[Bugfix] Fix mypy errors for StructuredOutputsParams by using stdlib dataclass

vllm/sampling_params.py

@@ -4,13 +4,12 @@
 
 import copy
 import json
-from dataclasses import field
+from dataclasses import dataclass
 from enum import Enum, IntEnum
 from functools import cached_property
 from typing import Annotated, Any
 
 import msgspec
-from pydantic.dataclasses import dataclass
 
 from vllm.config import ModelConfig, SpeculativeConfig, StructuredOutputsConfig
 from vllm.exceptions import VLLMValidationError
@@ -46,13 +45,12 @@ class StructuredOutputsParams:
     whitespace_pattern: str | None = None
     structural_tag: str | None = None
 
-    _backend: str | None = field(default=None, init=False)
-    """CAUTION: Should only be set by Processor._validate_structured_output"""
-    _backend_was_auto: bool = field(default=False, init=False)
-    """CAUTION: Should only be set by Processor._validate_structured_output"""
-
     def __post_init__(self):
         """Validate that some fields are mutually exclusive."""
+        # CAUTION: Should only be set by Processor._validate_structured_output
+        self._backend: str | None = None
+        self._backend_was_auto: bool = False
+
         count = sum(
             [
                 self.json is not None,
@@ -74,6 +72,17 @@ def __post_init__(self):
                 f"but none are specified: {self.__dict__}"
             )
 
+    def __replace__(self, **changes):
+        """Custom replace that preserves instance attributes not in dataclass fields."""
+        from dataclasses import replace
+
+        new_obj = replace(self, **changes)
+        # Preserve _backend and _backend_was_auto which are set in __post_init__
+        # but not part of the dataclass fields, so replace() doesn't copy them
+        new_obj._backend = self._backend
+        new_obj._backend_was_auto = self._backend_was_auto
+        return new_obj
+
     def all_constraints_none(self) -> bool:
         """
         Returns True if all structured-output constraint fields are None.