FIX: Reinstate PEFT_TYPE_TO_MODEL_MAPPING variable with deprecation

src/peft/peft_model.py

@@ -3022,3 +3022,19 @@ def check_irrgular(vals: list[bool | Literal["irregular"]]) -> bool | Literal["i
         devices=devices,
     )
     return adapter_model_status
+
+
+def __getattr__(name):
+    if name == "PEFT_TYPE_TO_MODEL_MAPPING":
+        # This is for backwards compatibility: In #2282, PEFT_TYPE_TO_MODEL_MAPPING was removed as it was redundant with
+        # PEFT_TYPE_TO_TUNER_MAPPING. However, third party code could still use this mapping, e.g.:
+        # https://github.com/AutoGPTQ/AutoGPTQ/blob/6689349625de973b9ee3016c28c11f32acf7f02c/auto_gptq/utils/peft_utils.py#L8
+        # TODO: Remove after 2026-01
+        msg = (
+            "PEFT_TYPE_TO_MODEL_MAPPING is deprecated, please use `from peft import PEFT_TYPE_TO_TUNER_MAPPING` instead. "
+            "The deprecated variable will be removed in 2026."
+        )
+        warnings.warn(msg, category=DeprecationWarning)
+        return PEFT_TYPE_TO_TUNER_MAPPING
+
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")

tests/test_initialization.py

@@ -2961,3 +2961,25 @@ def test_hotswap_extra_key_raises(self, tmp_path):
         msg = f"Hot swapping the adapter did not succeed. Unexpected keys: {new_key}"
         with pytest.raises(RuntimeError, match=msg):
             hotswap_adapter(model, tmp_path / "adapter1", adapter_name="default")
+
+
+def test_import_peft_type_to_model_mapping_deprecation_warning(recwarn):
+    # This is for backwards compatibility: In #2282, PEFT_TYPE_TO_MODEL_MAPPING was removed as it was redundant with
+    # PEFT_TYPE_TO_TUNER_MAPPING. However, third party code could still use this mapping, e.g.:
+    # https://github.com/AutoGPTQ/AutoGPTQ/blob/6689349625de973b9ee3016c28c11f32acf7f02c/auto_gptq/utils/peft_utils.py#L8
+    # TODO: Remove after 2026-01
+
+    # first check that there is no warning under normal circumstances
+    from peft.peft_model import PeftModel  # noqa
+
+    expected = (
+        "PEFT_TYPE_TO_MODEL_MAPPING is deprecated, please use `from peft import PEFT_TYPE_TO_TUNER_MAPPING` instead"
+    )
+    warnings = (w.message.args[0] for w in recwarn.list)
+    assert not any(w.startswith(expected) for w in warnings)
+
+    from peft.peft_model import PEFT_TYPE_TO_MODEL_MAPPING  # noqa
+
+    # check that there is a warning with this message after importing the variable
+    warnings = (w.message.args[0] for w in recwarn.list)
+    assert any(w.startswith(expected) for w in warnings)