diff --git a/bindings/python/src/pre_tokenizers.rs b/bindings/python/src/pre_tokenizers.rs
index 4ae7037da..b4cefe692 100644
--- a/bindings/python/src/pre_tokenizers.rs
+++ b/bindings/python/src/pre_tokenizers.rs
@@ -1,6 +1,7 @@
 use std::sync::{Arc, RwLock};
 
 use pyo3::exceptions;
+use pyo3::exceptions::PyException;
 use pyo3::prelude::*;
 use pyo3::types::*;
 use serde::ser::SerializeStruct;
@@ -48,12 +49,16 @@ impl PyPreTokenizer {
 
     pub(crate) fn get_as_subtype(&self, py: Python<'_>) -> PyResult<PyObject> {
         let base = self.clone();
-        Ok(match &self.pretok {
+        Ok(match self.pretok {
             PyPreTokenizerTypeWrapper::Sequence(_) => {
                 Py::new(py, (PySequence {}, base))?.into_py(py)
             }
             PyPreTokenizerTypeWrapper::Single(ref inner) => {
-                match &*inner.as_ref().read().unwrap() {
+                match &*inner
+                    .as_ref()
+                    .read()
+                    .map_err(|_| PyException::new_err("pre tokenizer rwlock is poisoned"))?
+                {
                     PyPreTokenizerWrapper::Custom(_) => Py::new(py, base)?.into_py(py),
                     PyPreTokenizerWrapper::Wrapped(inner) => match inner {
                         PreTokenizerWrapper::Whitespace(_) => {
@@ -467,20 +472,39 @@ impl PySequence {
     fn __getitem__(self_: PyRef<'_, Self>, py: Python<'_>, index: usize) -> PyResult<Py<PyAny>> {
         match &self_.as_ref().pretok {
             PyPreTokenizerTypeWrapper::Sequence(inner) => match inner.get(index) {
-                Some(item) => {
-                    PyPreTokenizer::new(PyPreTokenizerTypeWrapper::Single(Arc::clone(item)))
-                        .get_as_subtype(py)
-                }
+                Some(item) => PyPreTokenizer::new(PyPreTokenizerTypeWrapper::Single(item.clone()))
+                    .get_as_subtype(py),
                 _ => Err(PyErr::new::<pyo3::exceptions::PyIndexError, _>(
                     "Index not found",
                 )),
             },
             PyPreTokenizerTypeWrapper::Single(inner) => {
-                PyPreTokenizer::new(PyPreTokenizerTypeWrapper::Single(Arc::clone(inner)))
+                PyPreTokenizer::new(PyPreTokenizerTypeWrapper::Single(inner.clone()))
                     .get_as_subtype(py)
             }
         }
     }
+
+    fn __setitem__(self_: PyRef<'_, Self>, index: usize, value: Bound<'_, PyAny>) -> PyResult<()> {
+        let norm: PyPreTokenizer = value.extract()?;
+        let PyPreTokenizerTypeWrapper::Single(norm) = norm.pretok else { return Err(PyException::new_err("normalizer should not be a sequence")); };
+        match &self_.as_ref().pretok {
+            PyPreTokenizerTypeWrapper::Sequence(inner) => match inner.get(index) {
+                Some(item) => {
+                    *item.write().unwrap() = norm.read().unwrap().clone();
+                }
+                _ => {
+                    return Err(PyErr::new::<pyo3::exceptions::PyIndexError, _>(
+                        "Index not found",
+                    ))
+                }
+            },
+            PyPreTokenizerTypeWrapper::Single(_) => {
+                return Err(PyException::new_err("normalizer is not a sequence"))
+            }
+        };
+        Ok(())
+    }
 }
 
 pub(crate) fn from_string(string: String) -> Result<PrependScheme, PyErr> {