transit-python

Transit is a format and set of libraries for conveying values between applications written in different programming languages. The library provides support for marshalling data to/from Python.

transit-python is maintained by Vendekagon Labs, which has taken it over as the successor to Cognitect's original cognitect/transit-python, bringing it up to date: Python 3.10 and later, all three transit encodings, and a native extension for speed. It continues the same PyPI package, transit-python.

• Rationale
• Specification

This implementation's major.minor version number corresponds to the version of the Transit specification it supports.

NOTE: Transit is intended primarily as a wire protocol for transferring data between applications. If storing Transit data durably, readers and writers are expected to use the same version of Transit and you are responsible for migrating/transforming/re-storing that data when and if the transit format changes.

Installation

pip install transit-python             # JSON and JSON-verbose
pip install 'transit-python[msgpack]'  # also msgpack, when running as pure Python

transit-python releases up to 0.8.302 support Python 2.7 and 3.5; later releases, from this repository, support Python 3.10 and later.

The package includes a native extension, built from transit-c, that reads and writes transit 4-15 times faster than pure Python, with identical results. PyPI has wheels with it compiled for Linux, macOS and Windows (x86_64 and ARM); elsewhere pip builds it from the source distribution if there's a C compiler, and otherwise installs the package as pure Python. Set TRANSIT_PUREPYTHON=1 to use pure Python regardless (or, when installing from source, to not build the extension).

The native extension reads and writes msgpack itself. Pure Python needs the msgpack package for that, installed with the msgpack extra; without it, creating a msgpack Reader or Writer raises an ImportError saying so.

To install from GitHub:

pip install "transit-python[msgpack] @ git+https://github.com/vendekagon-labs/transit-python.git"

(add @<tag or commit> to the URL to pin a version, or use git+ssh://git@github.com/... to install over SSH). In a pyproject.toml or requirements.txt, use the same transit-python @ git+https://... form.

Usage

# io can be any Python file descriptor,
# like you would typically use with JSON's load/dump

from transit.writer import Writer
from transit.reader import Reader

writer = Writer(io, "json") # or "json-verbose", "msgpack"
writer.write(value)

reader = Reader("json") # or "msgpack"
val = reader.read(io)

For example:

>>> from transit.writer import Writer
>>> from transit.reader import Reader
>>> from io import StringIO
>>> io = StringIO()
>>> writer = Writer(io, "json")
>>> writer.write(["abc", 1234567890])
>>> s = io.getvalue()
>>> reader = Reader()
>>> vals = reader.read(StringIO(s))

To read a sequence of values as they arrive (e.g. from a pipe or socket), use readeach, which yields each value as soon as it has been read and stops at the end of the stream. A Writer can likewise write any number of values.

reader = Reader("json")
for value in reader.readeach(sys.stdin.buffer):
    writer.write(value)

Supported Python versions

• 3.10 and later

Type Mapping

Typed arrays, lists, and chars

The transit spec defines several semantic types that map to more general types in Python:

• lists map to Python Tuples
• typed arrays (ints, longs, doubles, floats, bools), chars and bytes are read as TaggedValues (e.g. TaggedValue("ints", (1, 2, 3))), so they are written back out unchanged

Use a TaggedValue to write these out if it will benefit a consuming app e.g.:

writer.write(TaggedValue("ints", [1,2,3]))

Python's bool and int

In Python, bools are subclasses of int (that is, True is actually 1).

>>> hash(1)
1
>>> hash(True)
1
>>> True == 1
True

This becomes problematic when decoding a map that contains bool and int keys. The bool keys may be overridden (ie: you'll only see the int key), and the value will be one of any possible bool/int keyed value.

>>> {1: "Hello", True: "World"}
{1: 'World'}

To counter this problem, the latest version of Transit Python introduces a Boolean type with singleton (by convention of use) instances of "true" and "false." A Boolean can be converted to a native Python bool with bool(x) where x is the "true" or "false" instance. Logical evaluation works correctly with Booleans (that is, they override the bool method and correctly evaluate as true and false in simple logical evaluation), but uses of a Boolean as an integer will fail.

Default type mapping

Development

Setup

The tests read the exemplar files from transit-format, which is expected to be checked out next to transit-python (or inside it, or set TRANSIT_FORMAT_DIR to its location).

pip install -e '.[test]'

The test extra includes msgpack. The msgpack tests are skipped when it isn't installed, so run the tests both with and without it.

Running the tests

pytest

Running the transit-format verify harness

transit-format's bin/verify drives bin/roundtrip, which runs with the python3 on your PATH, so activate an environment with transit-python's dependencies installed first.

cd ../transit-format
bin/verify -impls python

Benchmarks

python -m tests.seattle_benchmark

The native extension

transit/_native.c wraps transit-c, whose sources are in csrc/, copied from transit-c by bin/sync-transit-c (which records the commit in csrc/transit_c_version.h); change them there, not here. pip install -e . builds the extension in place. The tests check that native and pure Python read and write identically; run them both ways (TRANSIT_PUREPYTHON=1 pytest for pure Python).

bin/verify runs transit-format's verify harness in each encoding, with the clojure CLI, and fails if any roundtrip does.

Releasing

The version is 0.8.. To release:

bin/make-release    # stamps the version into transit/__init__.py
git commit -am "Release 0.8.N"
git tag v0.8.N
git push origin main v0.8.N

Pushing the tag runs .github/workflows/wheels.yml, which tests, builds the sdist and wheels (with cibuildwheel), and publishes them to PyPI with trusted publishing.

Contributing

transit-python is maintained by Vendekagon Labs. Report problems and suggest changes with GitHub issues.

Pull requests are welcome, but make sure they're well tested first: add tests for what you change, and check that the tests and transit-format's verify harness (bin/verify) pass, with both the native extension and pure Python (TRANSIT_PUREPYTHON=1).

Copyright and License

Copyright © 2026 Vendekagon Labs LLC

Copyright © 2014-2016 Cognitect

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.