Raise ValueError on Response.encoding being set after Response.text has been accessed

httpx/_models.py

@@ -603,6 +603,16 @@ def encoding(self) -> typing.Optional[str]:
 
     @encoding.setter
     def encoding(self, value: str) -> None:
+        """
+        Set the encoding to use for decoding the byte content into text.
+
+        If the `text` attribute has been accessed, attempting to set the
+        encoding will throw a ValueError.
+        """
+        if hasattr(self, "_text"):
+            raise ValueError(
+                "Setting encoding after `text` has been accessed is not allowed."
+            )
         self._encoding = value
 
     @property

tests/models/test_responses.py

@@ -298,6 +298,23 @@ def test_response_force_encoding():
     assert response.encoding == "iso-8859-1"
 
 
+def test_response_force_encoding_after_text_accessed():
+    response = httpx.Response(
+        200,
+        content=b"Hello, world!",
+    )
+    assert response.status_code == 200
+    assert response.reason_phrase == "OK"
+    assert response.text == "Hello, world!"
+    assert response.encoding == "utf-8"
+
+    with pytest.raises(ValueError):
+        response.encoding = "UTF8"
+
+    with pytest.raises(ValueError):
+        response.encoding = "iso-8859-1"
+
+
 def test_read():
     response = httpx.Response(
         200,