chore: Deprecate the old streaming engine (#20949)

crates/polars-error/src/warning.rs

@@ -12,6 +12,7 @@ pub unsafe fn set_warning_function(function: WarningFunction) {
 }
 #[derive(Debug)]
 pub enum PolarsWarning {
+    Deprecation,
     UserWarning,
     CategoricalRemappingWarning,
     MapWithoutReturnDtypeWarning,

crates/polars-plan/src/plans/optimizer/mod.rs

@@ -70,6 +70,19 @@ pub fn optimize(
     #[allow(dead_code)]
     let verbose = verbose();
 
+    if opt_state.streaming() {
+        polars_warn!(
+            Deprecation,
+            "\
+The old streaming engine is being deprecated and will soon be replaced by the new streaming \
+engine. Starting Polars version 1.23.0 and until the new streaming engine is released, the old \
+streaming engine may become less usable. For people who rely on the old streaming engine, it is \
+suggested to pin your version to before 1.23.0.
+
+More information on the new streaming engine: https://github.com/pola-rs/polars/issues/20947"
+        )
+    }
+
     // Gradually fill the rules passed to the optimizer
     let opt = StackOptimizer {};
     let mut rules: Vec<Box<dyn OptimizationRule>> = Vec::with_capacity(8);

crates/polars-python/src/error.rs

@@ -4,8 +4,8 @@ use std::io::{Error, ErrorKind};
 use polars::prelude::PolarsError;
 use polars_error::PolarsWarning;
 use pyo3::exceptions::{
-    PyFileExistsError, PyFileNotFoundError, PyIOError, PyPermissionError, PyRuntimeError,
-    PyUserWarning,
+    PyDeprecationWarning, PyFileExistsError, PyFileNotFoundError, PyIOError, PyPermissionError,
+    PyRuntimeError, PyUserWarning,
 };
 use pyo3::prelude::*;
 use pyo3::PyTypeInfo;
@@ -118,6 +118,7 @@ impl<'py> IntoPyObject<'py> for Wrap<PolarsWarning> {
                 Ok(MapWithoutReturnDtypeWarning::type_object(py).into_any())
             },
             PolarsWarning::UserWarning => Ok(PyUserWarning::type_object(py).into_any()),
+            PolarsWarning::Deprecation => Ok(PyDeprecationWarning::type_object(py).into_any()),
         }
     }
 }

py-polars/pyproject.toml

@@ -261,6 +261,9 @@ filterwarnings = [
   "ignore:Deprecated API features detected.*:DeprecationWarning",
   # numpy import can generate this warning in Python 3.13 on certain runners
   "ignore:.*numpy\\.longdouble.*:UserWarning",
+  # The old streaming engine is deprecated, until the new streaming engine is
+  # done just ignore the deprecation warnings.
+  "ignore:The old streaming engine*:DeprecationWarning",
 ]
 xfail_strict = true