Repository: huggingface/transformers
Pull request: Support `typing.Literal` as type of tool parameters or return value

# What does this PR do?

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This PR adds support for using `typing.Literal` as a type hint of tool function written in python.

I realized that using `typing.Literal` for argument's type hint is currently not supported during my tries to use tools as the python function(referring [Document 'Tools and RAG'](https://huggingface.co/docs/transformers/v4.51.3/en/chat_extras)).

The error I got is:
```
in _parse_type_hint
    raise TypeHintParsingException("Couldn't parse this type hint, likely due to a custom class or object: ", hint)
transformers.utils.chat_template_utils.TypeHintParsingException: ("Couldn't parse this type hint, likely due to a custom class or object: ", typing.Literal[...])
```

Fortunately, there is already a feature to deal with the 'complex' type hints like `list[str]`, `Union[List[int], Dict[int, str]]`.
So I just added case for `origin is Literal`, just putting its `args` as value of the key `"enum"` in json schema is fine.

Putting `"type"` in that was not a good idea, because we could accidentally filter a valid case when the types are heterogeneous in `args`. But a rough type validation could be valuable for the case using no static typing tool.

Additionally, as I see, there was a glitch in the related function `_get_json_schema_type(param_type: str)`, which is that the type hint of argument `param_type` need to be `type` instead of `str`. This tiny change is also in this PR.

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@ArthurZucker @Rocketknight1

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Changed files:
filename: src/transformers/utils/chat_template_utils.py
patch: @@ -20,7 +20,16 @@
 from datetime import datetime
 from functools import lru_cache
 from inspect import isfunction
-from typing import Any, Callable, Optional, Union, get_args, get_origin, get_type_hints
+from typing import (
+    Any,
+    Callable,
+    Literal,
+    Optional,
+    Union,
+    get_args,
+    get_origin,
+    get_type_hints,
+)
 
 from packaging import version
 
@@ -75,7 +84,7 @@ class DocstringParsingException(Exception):
     pass
 
 
-def _get_json_schema_type(param_type: str) -> dict[str, str]:
+def _get_json_schema_type(param_type: type) -> dict[str, str]:
     type_mapping = {
         int: {"type": "integer"},
         float: {"type": "number"},
@@ -119,6 +128,20 @@ def _parse_type_hint(hint: str) -> dict:
             return_dict["nullable"] = True
         return return_dict
 
+    elif origin is Literal and len(args) > 0:
+        LITERAL_TYPES = (int, float, str, bool, type(None))
+        args_types = []
+        for arg in args:
+            if type(arg) not in LITERAL_TYPES:
+                raise TypeHintParsingException("Only the valid python literals can be listed in typing.Literal.")
+            arg_type = _get_json_schema_type(type(arg)).get("type")
+            if arg_type is not None and arg_type not in args_types:
+                args_types.append(arg_type)
+        return {
+            "type": args_types.pop() if len(args_types) == 1 else list(args_types),
+            "enum": list(args),
+        }
+
     elif origin is list:
         if not args:
             return {"type": "array"}

filename: tests/utils/test_chat_template_utils.py
patch: @@ -13,7 +13,7 @@
 # limitations under the License.
 
 import unittest
-from typing import Optional, Union
+from typing import Literal, Optional, Union
 
 from transformers.utils import DocstringParsingException, TypeHintParsingException, get_json_schema
 
@@ -384,6 +384,49 @@ def fn(temperature_format: str):
 
         self.assertEqual(schema["function"], expected_schema)
 
+    def test_literal(self):
+        def fn(
+            temperature_format: Literal["celsius", "fahrenheit"],
+            booleanish: Literal[True, False, 0, 1, "y", "n"] = False,
+        ):
+            """
+            Test function
+
+            Args:
+                temperature_format: The temperature format to use
+                booleanish: A value that can be regarded as boolean
+
+
+            Returns:
+                The temperature
+            """
+            return -40.0
+
+        # Let's see if that gets correctly parsed as an enum
+        schema = get_json_schema(fn)
+        expected_schema = {
+            "name": "fn",
+            "description": "Test function",
+            "parameters": {
+                "type": "object",
+                "properties": {
+                    "temperature_format": {
+                        "type": "string",
+                        "enum": ["celsius", "fahrenheit"],
+                        "description": "The temperature format to use",
+                    },
+                    "booleanish": {
+                        "type": ["boolean", "integer", "string"],
+                        "enum": [True, False, 0, 1, "y", "n"],
+                        "description": "A value that can be regarded as boolean",
+                    },
+                },
+                "required": ["temperature_format"],
+            },
+        }
+
+        self.assertEqual(schema["function"], expected_schema)
+
     def test_multiline_docstring_with_types(self):
         def fn(x: int, y: int):
             """