fastsimdjson

A Python binding for simdjson that parses JSON into native Python objects (dict, list, str, int, float, bool, None) and serializes them back. It is a drop-in replacement for the json module's loads, load, dumps and dump, and it can be over 3 times faster than the standard json.loads and json.dumps. When you only need part of a document, its lazy parse function is faster still. It also reads streams of documents (NDJSON, JSON Lines), on many threads with free-threaded Python, and can extract only the fields you need from each document. It writes JSON in two ways: dumps returns the same str as json.dumps, and dumpb returns the same bytes as orjson.dumps, as fast as orjson; dumpb_many writes NDJSON, on many threads with free-threaded Python.

With pip:

pip install fastsimdjson

With uv:

uv pip install fastsimdjson      # in a uv project: uv add fastsimdjson

Wheels are available for Linux, macOS and Windows, for Python 3.10 to 3.14, including free-threaded Python 3.14.

Usage

loads: the whole document

import fastsimdjson
fastsimdjson.loads(b'{"a": [1, 2.5, "x", true, null]}')
# {'a': [1, 2.5, 'x', True, None]}

loads(data) accepts bytes, bytearray, memoryview and str, and returns the same value as json.loads, with the same types and key order. Integers that do not fit in 64 bits become exact Python ints. NaN, Infinity and -Infinity are accepted, in any capitalization.

Invalid input raises fastsimdjson.JSONDecodeError, a subclass of json.JSONDecodeError. When simdjson rejects a document, the same input is parsed with json.loads. If that succeeds, loads returns its value (this is how a number that overflows a double becomes inf). If it raises JSONDecodeError, the exception is re-raised with Python's message and byte position. Any other exception from json.loads propagates.

parse: lazy views

When you need only part of a document, parse avoids building the rest. It accepts the same inputs as loads and returns read-only views: fastsimdjson.Object (a Mapping) and fastsimdjson.Array (a Sequence). Values are converted when you access them; nested objects and arrays are returned as views. A scalar root is returned as a plain value.

doc = fastsimdjson.parse(open("twitter.json", "rb").read())
ids = [(s["id"], s["user"]["screen_name"]) for s in doc["statuses"]]
doc.at_pointer("/statuses/0/user/name")   # JSON Pointer (RFC 6901)
doc["search_metadata"].as_dict()          # convert a subtree, like loads

Object supports obj[key], get, in, len, iteration over the keys, keys(), values(), items() (iterators), at_pointer and as_dict(). Array supports arr[i] (negative indexes and slices), len, iteration, at_pointer and as_list(). Both work with match statements.

• A view keeps its document alive; the document owns its own buffers, so it remains valid while other documents are parsed.
• A key lookup scans the object. With duplicate keys, lookups return the first value, whereas as_dict() (like json.loads) keeps the last.
• Indexing an array walks it from the last index reached, so a loop over arr[i] is linear; iteration is the fastest way to visit an array.
• Errors are handled as in loads. A document that simdjson rejects but json.loads accepts (an overflowing number, an unpaired surrogate) is returned as plain Python objects, as loads would return it.

dumps and dump: like json.dumps

fastsimdjson.dumps({"a": [1, 2.5, None]})            # '{"a": [1, 2.5, null]}'
fastsimdjson.dumps(obj, indent=2, sort_keys=True)
with open("out.json", "w", encoding="utf-8") as f:
    fastsimdjson.dump(obj, f)

dumps(obj, **kw) takes the arguments of json.dumps and returns the same str, character for character, including the float format (repr), the escapes and the default separators. ensure_ascii, indent, separators, sort_keys, allow_nan and default are handled in C. Everything else is passed to json.dumps itself, which produces the result or raises its usual exception: a cls argument or other encoder options, skipkeys, a circular reference, NaN with allow_nan=False, a key or a value that json cannot serialize. dump(obj, fp, **kw) writes dumps(obj, **kw) to fp.

