From eea22949739ecc9b6288895fa4a25ac4ca1b52c5 Mon Sep 17 00:00:00 2001
From: cmdlineluser <99486669+cmdlineluser@users.noreply.github.com>
Date: Sun, 30 Jul 2023 13:37:31 +0100
Subject: [PATCH] fix(rust, python): prevent re-ordering of dict keys inside
 `.apply`

---
 py-polars/src/apply/mod.rs                    | 10 ++++++++--
 py-polars/tests/unit/operations/test_apply.py |  5 +++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/py-polars/src/apply/mod.rs b/py-polars/src/apply/mod.rs
index f9d0a10b2d26..4d5fdc3d04ee 100644
--- a/py-polars/src/apply/mod.rs
+++ b/py-polars/src/apply/mod.rs
@@ -54,6 +54,10 @@ fn iterator_to_struct<'a>(
     // ]
     let mut struct_fields: BTreeMap<&str, Vec<AnyValue>> = BTreeMap::new();
 
+    // as a BTreeMap sorts its keys, we also need to track the original
+    // order of the field names
+    let mut field_names_ordered = Vec::with_capacity(flds.len());
+
     // use the first value and the known null count to initialize the buffers
     // if we find a new key later on, we make a new entry in the BTree
     for (value, fld) in vals.into_iter().zip(flds) {
@@ -62,6 +66,7 @@ fn iterator_to_struct<'a>(
             buf.push(AnyValue::Null);
         }
         buf.push(value);
+        field_names_ordered.push(fld.name() as &str);
         struct_fields.insert(fld.name(), buf);
     }
 
@@ -86,6 +91,7 @@ fn iterator_to_struct<'a>(
                 for (key, val) in dict.iter() {
                     let key = key.str().unwrap().to_str().unwrap();
                     let buf = struct_fields.entry(key).or_insert_with(|| {
+                        field_names_ordered.push(key);
                         let mut buf = Vec::with_capacity(capacity);
                         for _ in 0..(init_null_count + current_len) {
                             buf.push(AnyValue::Null);
@@ -110,9 +116,9 @@ fn iterator_to_struct<'a>(
     }
 
     let fields = POOL.install(|| {
-        struct_fields
+        field_names_ordered
             .par_iter()
-            .map(|(name, avs)| Series::new(name, avs))
+            .map(|name| Series::new(name, struct_fields.get(name).unwrap()))
             .collect::<Vec<_>>()
     });
 
diff --git a/py-polars/tests/unit/operations/test_apply.py b/py-polars/tests/unit/operations/test_apply.py
index 840a2754830e..b8bfe480b738 100644
--- a/py-polars/tests/unit/operations/test_apply.py
+++ b/py-polars/tests/unit/operations/test_apply.py
@@ -381,3 +381,8 @@ def test_apply_shifted_chunks() -> None:
         "column_0": ["test", "test123", "tests"],
         "column_1": [None, "test", "test123"],
     }
+
+
+def test_apply_dict_order_10128() -> None:
+    df = pl.select(pl.lit("").apply(lambda x: {"c": 1, "b": 2, "a": 3}))
+    assert df.to_dict(False) == {"literal": [{"c": 1, "b": 2, "a": 3}]}