  # Full weekday name
-        "%j": "D",  # Day of the year
-        "%z": "Z",  # Timezone offset
-        "%s": "X",  # Unix timestamp
-    }
-
-    # Replace Python format specifiers with PySpark specifiers
-    pyspark_format = format
-    for py_format, spark_format in format_mapping.items():
-        pyspark_format = pyspark_format.replace(py_format, spark_format)
-    return pyspark_format.replace("T", " ")

narwhals/_spark_like/utils.py

@@ -3,6 +3,7 @@
 from functools import lru_cache
 from typing import TYPE_CHECKING
 from typing import Any
+from typing import overload
 
 from pyspark.sql import functions as F  # noqa: N812
 
@@ -195,3 +196,43 @@ def _var(_input: Column | str, ddof: int, np_version: tuple[int, ...]) -> Column
 
     input_col = F.col(_input) if isinstance(_input, str) else _input
     return var(input_col, ddof=ddof)
+
+
+@overload
+def strptime_to_pyspark_format(format: None) -> None: ...
+
+
+@overload
+def strptime_to_pyspark_format(format: str) -> str: ...
+
+
+def strptime_to_pyspark_format(format: str | None) -> str | None:  # noqa: A002
+    """Converts a Python strptime datetime format string to a PySpark datetime format string."""
+    if format is None:
+        return None
+
+    # see https://spark.apache.org/docs/latest/sql-ref-datetime-pattern.html
+    # and https://docs.python.org/3/library/datetime.html#strftime-strptime-behavior
+    format_mapping = {
+        "%Y": "yyyy",  # Year with century (4 digits)
+        "%y": "yy",  # Year without century (2 digits)
+        "%m": "MM",  # Month (01-12)
+        "%d": "dd",  # Day of the month (01-31)
+        "%H": "HH",  # Hour (24-hour clock) (00-23)
+        "%I": "hh",  # Hour (12-hour clock) (01-12)
+        "%M": "mm",  # Minute (00-59)
+        "%S": "ss",  # Second (00-59)
+        "%f": "S",  # Microseconds -> Milliseconds
+        "%p": "a",  # AM/PM
+        "%a": "E",  # Abbreviated weekday name
+        "%A": "E",  # Full weekday name
+        "%j": "D",  # Day of the year
+        "%z": "Z",  # Timezone offset
+        "%s": "X",  # Unix timestamp
+    }
+
+    # Replace Python format specifiers with PySpark specifiers
+    pyspark_format = format
+    for py_format, spark_format in format_mapping.items():
+        pyspark_format = pyspark_format.replace(py_format, spark_format)
+    return pyspark_format.replace("T", " ")

tests/conftest.py

@@ -150,15 +150,16 @@ def pyspark_lazy_constructor() -> Callable[[Any], IntoFrame]:  # pragma: no cove
             "ignore", r"Using fork\(\) can cause Polars", category=RuntimeWarning
         )
 
+        # common timezone for all tests environments
+        os.environ["TZ"] = "UTC"
+
         session = (
             SparkSession.builder.appName("unit-tests")
             .master("local[1]")
             .config("spark.ui.enabled", "false")
             # executing one task at a time makes the tests faster
             .config("spark.default.parallelism", "1")
             .config("spark.sql.shuffle.partitions", "2")
-            # common timezone for all tests environments
-            .config("spark.sql.session.timeZone", "UTC")
             .getOrCreate()
         )
 

tests/expr_and_series/dt/to_string_test.py

@@ -59,11 +59,7 @@ def test_dt_to_string_series(constructor_eager: ConstructorEager, fmt: str) -> N
     ],
 )
 @pytest.mark.skipif(is_windows(), reason="pyarrow breaking on windows")
-def test_dt_to_string_expr(
-    constructor: Constructor, fmt: str, request: pytest.FixtureRequest
-) -> None:
-    if "pyspark" in str(constructor):
-        request.applymarker(pytest.mark.xfail)
+def test_dt_to_string_expr(constructor: Constructor, fmt: str) -> None:
     input_frame = nw.from_native(constructor(data))
 
     expected_col = [datetime.strftime(d, fmt) for d in data["a"]]
@@ -141,7 +137,7 @@ def test_dt_to_string_iso_local_datetime_expr(
     expected: str,
     request: pytest.FixtureRequest,
 ) -> None:
-    if ("pyspark" in str(constructor)) or "duckdb" in str(constructor):
+    if "duckdb" in str(constructor):
         request.applymarker(pytest.mark.xfail)
     df = constructor({"a": [data]})
 
@@ -178,10 +174,7 @@ def test_dt_to_string_iso_local_date_expr(
     constructor: Constructor,
     data: datetime,
     expected: str,
-    request: pytest.FixtureRequest,
 ) -> None:
-    if "pyspark" in str(constructor):
-        request.applymarker(pytest.mark.xfail)
     df = constructor({"a": [data]})
     result = nw.from_native(df).with_columns(
         nw.col("a").dt.to_string("%Y-%m-%d").alias("b")