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json module

Monty’s json provides loads, dumps, and the JSONDecodeError exception. Parsing is backed by jiter; serialization is a hand-written encoder matching CPython byte-for-byte for the supported keyword set.

What’s NOT in the module

json.JSONEncoder and json.JSONDecoder are not implemented, and the cls= keyword is rejected. json.load(fp) and json.dump(obj, fp) are not implemented (no file-object protocol).

json.loads(s, **kwargs)

  • Accepts str or bytes as input.
  • No keyword arguments are accepted. Passing any of cls, object_hook, parse_float, parse_int, parse_constant, or object_pairs_hook raises TypeError: ... unexpected keyword argument.
  • NaN, Infinity, -Infinity are always accepted, with no toggle to reject them.
  • Nesting depth is capped at 200 levels; deeper inputs raise json.JSONDecodeError.
  • JSON integers that would exceed Monty’s BigInt digit limit are rejected with ValueError (matching CPython’s int_max_str_digits behaviour) rather than JSONDecodeError.

json.dumps(obj, **kwargs)

Supported kwargs, with CPython semantics: indent, sort_keys, ensure_ascii, allow_nan, separators, skipkeys.

Rejected with TypeError if passed:

  • cls — custom encoder classes are not supported.
  • default — there is no fallback encoder callback. Non-serializable values raise TypeError instead of routing through one.
  • check_circular — circular reference detection is always on.

JSONDecodeError

Inherits from ValueError (catchable as except ValueError:). The class qualified name is json.JSONDecodeError; __name__ matches CPython. Error messages use the same line N column M (char K) suffix as CPython, counting characters, not bytes.

When the input ends inside an unclosed array or object ('[', '{', '{"a"', …), Monty always reports Expecting ',' delimiter, where CPython distinguishes what was expected at that point (Expecting value, Expecting property name enclosed in double quotes, Expecting ':' delimiter). Positions match; only the message text differs.

Inside the sandbox the exception is message-only: the msg, doc, pos, lineno and colno attributes CPython sets are not available. When a sandbox JSONDecodeError surfaces to the host (e.g. via pydantic_monty), it is rebuilt as a real json.JSONDecodeError with all five attributes from a structured payload attached at raise time, except that documents larger than 64 KiB are not carried, in which case doc is ''. A JSONDecodeError raised manually inside the sandbox has no payload and surfaces as a plain ValueError carrying the message.