Adding default max json array length, preventing LLM infinite loops i…

…n some cases

README.md

@@ -197,6 +197,7 @@ There are several environment variables that can be set, that affect the operati
 
 - `LMFE_MAX_CONSECUTIVE_WHITESPACES` - How many consecutive whitespaces are allowed when parsing JsonSchemaObjects. Default: 12.
 - `LMFE_STRICT_JSON_FIELD_ORDER` - Should the JsonSchemaParser force the properties to appear in the same order as they appear in the 'required' list of the JsonSchema? (Note: this is consistent with the order of declaration in Pydantic models). Default: False.
+- `LMFE_MAX_JSON_ARRAY_LENGTH` - What is the maximal JSON array length, if not specified by the schema. Helps LLM Avoid infinite loops. Default: 20.
 
 ### Option 2: via the CharacterLevelParserConfig class
 When using the library through code, any `CharacterLevelParser` (`JsonSchemaParser`, `RegexParser` etc) constructor receives an optional `CharacterLevelParserConfig` object. 

lmformatenforcer/characterlevelparser.py

@@ -4,7 +4,8 @@
 from typing import Hashable, List, Optional, TypeVar
 from .consts import (COMPLETE_ALPHABET, WHITESPACE_CHARACTERS, DEFAULT_MAX_CONSECUTIVE_WHITESPACES, 
                      DEFAULT_FORCE_JSON_FIELD_ORDER, CONFIG_ENV_VAR_MAX_CONSECUTIVE_WHITESPACES, 
-                     CONFIG_ENV_VAR_STRICT_JSON_FIELD_ORDER)
+                     CONFIG_ENV_VAR_STRICT_JSON_FIELD_ORDER, CONFIG_ENV_VAR_MAX_JSON_ARRAY_LENGTH,
+                     DEFAULT_MAX_JSON_ARRAY_LENGTH)
 
 
 def _parse_bool(s: str) -> bool:
@@ -29,6 +30,10 @@ class CharacterLevelParserConfig:
                                                          DEFAULT_FORCE_JSON_FIELD_ORDER)
     """Whether the JsonSchemaParser will force fields to appear in the 
     order of the 'required' field in the schema"""
+    max_json_array_length: int = _env_or_default_field(CONFIG_ENV_VAR_MAX_JSON_ARRAY_LENGTH,
+                                                       DEFAULT_MAX_JSON_ARRAY_LENGTH)
+    """What is the maximum json array length if not specified by the schema. Helps the LLM
+    avoid infinite loops."""
 
 
 class CharacterLevelParser(abc.ABC):

lmformatenforcer/consts.py

@@ -1,6 +1,7 @@
 COMPLETE_ALPHABET = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!@#$%^&*()_+-=[]{};:,./<>? `'\""
 DEFAULT_MAX_CONSECUTIVE_WHITESPACES = 12
 DEFAULT_FORCE_JSON_FIELD_ORDER = False
+DEFAULT_MAX_JSON_ARRAY_LENGTH = 20
 WHITESPACE_CHARACTERS = " \t\n\r"
 BACKSLASH = "\\"
 BACKSLASH_ESCAPING_CHARACTERS = '"\\/bfnrt'  # Characters allowed after an escaping backslash, except unicode
@@ -12,4 +13,8 @@
 
 CONFIG_ENV_VAR_STRICT_JSON_FIELD_ORDER = 'LMFE_STRICT_JSON_FIELD_ORDER'
 """Environment variable for externally controlling whether the JsonSchemaParser will force 
-fields to appear in the order of the 'required' field in the schema. Default: false"""
+fields to appear in the order of the 'required' field in the schema. Default: false"""
+
+CONFIG_ENV_VAR_MAX_JSON_ARRAY_LENGTH = 'LMFE_MAX_JSON_ARRAY_LENGTH'
+"""Environment variable for externally controlling what is the maximal JSON array length,
+if not specified by the schema. Default: 20"""

lmformatenforcer/jsonschemaparser.py

@@ -611,6 +611,9 @@ def __init__(
         self.list_member_type = list_member_type
         self.min_items = min_items
         self.max_items = max_items
+        default_max = root.config.max_json_array_length
+        if self.max_items is None and default_max > 0 and (min_items is None or min_items < default_max):
+            self.max_items = default_max
 
     def _clone(self) -> PrimitiveParsingState:
         new = ListParsingState(self.root, self.list_member_type, self.min_items, self.max_items)

tests/test_jsonschemaparser.py

@@ -6,7 +6,7 @@
 from lmformatenforcer import JsonSchemaParser
 from enum import Enum
 import pytest
-from lmformatenforcer.consts import BACKSLASH, BACKSLASH_ESCAPING_CHARACTERS, CONFIG_ENV_VAR_STRICT_JSON_FIELD_ORDER, CONFIG_ENV_VAR_MAX_CONSECUTIVE_WHITESPACES
+from lmformatenforcer.consts import BACKSLASH, BACKSLASH_ESCAPING_CHARACTERS, CONFIG_ENV_VAR_STRICT_JSON_FIELD_ORDER, CONFIG_ENV_VAR_MAX_CONSECUTIVE_WHITESPACES, CONFIG_ENV_VAR_MAX_JSON_ARRAY_LENGTH
 
 from .common import assert_parser_with_string, CharacterNotAllowedException
 
@@ -634,3 +634,42 @@ def test_max_whitespaces_via_env_var():
         for num_spaces in range(12):
             expect_success = num_spaces <= 8
             _test_json_schema_parsing_with_string(base_answer.replace("$", " " * num_spaces), schema, expect_success)
+
+
+def test_max_json_array_length_via_env_var():
+    env_var_name = CONFIG_ENV_VAR_MAX_JSON_ARRAY_LENGTH
+
+    class IntListModel(BaseModel):
+        nums: List[int]
+
+    schema = IntListModel.model_json_schema()
+    with _temp_replace_env_var(env_var_name, '8'):
+        for num_numbers in range(12):
+            instance = IntListModel(nums=list(range(num_numbers)))
+            instance_str = instance.model_dump_json()
+            expect_success = num_numbers <= 8
+            _test_json_schema_parsing_with_string(instance_str, schema, expect_success)
+
+
+def test_top_level_object_inheritance():
+    schema = {
+        "$defs": {
+            "ParentObject": {
+                "properties": {
+                    "child": {
+                        "type": "string"
+                    }
+            },
+            "type": "object"
+            }
+        },
+        "properties": {
+            "parent": {
+                "$ref": "#/$defs/ParentObject"
+            }
+        },
+        "type": "object"
+    }
+    valid_object = '{"parent": {"child": "test"}}'
+    _test_json_schema_parsing_with_string(valid_object, schema, True)
+