From 2097e7ec116a1493a4b402fc788a495cab0c0506 Mon Sep 17 00:00:00 2001
From: alexander-beedie <alexander.m.beedie@icloud.com>
Date: Thu, 30 May 2024 00:33:43 +0400
Subject: [PATCH] feat: Add SQL support for `TRY_CAST` function

---
 crates/polars-sql/src/sql_expr.rs     | 18 ++++++++++++++----
 py-polars/tests/unit/sql/test_cast.py |  6 ++++++
 2 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/crates/polars-sql/src/sql_expr.rs b/crates/polars-sql/src/sql_expr.rs
index 7a31f8bbb988..7d080d6d04d9 100644
--- a/crates/polars-sql/src/sql_expr.rs
+++ b/crates/polars-sql/src/sql_expr.rs
@@ -209,7 +209,7 @@ impl SQLExprVisitor<'_> {
                 expr,
                 data_type,
                 format,
-            } => self.visit_cast(expr, data_type, format),
+            } => self.visit_cast(expr, data_type, format, true),
             SQLExpr::Ceil { expr, .. } => Ok(self.visit_expr(expr)?.ceil()),
             SQLExpr::CompoundIdentifier(idents) => self.visit_compound_identifier(idents),
             SQLExpr::Extract { field, expr } => parse_extract(self.visit_expr(expr)?, field),
@@ -280,6 +280,11 @@ impl SQLExprVisitor<'_> {
                 trim_what,
                 trim_characters,
             } => self.visit_trim(expr, trim_where, trim_what, trim_characters),
+            SQLExpr::TryCast {
+                expr,
+                data_type,
+                format,
+            } => self.visit_cast(expr, data_type, format, false),
             SQLExpr::UnaryOp { op, expr } => self.visit_unary_op(op, expr),
             SQLExpr::Value(value) => self.visit_literal(value),
             e @ SQLExpr::Case { .. } => self.visit_case_when_then(e),
@@ -610,14 +615,15 @@ impl SQLExprVisitor<'_> {
         }
     }
 
-    /// Visit a SQL `CAST` expression.
+    /// Visit a SQL `CAST` or `TRY_CAST` expression.
     ///
-    /// e.g. `CAST(column AS INT)` or `column::INT`
+    /// e.g. `CAST(col AS INT)`, `col::int4`, or `TRY_CAST(col AS VARCHAR)`,
     fn visit_cast(
         &mut self,
         expr: &SQLExpr,
         data_type: &SQLDataType,
         format: &Option<CastFormat>,
+        strict: bool,
     ) -> PolarsResult<Expr> {
         if format.is_some() {
             return Err(polars_err!(ComputeError: "unsupported use of FORMAT in CAST expression"));
@@ -629,7 +635,11 @@ impl SQLExprVisitor<'_> {
             return Ok(expr.str().json_decode(None, None));
         }
         let polars_type = map_sql_polars_datatype(data_type)?;
-        Ok(expr.strict_cast(polars_type))
+        Ok(if strict {
+            expr.strict_cast(polars_type)
+        } else {
+            expr.cast(polars_type)
+        })
     }
 
     /// Visit a SQL literal.
diff --git a/py-polars/tests/unit/sql/test_cast.py b/py-polars/tests/unit/sql/test_cast.py
index 0f5cc61c1dc9..baccfbd74b06 100644
--- a/py-polars/tests/unit/sql/test_cast.py
+++ b/py-polars/tests/unit/sql/test_cast.py
@@ -161,9 +161,15 @@ def test_cast() -> None:
 def test_cast_errors(values: Any, cast_op: str, error: str) -> None:
     df = pl.DataFrame({"values": values})
 
+    # invalid CAST should raise an error...
     with pytest.raises(ComputeError, match=error):
         df.sql(f"SELECT {cast_op} FROM df")
 
+    # ... or return `null` values if using TRY_CAST
+    target_type = cast_op.split("::")[1]
+    res = df.sql(f"SELECT TRY_CAST(values AS {target_type}) AS cast_values FROM df")
+    assert None in res.to_series()
+
 
 def test_cast_json() -> None:
     df = pl.DataFrame({"txt": ['{"a":[1,2,3],"b":["x","y","z"],"c":5.0}']})