dumpb: like orjson.dumps

fastsimdjson.dumpb({"a": [1, 2.5, None]})            # b'{"a":[1,2.5,null]}'
fastsimdjson.dumpb(obj, option=fastsimdjson.OPT_INDENT_2 | fastsimdjson.OPT_SORT_KEYS)
fastsimdjson.dumpb({1, 2}, default=sorted)           # b'[1,2]'

dumpb(obj, default=None, option=None) has the arguments, the output and the errors of orjson.dumps (orjson 3.12): orjson.dumps(obj, ...) can be replaced by fastsimdjson.dumpb(obj, ...) for the types below. It returns compact UTF-8 bytes, writes floats as orjson does (1e-6, 1e+16, NaN and infinities as null), and raises TypeError with orjson's messages: integers beyond 64 bits, a dict key that is not a str, invalid UTF-8 (a lone surrogate), nesting deeper than 254, a type it cannot serialize. orjson.JSONEncodeError is a subclass of TypeError, so except TypeError catches the errors of both.

It serializes str, int, float, bool, None, dict, list, tuple, enums, and subclasses of str, int, dict and list. Anything else goes to default, as in orjson: its result is serialized in place of the object, and an exception it raises becomes the __cause__ of the TypeError. The options are exported under orjson's names and values (OPT_APPEND_NEWLINE, OPT_INDENT_2, OPT_NON_STR_KEYS, OPT_PASSTHROUGH_SUBCLASS, OPT_SORT_KEYS, OPT_STRICT_INTEGER, ...). Unlike orjson, dumpb does not serialize dataclasses, datetime, date, time, UUID, numpy arrays or orjson.Fragment itself: they go to default, as if OPT_PASSTHROUGH_DATACLASS and OPT_PASSTHROUGH_DATETIME were set, and the options that concern only these types are accepted and have no effect.

dumpb_many: NDJSON, on several threads

data = fastsimdjson.dumpb_many(records)               # b'{...}\n{...}\n'
data = fastsimdjson.dumpb_many(records, threads=16)   # free-threaded Python

dumpb_many(records, default=None, option=None, *, threads=None) returns the records (a list, a tuple or any iterable) as NDJSON bytes: each record as dumpb writes it, followed by a newline, so that the output reads back with loads_many(data, format="lines"). default and option are those of dumpb, except OPT_INDENT_2, which would spread a record over several lines (ValueError); OPT_APPEND_NEWLINE changes nothing.

On free-threaded Python, threads=N writes chunks of records on N threads at once (threads=0 or None: one per processor), with the worker threads of map_many. The output and the errors are those of one thread: when records cannot be serialized, the error of the first of them is raised (default may then have been called on later records, in other threads, and it must be safe to call from several threads at once). With the GIL, the calling thread does all the work. Like dumpb, dumpb_many does not crash when other threads change the records meanwhile, but the output then reflects some of the changes and not others.

Files

with open("data.json", "rb") as f:
    doc = fastsimdjson.load(f)          # like json.load: f.read(), then loads
doc = fastsimdjson.load_file("data.json")
view = fastsimdjson.parse_file("data.json")   # lazy, like parse

load_file(path) and parse_file(path) accept a str, bytes or os.PathLike path and raise OSError (e.g. FileNotFoundError) when the file cannot be read.

loads_many and parse_many: streams of documents

for record in fastsimdjson.loads_many(open("log.ndjson", "rb").read()):
    ...
for view in fastsimdjson.parse_many(data):    # lazy views, like parse
    ...

Both return an iterator over the documents of data (bytes, bytearray, memoryview or str). The format keyword selects how documents are separated:

format	input
"whitespace" (default)	documents separated by white space, including NDJSON and JSON Lines
"lines"	one document per line (NDJSON, JSON Lines)
"json_seq"	RFC 7464 JSON text sequences (each document preceded by \x1e)
"comma"	documents separated by commas: {...}, {...} (simdjson also accepts white space between them)
"array"	the elements of one array: [{...}, {...}]

simdjson parses the input in batches (batch_size, 1 MB by default); a larger document is handled automatically. In every format, documents that simdjson rejects are handled as in loads: a document that json accepts is returned, otherwise JSONDecodeError reports json's message and the position in the whole input. A truncated last document is an error. The views returned by parse_many remain valid after the iterator moves on.

