core[patch]: fix validation of @deprecated decorator (#22513)

This PR moves the validation of the decorator to a better place to avoid
creating bugs while deprecating code.

Prevent issues like this from arising:
https://github.com/langchain-ai/langchain/issues/22510

we should replace with a linter at some point that just does static
analysis

libs/community/langchain_community/chat_models/huggingface.py

@@ -39,7 +39,7 @@ DEFAULT_SYSTEM_PROMPT = """You are a helpful, respectful, and honest assistant."
 @deprecated(
     since="0.0.37",
     removal="0.3",
-    alternative_import=("from langchain_huggingface import ChatHuggingFace"),
+    alternative_import="langchain_huggingface.ChatHuggingFace",
 )
 class ChatHuggingFace(BaseChatModel):
     """

libs/core/langchain_core/_api/deprecation.py

@@ -33,6 +33,25 @@ class LangChainPendingDeprecationWarning(PendingDeprecationWarning):
 T = TypeVar("T", bound=Union[Type, Callable[..., Any]])
 
 
+def _validate_deprecation_params(
+    pending: bool,
+    removal: str,
+    alternative: str,
+    alternative_import: str,
+) -> None:
+    """Validate the deprecation parameters."""
+    if pending and removal:
+        raise ValueError("A pending deprecation cannot have a scheduled removal")
+    if alternative and alternative_import:
+        raise ValueError("Cannot specify both alternative and alternative_import")
+
+    if alternative_import and "." not in alternative_import:
+        raise ValueError(
+            "alternative_import must be a fully qualified module path. Got "
+            f" {alternative_import}"
+        )
+
+
 def deprecated(
     since: str,
     *,
@@ -99,6 +118,7 @@ def deprecated(
             def the_function_to_deprecate():
                 pass
     """
+    _validate_deprecation_params(pending, removal, alternative, alternative_import)
 
     def deprecate(
         obj: T,
@@ -337,13 +357,6 @@ def warn_deprecated(
             since. Set to other Falsy values to not schedule a removal
             date. Cannot be used together with pending.
     """
-    if pending and removal:
-        raise ValueError("A pending deprecation cannot have a scheduled removal")
-    if alternative and alternative_import:
-        raise ValueError("Cannot specify both alternative and alternative_import")
-    if alternative_import and "." not in alternative_import:
-        raise ValueError("alternative_import must be a fully qualified module path")
-
     if not pending:
         if not removal:
             removal = f"in {removal}" if removal else "within ?? minor releases"

libs/core/tests/unit_tests/_api/test_deprecation.py

@@ -401,3 +401,14 @@ def test_deprecated_method_pydantic() -> None:
         doc = obj.deprecated_method.__doc__
         assert isinstance(doc, str)
         assert doc.startswith("[*Deprecated*] original doc")
+
+
+def test_raise_error_for_bad_decorator() -> None:
+    """Verify that errors raised on init rather than on use."""
+    # Should not specify both `alternative` and `alternative_import`
+    with pytest.raises(ValueError):
+
+        @deprecated(since="2.0.0", alternative="NewClass", alternative_import="hello")
+        def deprecated_function() -> str:
+            """original doc"""
+            return "This is a deprecated function."