fix: Fix union

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

@@ -22,6 +22,7 @@ use std::fmt::{Display, Formatter};
 use std::hash::{Hash, Hasher};
 use std::ops::{Add, AddAssign, Div, Mul, Rem, Sub, SubAssign};
 
+mod schema;
 pub use aliases::*;
 pub use any_value::*;
 pub use arrow::array::{ArrayCollectIterExt, ArrayFromIter, ArrayFromIterDtype, StaticArray};
@@ -42,6 +43,7 @@ use polars_utils::abs_diff::AbsDiff;
 use polars_utils::float::IsFloat;
 use polars_utils::min_max::MinMax;
 use polars_utils::nulls::IsNull;
+pub use schema::SchemaExtPl;
 #[cfg(feature = "serde")]
 use serde::de::{EnumAccess, Error, Unexpected, VariantAccess, Visitor};
 #[cfg(any(feature = "serde", feature = "serde-lazy"))]

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

@@ -0,0 +1,22 @@
+use super::*;
+
+pub trait SchemaExtPl {
+    // Answers if this schema matches the given schema.
+    //
+    // Allows (nested) Null types in this schema to match any type in the schema,
+    // but not vice versa. In such a case Ok(true) is returned, because a cast
+    // is necessary. If no cast is necessary Ok(false) is returned, and an
+    // error is returned if the types are incompatible.
+    fn matches_schema(&self, other: &Schema) -> PolarsResult<bool>;
+}
+
+impl SchemaExtPl for Schema {
+    fn matches_schema(&self, other: &Schema) -> PolarsResult<bool> {
+        polars_ensure!(self.len() == other.len(), SchemaMismatch: "found different number of fields in schema's\n\nLeft schema: {} fields, right schema: {} fields.", self.len(), other.len());
+        let mut cast = false;
+        for (a, b) in self.iter_values().zip(other.iter_values()) {
+            cast |= a.matches_schema_type(b)?;
+        }
+        Ok(cast)
+    }
+}

crates/polars-core/src/lib.rs

@@ -31,6 +31,7 @@ mod tests;
 use std::sync::Mutex;
 use std::time::{SystemTime, UNIX_EPOCH};
 
+pub use datatypes::SchemaExtPl;
 pub use hashing::IdBuildHasher;
 use once_cell::sync::Lazy;
 use rayon::{ThreadPool, ThreadPoolBuilder};

crates/polars-plan/src/plans/conversion/dsl_to_ir.rs

@@ -384,8 +384,10 @@ pub fn to_alp_impl(lp: DslPlan, ctxt: &mut DslConversionContext) -> PolarsResult
             let schema = ctxt.lp_arena.get(first).schema(ctxt.lp_arena);
             for n in &inputs[1..] {
                 let schema_i = ctxt.lp_arena.get(*n).schema(ctxt.lp_arena);
-                polars_ensure!(schema == schema_i, InvalidOperation: "'union'/'concat' inputs should all have the same schema,\
+                // The first argument
+                schema_i.matches_schema(schema.as_ref()).map_err(|_| polars_err!(InvalidOperation:  "'union'/'concat' inputs should all have the same schema,\
                     got\n{:?} and \n{:?}", schema, schema_i)
+                )?;
             }
 
             let options = args.into();

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

@@ -97,3 +97,13 @@ def test_concat_series() -> None:
     assert pl.concat([s, s]).len() == 6
     # check if s remains unchanged
     assert s.len() == 3
+
+
+def test_concat_null_20501() -> None:
+    a = pl.DataFrame({"id": [1], "value": ["foo"]})
+    b = pl.DataFrame({"id": [2], "value": [None]})
+
+    assert pl.concat([a.lazy(), b.lazy()]).collect().to_dict(as_series=False) == {
+        "id": [1, 2],
+        "value": ["foo", None],
+    }