Streams on several threads: threads=N and map_many

On free-threaded Python (3.14t and newer), a stream can be parsed by several threads at once:

docs = fastsimdjson.loads_many(data, threads=16)        # a list, in order
views = fastsimdjson.parse_many(data, threads=16)       # a list of views
# fn runs in the threads too, on each document; its results, in order:
scores = fastsimdjson.map_many(score, data, threads=16)

With threads, loads_many and parse_many return a list of all the documents instead of an iterator. map_many(fn, data, **kw) returns [fn(doc) for doc in loads_many(data, **kw)] (parse_many with lazy=True), with fn called in the threads. threads=0 (or None) uses one thread per processor; small inputs use fewer threads (about one per 512 KB). The input is cut into chunks at newlines (before each \x1e for json_seq), so threads works with the formats "whitespace", "lines" and "json_seq".

The results, and the exceptions, are those of one thread: when a chunk contains an error or a document that only json accepts, the documents before it are kept and the rest is parsed in the calling thread, as without threads. An exception raised by fn is raised again, for the first document where it happened; fn may then have been called on later documents, in other threads, and it must be safe to call from several threads at once.

The worker threads are started by the first call that needs them and kept for later calls (they wait, detached from the interpreter, between calls, and stop at exit). map_many scales best: each document is built, used and freed in the same thread, and only fn's results come back. loads_many must return every document, and the Python objects of all of them, to the calling thread. With the GIL (the default build of Python), the calling thread does all the work: threads has no effect on speed.

Only some fields: fields=

When you need a few values from each document, fields extracts them without building the documents: only the values found become Python objects.

events = fastsimdjson.loads_many(data, fields=["/type", "/payload/action"])
# (type, action) for each document

def count(t):
    kind, action, language = t
    ...
fastsimdjson.map_many(count, data, threads=64,
                      fields=["/type", "/payload/action",
                              "/payload/pull_request/base/repo/language"])

loads_many and map_many accept fields: a JSON Pointer (RFC 6901), or a list or tuple of them. Each document then becomes the value at the pointer, or a tuple of the values at the pointers, in order. A value that is an object or an array is built in full. A pointer that leads nowhere (a missing key, an index past the end, or a step into a scalar) gives default, None unless you pass another value; a JSON null gives None. As with at_pointer, a key that appears twice in an object refers to its first occurrence. fields cannot be combined with lazy=True.

On 7.6 GB of GitHub events, map_many with three fields ran at 2.8 GB/s on one core of an Intel Xeon Gold 6548N, against 2.2 GB/s with lazy views (lazy=True). With 64 threads, it ran at 78 GB/s (lazy views: 61 GB/s).

release

release() frees the simdjson parser and the string caches kept by the calling thread. Views returned by parse remain valid.

Build and test

Python 3.10 or newer, and a C++17 compiler (clang, GCC or MSVC). The simdjson 5.0.7 and simdutf 9.2.1 amalgamations, and zmij 1.2 (shortest float formatting, MIT license), are already in vendor/.

pip:

python3 -m venv .venv
. .venv/bin/activate
python -m pip install -U pip setuptools
python -m pip install -e ".[test]"
pytest tests

uv:

uv venv
. .venv/bin/activate
uv pip install -e ".[test]"
pytest tests

Either one builds the extension and makes import fastsimdjson work in the virtualenv. uv uses the setuptools build requirement from pyproject.toml, so it does not need a separate setuptools install for this path.

