fix: Ensure same fmt in Series/AnyValue to string cast

crates/polars-arrow/src/compute/cast/primitive_to.rs

@@ -13,7 +13,7 @@ use crate::offset::{Offset, Offsets};
 use crate::temporal_conversions::*;
 use crate::types::{days_ms, f16, months_days_ns, NativeType};
 
-pub(super) trait SerPrimitive {
+pub trait SerPrimitive {
     fn write(f: &mut Vec<u8>, val: Self) -> usize
     where
         Self: Sized;

crates/polars-core/src/datatypes/any_value.rs

@@ -1,7 +1,7 @@
 use std::borrow::Cow;
 
+use arrow::compute::cast::SerPrimitive;
 use arrow::types::PrimitiveType;
-use polars_utils::format_pl_smallstr;
 #[cfg(feature = "dtype-categorical")]
 use polars_utils::sync::SyncPtr;
 use polars_utils::total_ord::ToTotalOrd;
@@ -563,19 +563,22 @@ impl<'a> AnyValue<'a> {
             (AnyValue::Float64(v), DataType::Boolean) => AnyValue::Boolean(*v != f64::default()),
 
             // to string
-            (AnyValue::String(v), DataType::String) => {
-                AnyValue::StringOwned(PlSmallStr::from_str(v))
-            },
+            (AnyValue::String(v), DataType::String) => AnyValue::String(v),
             (AnyValue::StringOwned(v), DataType::String) => AnyValue::StringOwned(v.clone()),
 
             (av, DataType::String) => {
+                let mut tmp = vec![];
                 if av.is_unsigned_integer() {
-                    AnyValue::StringOwned(format_pl_smallstr!("{}", av.extract::<u64>()?))
+                    let val = av.extract::<u64>()?;
+                    SerPrimitive::write(&mut tmp, val);
                 } else if av.is_float() {
-                    AnyValue::StringOwned(format_pl_smallstr!("{}", av.extract::<f64>()?))
+                    let val = av.extract::<f64>()?;
+                    SerPrimitive::write(&mut tmp, val);
                 } else {
-                    AnyValue::StringOwned(format_pl_smallstr!("{}", av.extract::<i64>()?))
+                    let val = av.extract::<i64>()?;
+                    SerPrimitive::write(&mut tmp, val);
                 }
+                AnyValue::StringOwned(PlSmallStr::from_str(std::str::from_utf8(&tmp).unwrap()))
             },
 
             // to binary

py-polars/tests/unit/operations/test_cast.py

@@ -686,3 +686,9 @@ def test_bool_numeric_supertype(dtype: PolarsDataType) -> None:
     df = pl.DataFrame({"v": [1, 2, 3, 4, 5, 6]})
     result = df.select((pl.col("v") < 3).sum().cast(dtype) / pl.len())
     assert result.item() - 0.3333333 <= 0.00001
+
+
+def test_cast_consistency() -> None:
+    assert pl.DataFrame().with_columns(a=pl.lit(0.0)).with_columns(
+        b=pl.col("a").cast(pl.String), c=pl.lit(0.0).cast(pl.String)
+    ).to_dict(as_series=False) == {"a": [0.0], "b": ["0.0"], "c": ["0.0"]}