simplejson Files

What's Changed

Version 4.0.0 released 2026-04-18

• simplejson 4 requires Python 2.7 or Python 3.8+. Older Python versions (2.5, 2.6, 3.0-3.7) are no longer supported. pip will not install simplejson 4 on unsupported versions.

• The C extension now uses heap types and per-module state instead of static types and global state. This is required for free-threading support and sub-interpreter isolation. The Python-level API is unchanged.

• Full support for Python 3.13+ free-threading (PEP 703). The C extension is now safe to use with the GIL disabled (python3.14t):

• Converted all static types to heap types with per-module state
• Added per-object critical sections to scanner and encoder
• Added free-threading-safe dict operations for Python 3.13+

• Unified per-module state management and templated parser https://github.com/simplejson/simplejson/pull/363 https://github.com/simplejson/simplejson/pull/364 https://github.com/simplejson/simplejson/pull/365 https://github.com/simplejson/simplejson/pull/367 https://github.com/simplejson/simplejson/pull/369

• Numerous C extension memory safety fixes:

• Fix use-after-free and leak in encoder ident handling
• Fix NULL dereferences on OOM in module init and static string init
• Fix reference leaks in dict encoder (skipkeys item, variable shadowing)
• Fix member table copy-paste, exception clobbering, missing Py_VISIT
• Fix error-as-truthy bugs in maybe_quote_bigint and is_raw_json
• Fix iterable_as_array swallowing MemoryError and KeyboardInterrupt

• Fix for_json and _asdict swallowing MemoryError, KeyboardInterrupt, and other non-AttributeError exceptions raised by user getattr https://github.com/simplejson/simplejson/pull/355 https://github.com/simplejson/simplejson/pull/356 https://github.com/simplejson/simplejson/pull/357 https://github.com/simplejson/simplejson/pull/358 https://github.com/simplejson/simplejson/pull/359 https://github.com/simplejson/simplejson/pull/360 https://github.com/simplejson/simplejson/pull/373

• C/Python parity fixes:

• Fix C scanstring off-by-one bounds checks that caused truncated or boundary \uXXXX escapes to raise "Invalid \uXXXX escape sequence" instead of "Unterminated string", and report error position at the 'u' instead of the leading backslash. The C and Python decoders now agree on exception class, message, and position across all tested edge cases.
• Align the Python encoder's dispatch order with the C encoder for objects that define _asdict(). Previously a list/tuple/dict subclass with an _asdict() method encoded as its container type under the Python encoder and as the _asdict() return value under the C encoder; both now check _asdict() before list/tuple/dict. for_json() continues to outrank _asdict() in both.

• Fix C scanstring raising a plain ValueError ("end is out of bounds") instead of JSONDecodeError for out-of-range end indices. User code with except JSONDecodeError: now catches both the C and pure-Python paths consistently. https://github.com/simplejson/simplejson/pull/372

• C extension performance and correctness improvements:

• Add PyDict_Next fast path for unsorted exact-dict encoding, avoiding intermediate items list and N tuple allocations
• Add indexed fast path for exact list/tuple encoding, avoiding iterator allocation and per-item PyIter_Next overhead
• Use PyUnicodeWriter as JSON_Accu backend on Python 3.14+, eliminating intermediate string objects and ''.join calls
• Fix integer overflow in ascii_escape output_size calculation that could cause buffer overwrite on pathologically large strings
• Fix list encoder separator counter overflow (int to Py_ssize_t)

• Dead code cleanup (unreachable NULL checks, do-while wrappers) https://github.com/simplejson/simplejson/pull/370

• Added Python 3.14 support and updated to cibuildwheel 3.2.1. CI now tests free-threaded (3.14t) and debug builds with -Werror, refcount leak detection, and GIL-disabled mode. https://github.com/simplejson/simplejson/pull/343

• Added a ThreadSanitizer (TSan) stress test CI job. Builds a TSan-instrumented free-threaded CPython (cached between runs) and runs a concurrent stress test script against the C extension to catch data races under free-threading. https://github.com/simplejson/simplejson/pull/373

• Replace deprecated license classifiers with SPDX license expression https://github.com/simplejson/simplejson/pull/347

• Documented RawJSON usage with examples and caveats https://github.com/simplejson/simplejson/pull/346

• Added pyproject.toml for PEP 517 build support. setup.py is retained for Python 2.7 wheel builds and backwards compatibility.

• Migrated build_ext import from distutils to setuptools in setup.py. The distutils.errors imports are kept since setuptools vendors distutils on Python 3.12+ where stdlib distutils was removed.

• CI now tests PEP 517 builds (pyproject.toml) alongside the existing setup.py-based builds.

• Added Pyodide (wasm32) wheel builds with C speedups via cibuildwheel. Previously Pyodide users fell back to the pure-Python wheel; now they get the compiled C extension cross-compiled to WebAssembly. Thread and subprocess tests are skipped on Emscripten where those APIs are unavailable.

• Test suite now fails (instead of skipping) when C speedups are missing during cibuildwheel runs, catching broken extension builds early.

• New array_hook parameter for loads(), load(), and JSONDecoder. Called with each decoded JSON array (as a list), its return value replaces the list. Analogous to object_hook for dicts. Works with both the Python decoder and C scanner. (Matches CPython 3.15 json module.)

• Trailing comma detection: the decoder now raises JSONDecodeError with "Illegal trailing comma before end of object/array" for inputs like [1,] and {"a": 1,} instead of generic error messages. Both the Python decoder and C scanner are updated. (Matches CPython 3.13+ json module.)

• frozendict encoding support: when frozendict is available (CPython 3.15+ PEP 814), it is encoded as a JSON object just like dict. No effect on older Python versions.

• Serialization errors now include add_note() context on Python 3.11+ (PEP 678), annotating exceptions with the path to the error, e.g. "when serializing list item 1" / "when serializing dict item 'key'". Only applies to the Python encoder.

• New C fast path for encode_basestring (ensure_ascii=False). Previously the non-ASCII string encoder fell back to pure Python; now it has a C implementation matching the existing encode_basestring_ascii fast path. https://github.com/simplejson/simplejson/issues/207

• The Python decoder now rejects non-ASCII digits (e.g. fullwidth \uff10) in JSON numbers, matching the C scanner behavior. The NUMBER_RE regex was changed from \d to [0-9].

• Removed dead single-phase init code for Python 3.3/3.4 from the C extension (these versions are no longer supported).

New Contributors

• @Shreecharana24 made their first contribution in https://github.com/simplejson/simplejson/pull/346
• @veeceey made their first contribution in https://github.com/simplejson/simplejson/pull/347
• @devdanzin made their first contribution in https://github.com/simplejson/simplejson/pull/355
• @CarsonZuniga made their first contribution in https://github.com/simplejson/simplejson/pull/343

Full Changelog: https://github.com/simplejson/simplejson/compare/v3.20.2...v4.0.0