To compile the extension in the tree instead, install setuptools and pytest into the same virtualenv, then:

python -m pip install setuptools pytest
python setup.py build_ext --inplace
PYTHONPATH=src python -m pytest tests

uv pip install setuptools pytest
python setup.py build_ext --inplace
PYTHONPATH=src python -m pytest tests

Recent setuptools copies the .so next to src/fastsimdjson.cpp, which is why PYTHONPATH=src is required for the in-place build.

tests/test_loads.py compares loads with json.loads on types and key order; tests/test_lazy.py checks the views returned by parse the same way, tests/test_dumps.py compares dumps with json.dumps and dumpb with orjson (output and exceptions; the orjson comparisons are skipped when orjson is not installed, and recorded orjson results are checked either way), and tests/test_stream.py and tests/test_files.py cover streams and files. It covers scalars, integers past 64 bits, UTF-8 strings at every length from 0 to 199, the key cache, random documents, rejected input, deep nesting, padding at a page boundary, a saturated array count, reference counts, and release of a parser that has grown past 64 MB. The corpus test is skipped until simdjson-data is checked out beside the project:

git clone --depth 1 https://github.com/simdjson/simdjson-data.git
pytest tests
# or: JSONDIR=/path/to/jsonexamples pytest tests

The suite builds an ~80 MB document and a list of 16,777,221 integers, so give it some RAM.

The benchmark scripts need simdjson-data and the bench extra (orjson, jiter, msgspec, pysimdjson, cysimdjson). bench.py times loads against json.loads, orjson and jiter, bench_lazy.py times parse against pysimdjson and cysimdjson, bench_dumps.py times dumps against json.dumps and dumpb against orjson and msgspec, and bench_many.py times loads_many.

pip:

python -m pip install -e ".[bench]"
python bench.py
python bench_lazy.py
python bench_dumps.py
python bench_many.py

uv:

uv pip install -e ".[bench]"
python bench.py
python bench_lazy.py
python bench_dumps.py
python bench_many.py

Benchmarks

Intel Xeon Gold 6548N (Emerald Rapids), one core, Python 3.14.6, fastsimdjson 0.3.0 (simdjson 5.0.2), the 22 files of simdjson-data. The scripts are in this repository and in the blog repository.

Whole documents

Each parser produces the whole document as Python objects. Speed is the geometric mean over the 22 files (higher is better).

parser	GB/s	vs json.loads
json (standard library)	0.22	1.00×
simplejson 4.2.0	0.23	1.05×
python-rapidjson 1.25	0.24	1.10×
ujson 6.0.0	0.37	1.66×
cysimdjson 26.27	0.43	1.96×
pysimdjson 7.0.2	0.44	1.98×
jiter 0.17.0 (no string cache)	0.44	2.00×
jiter 0.17.0 (string cache, the default)	0.48	2.21×
msgspec 0.22.0	0.53	2.42×
orjson 3.12.0	0.60	2.74×
fastsimdjson loads	0.78	3.53×

fastsimdjson is the fastest on 21 of the 22 files, and ties with orjson on numbers.json, an array of floating-point numbers (within 2%). On numbers, both spend most of their time creating Python floats, which costs the same in both: simdjson parses the numbers of these files 15% to 30% faster than orjson's parser (yyjson), but that is a small part of the total. Part of the gain comes from pausing the garbage collector while the objects are built. Timing each call on its own, fastsimdjson's lead over orjson is 1.33× with the collector enabled and 1.22× with it disabled (geometric means); without the collector, canada.json, mesh.json and numbers.json are ties. yyjson 4.0.6 is left out: it returns wrong strings for non-ASCII text. jiter (the parser of pydantic) caches strings by default; turning the cache off (cache_mode=False) makes it slower here.

Parsing is no longer the bottleneck. simdjson alone parses these files at 3.1 GB/s. It accounts for about a third of the time of loads; the rest goes into creating Python objects. Freeing those objects later costs about a sixth of the total. Even if parsing took no time at all, loads would be less than 1.5 times faster.

