From 954538e3356fe43959acdfe88d51ae7d0c6693f1 Mon Sep 17 00:00:00 2001
From: Ritchie Vink <ritchie46@gmail.com>
Date: Wed, 24 Jul 2024 14:58:35 +0200
Subject: [PATCH] fix: Infer decimal scales on mixed scale input (#17840)

---
 crates/polars-core/src/series/any_value.rs         | 14 ++++++++++++--
 .../unit/constructors/test_any_value_fallbacks.py  |  1 -
 py-polars/tests/unit/datatypes/test_decimal.py     |  7 +++++++
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/crates/polars-core/src/series/any_value.rs b/crates/polars-core/src/series/any_value.rs
index 3eedb866118b..83abf75e980d 100644
--- a/crates/polars-core/src/series/any_value.rs
+++ b/crates/polars-core/src/series/any_value.rs
@@ -68,7 +68,7 @@ impl Series {
 
         // TODO: Remove this when Decimal data type equality is implemented.
         #[cfg(feature = "dtype-decimal")]
-        if !strict && dtype.is_decimal() {
+        if dtype.is_decimal() {
             let dtype = DataType::Decimal(None, None);
             return Self::from_any_values_and_dtype(name, values, &dtype, strict);
         }
@@ -488,7 +488,17 @@ fn any_values_to_decimal(
     let mut builder = PrimitiveChunkedBuilder::<Int128Type>::new("", values.len());
     for av in values {
         match av {
-            AnyValue::Decimal(v, s) if *s == scale => builder.append_value(*v),
+            // Allow equal or less scale. We do want to support different scales even in 'strict' mode.
+            AnyValue::Decimal(v, s) if *s <= scale => {
+                if *s == scale {
+                    builder.append_value(*v)
+                } else {
+                    match av.strict_cast(&target_dtype) {
+                        Some(AnyValue::Decimal(i, _)) => builder.append_value(i),
+                        _ => builder.append_null(),
+                    }
+                }
+            },
             AnyValue::Null => builder.append_null(),
             av => {
                 if strict {
diff --git a/py-polars/tests/unit/constructors/test_any_value_fallbacks.py b/py-polars/tests/unit/constructors/test_any_value_fallbacks.py
index e9413edfb060..0f3c40f21925 100644
--- a/py-polars/tests/unit/constructors/test_any_value_fallbacks.py
+++ b/py-polars/tests/unit/constructors/test_any_value_fallbacks.py
@@ -299,7 +299,6 @@ def test_fallback_without_dtype(
         [timedelta(hours=0), 1_000],
         [D("12.345"), 100],
         [D("12.345"), 3.14],
-        [D("0.12345"), D("6789.0")],
         [{"a": 1, "b": "foo"}, {"a": -1, "b": date(2020, 12, 31)}],
         [{"a": None}, {"a": 1.0}, {"a": 1}],
     ],
diff --git a/py-polars/tests/unit/datatypes/test_decimal.py b/py-polars/tests/unit/datatypes/test_decimal.py
index c6cdaf247524..13acb7d66741 100644
--- a/py-polars/tests/unit/datatypes/test_decimal.py
+++ b/py-polars/tests/unit/datatypes/test_decimal.py
@@ -515,3 +515,10 @@ def test_decimal_dynamic_float_st() -> None:
     assert pl.LazyFrame({"a": [D("2.0"), D("0.5")]}).filter(
         pl.col("a").is_between(0.45, 0.9)
     ).collect().to_dict(as_series=False) == {"a": [D("0.5")]}
+
+
+def test_decimal_strict_scale_inference_17770() -> None:
+    values = [D("0.1"), D("0.10"), D("1.0121")]
+    s = pl.Series(values, strict=True)
+    assert s.dtype == pl.Decimal(precision=None, scale=4)
+    assert s.to_list() == values