refactor: Rework _readers.py

- Moved `_Reader._metadata` -> module-level constant `_METADATA`.
  - It was never modified and is based on the relative directory of this module
- Generally improved the readability with more method-chaining (less assignment)
- Renamed, improved doc `_filter_reduce` -> `_parse_predicates_constraints`

altair/datasets/_readers.py

@@ -22,6 +22,7 @@
     Any,
     Callable,
     ClassVar,
+    Final,
     Generic,
     Literal,
     Protocol,
@@ -76,6 +77,8 @@
 
 __all__ = ["get_backend"]
 
+_METADATA: Final[Path] = Path(__file__).parent / "_metadata" / "metadata.parquet"
+
 
 def _identity(_: _T, /) -> _T:
     return _
@@ -105,8 +108,6 @@ class _Reader(Generic[IntoDataFrameT, IntoFrameT], Protocol):
         https://docs.python.org/3/library/pathlib.html#pathlib.Path
     """
 
-    _metadata: Path = Path(__file__).parent / "_metadata" / "metadata.parquet"
-
     def read_fn(self, source: StrPath, /) -> Callable[..., IntoDataFrameT]:
         suffix = validate_suffix(source, is_ext_read)
         return self._read_fn[suffix]
@@ -159,20 +160,13 @@ def url(
         /,
         tag: VersionTag | None = None,
     ) -> str:
-        df = self.query(**validate_constraints(name, suffix, tag))
-        url = df.item(0, "url_npm")
+        frame = self.query(**validate_constraints(name, suffix, tag))
+        url = nw.to_py_scalar(frame.item(0, "url_npm"))
         if isinstance(url, str):
             return url
         else:
-            converted = nw.to_py_scalar(url)
-            if isinstance(converted, str):
-                return converted
-            else:
-                msg = (
-                    f"Expected 'str' but got {type(converted).__name__!r}\n"
-                    f"from {converted!r}."
-                )
-                raise TypeError(msg)
+            msg = f"Expected 'str' but got {type(url).__name__!r}\n" f"from {url!r}."
+            raise TypeError(msg)
 
     def query(
         self, *predicates: OneOrSeq[IntoExpr], **constraints: Unpack[Metadata]
@@ -188,15 +182,14 @@ def query(
         .. _pl.LazyFrame.filter:
             https://docs.pola.rs/api/python/stable/reference/lazyframe/api/polars.LazyFrame.filter.html
         """
-        source = self._metadata
-        fn = self.scan_fn(source)
-        frame = nw.from_native(fn(source))
-        result = frame.filter(_filter_reduce(predicates, constraints))
-        df: nw.DataFrame[Any] = (
-            result.collect() if isinstance(result, nw.LazyFrame) else result
+        frame = (
+            nw.from_native(self.scan_fn(_METADATA)(_METADATA))
+            .filter(_parse_predicates_constraints(predicates, constraints))
+            .lazy()
+            .collect()
         )
-        if not df.is_empty():
-            return df
+        if not frame.is_empty():
+            return frame
         else:
             terms = "\n".join(f"{t!r}" for t in (predicates, constraints) if t)
             msg = f"Found no results for:\n{terms}"
@@ -208,12 +201,12 @@ def _read_metadata(self) -> IntoDataFrameT:
 
         Effectively an eager read, no filters.
         """
-        fn = self.scan_fn(self._metadata)
-        frame = nw.from_native(fn(self._metadata))
-        df: nw.DataFrame[Any] = (
-            frame.collect() if isinstance(frame, nw.LazyFrame) else frame
+        return (
+            nw.from_native(self.scan_fn(_METADATA)(_METADATA))
+            .lazy()
+            .collect()
+            .to_native()
         )
-        return df.to_native()
 
     @property
     def _cache(self) -> Path | None:  # type: ignore[return]
@@ -351,11 +344,15 @@ def __init__(self, name: _PyArrow, /) -> None:
         self._scan_fn = {".parquet": pa_read_parquet}
 
 
-def _filter_reduce(predicates: tuple[Any, ...], constraints: Metadata, /) -> nw.Expr:
+def _parse_predicates_constraints(
+    predicates: tuple[Any, ...], constraints: Metadata, /
+) -> nw.Expr:
     """
-    ``narwhals`` only accepts ``filter(*predicates)`.
+    ``narwhals`` only accepts ``filter(*predicates)``.
+
+    So we convert each item in ``**constraints`` here as::
 
-    Manually converts the constraints into ``==``
+       col("column_name") == literal_value
     """
     return nw.all_horizontal(
         chain(predicates, (nw.col(name) == v for name, v in constraints.items()))