Parts of documents with parse

If you only need a few values, parse creates only those. Extracting the id and the screen name of the 100 statuses of twitter.json:

method	µs
json.loads	3922
jiter (no string cache)	1592
jiter (string cache)	1281
orjson	1009
fastsimdjson loads	861
msgspec (typed Struct)	336
cysimdjson (lazy)	229
pysimdjson (lazy)	179
fastsimdjson parse	158

Here parse is about 25 times faster than json.loads and 5.5 times faster than loads. Most of its time is the simdjson parse itself.

dumps against json.dumps

dumps returns exactly the str of json.dumps and is 4.6 times faster (geometric mean over the 22 files; from 3.1 times on citm_catalog.json to 12 times on numbers.json).

dumpb against orjson and msgspec

dumpb, orjson.dumps and msgspec.json.encode produce compact UTF-8 bytes; dumpb and orjson produce identical bytes on every file. With the standard library, the same compact bytes come from json.dumps(obj, separators=(",", ":"), ensure_ascii=False).encode(). Microseconds:

file	json.dumps(...).encode()	fastsimdjson dumpb	orjson	msgspec
twitter	1827	202	204	330
citm_catalog	2921	432	432	499
github_events	177	17	19	31
gsoc-2018	15386	595	554	1462
update-center	2482	254	232	477
canada	38101	2432	2936	3640
mesh	8726	785	999	1370
numbers	2530	178	200	332

dumpb and orjson are on par: over the 22 files, dumpb is 4% faster (geometric mean), from 10% slower on text-heavy or tiny files (gsoc-2018.json, update-center.json, repeat.json) to 27% faster on files full of numbers. Both are 1.7 times faster than msgspec and 10 times faster than json. The comparison was run on a processor with AVX-512, which dumpb and orjson both use to escape strings; other x64 processors use SSE2 and ARM processors NEON.

Writing NDJSON with dumpb_many

The 593,945 events of two hours of GH Archive (1.87 GB of NDJSON), written back as NDJSON from Python objects. With the GIL (Python 3.14.6), one thread: orjson.dumps on each record, joined, runs at 0.94 GB/s, dumpb on each record at 1.07 GB/s, and dumpb_many at 1.41 GB/s. On free-threaded Python 3.14.6t, GB/s:

threads	1	8	32	64
dumpb_many(records, threads=N)	1.3	6.0	11	12

Streams with loads_many

20 MB of NDJSON (5268 objects and arrays, one per line), made from the same files; best of three runs:

method	ms	GB/s
json.loads on each line	169	0.12
jiter on each line (no string cache)	98	0.20
jiter on each line (string cache)	90	0.22
orjson on each line	77	0.26
msgspec decode on each line	74	0.27
fastsimdjson loads on each line	60	0.33
msgspec decode_lines	53	0.37
fastsimdjson loads_many	52	0.38
fastsimdjson parse_many (views only)	21	0.94

msgspec's decode_lines and loads_many are on par (loads_many is about 2% faster). parse_many only creates the views; reading values from them adds to its time.

Streams on several threads

300 MB of NDJSON (146,624 records: tweets, GitHub events, citm performances, gsoc and update-center entries), free-threaded Python 3.14.6, collector disabled (see below), Intel Xeon Gold 6548N (64 cores); GB/s:

threads	1	2	4	8	16	32	64
map_many(len, data, threads=N)	1.03	1.78	3.66	6.82	13.0	23.7	28.8
parse_many(data, threads=N)	2.00	2.84	5.28	9.26	16.1	22.7	25.8
loads_many(data, threads=N)	0.90	1.44	2.79	4.99	8.98	13.3	15.4

map_many is 28 times faster on 64 threads than on one (up to 38 GB/s when nothing else runs in the process). loads_many builds every document for the calling thread and is 17 times faster. With the collector enabled, Python collects after the call returns: it examines the millions of objects just created, which takes about 0.1 s here, with or without threads.

Limitations

• The simdjson parser and the key and string caches are thread-local. release() frees the parser and the cached strings retained by the calling thread, and they are freed when a Python thread exits. A parser that grows past 64 MB is freed on its own at the end of that call; its caches stay.
• The module is marked free-threading compatible (Py_MOD_GIL_NOT_USED on Python 3.13 and newer), so importing it on a free-threaded build does not re-enable the GIL. It is tested with many threads parsing, serializing and sharing views and stream iterators on free-threaded Python 3.14 and 3.15. On a free-threaded build, bytearray and memoryview inputs are copied before parsing, and dumps and dumpb hold references to the items of the containers they serialize, since other threads may change them. Subinterpreters are not supported.