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Minimal, modern embedded V8 for Python.

MiniRacer logo: a V8 with a very snakey 8

Full documentation.


  • Latest ECMAScript support
  • Web Assembly support
  • Unicode support
  • Thread safe
  • Re-usable contexts

MiniRacer can be easily used by Django or Flask projects to minify assets, run babel or WASM modules.

New home! (As of March 2024)

PyMiniRacer was created by Sqreen, and originally lived at with the PyPI package py-mini-racer.

As of March 2024, after a few years without updates, I have reached out to the original Sqreen team. We agreed that I should fork PyMiniRacer, giving it a new home at with a new PyPI package mini-racer (note: no py-). It now has a new version for the first time since 2021!


MiniRacer is straightforward to use:

    $ pip install mini-racer

and then:

    $ python3
    >>> from py_mini_racer import MiniRacer
    >>> ctx = MiniRacer()
    >>> ctx.eval("1+1")
    >>> ctx.eval("var x = {company: 'Sqreen'};")
    >>> print(ctx.eval("'❤'"))
    >>> ctx.eval("var fun = () => ({ foo: 1 });")

Variables are kept inside of a context:

    >>> ctx.eval("")

You can evaluate whole scripts within JavaScript, or define and return JavaScript function objects and call them from Python (new in v0.11.0):

    >>> square = ctx.eval("a => a*a")
    >>> square(4)

JavaScript Objects and Arrays are modeled in Python as dictionaries and lists (or, more precisely, MutableMapping and MutableSequence instances), respectively (new in v0.11.0):

    >>> obj = ctx.eval("var obj = {'foo': 'bar'}; obj")
    >>> obj["foo"]
    >>> list(obj.keys())
    >>> arr = ctx.eval("var arr = ['a', 'b']; arr")
    >>> arr[1]
    >>> 'a' in arr
    >>> arr.append(obj)
    >>> ctx.eval("JSON.stringify(arr)")

Meanwhile, call uses JSON to transfer data between JavaScript and Python, and converts data in bulk:

    {'foo': 1}

Composite values are serialized using JSON. Use a custom JSON encoder when sending non-JSON encodable parameters:

    import json

    from datetime import datetime

    class CustomEncoder(json.JSONEncoder):

            def default(self, obj):
                if isinstance(obj, datetime):
                    return obj.isoformat()

                return json.JSONEncoder.default(self, obj)
    >>> ctx.eval("var f = function(args) { return args; }")
    >>>"f",, encoder=CustomEncoder)

MiniRacer is ES6 capable:

    >>> ctx.execute("[1,2,3].includes(5)")

MiniRacer supports asynchronous execution using JS Promise instances (new in v0.10.0):

    >>> promise = ctx.eval(
    ...     "new Promise((res, rej) => setTimeout(() => res(42), 10000))")
    >>> promise.get()  # blocks for 10 seconds, and then:

You can use JS Promise instances with Python async (new in v0.10.0):

    >>> import asyncio
    >>> async def demo():
    ...     promise = ctx.eval(
    ...         "new Promise((res, rej) => setTimeout(() => res(42), 10000))")
    ...     return await promise
    >>>  # blocks for 10 seconds, and then:

JavaScript null and undefined are modeled in Python as None and JSUndefined, respectively:

    >>> list(ctx.eval("[undefined, null]"))
    [JSUndefined, None]

You can install callbacks from JavaScript to Python (new in v0.12.0):

    >>> async def read_file(fn):
    ...     with open(fn) as f:  # (or aiofiles would be even better here)
    ...         return
    >>> async def get_dictionary():
    ...    async with ctx.wrap_py_function(read_file) as jsfunc:
    ...        # "Install" our JS function on the global "this" object:
    ...        ctx.eval('this')['read_file'] = jsfunc
    ...        d = await ctx.eval('this.read_file("/usr/share/dict/words")')
    ...        return d.split()
    >>> dictionary =
    >>> print(dictionary[0:10])
    ['A', 'AA', 'AAA', "AA's", 'AB', 'ABC', "ABC's", 'ABCs', 'ABM', "ABM's"]

Note that adding Python callbacks may degrade the security properties of PyMiniRacer! See PyMiniRacer's security goals.

MiniRacer supports the ECMA Intl API:

    # Indonesian dates!
    >>> ctx.eval('Intl.DateTimeFormat(["ban", "id"]).format(new Date())')

V8 heap information can be retrieved:

    >>> ctx.heap_stats()
    {'total_physical_size': 1613896,
     'used_heap_size': 1512520,
     'total_heap_size': 3997696,
     'total_heap_size_executable': 3145728,
     'heap_size_limit': 1501560832}

A WASM example is available in the tests.


PyMiniRacer is compatible with Python 3.8-3.12 and is based on ctypes.

PyMiniRacer is distributed using wheels on PyPI. The wheels are intended to provide compatibility with:

OS x86_64 aarch64
macOS ≥ 10.9
Windows ≥ 10
Ubuntu ≥ 20.04
Debian ≥ 11
RHEL ≥ 9
other Linuxes with glibc ≥ 2.31
Alpine ≥ 3.19
other Linux with musl ≥ 1.2

If you have a up-to-date pip and it doesn't use a wheel, you might have an environment for which no wheel is built. Please open an issue.

Developing and releasing PyMiniRacer

See the contribution guide.


Built with love by Sqreen.

PyMiniRacer launch was described in this blog post.

PyMiniRacer is inspired by mini_racer, built for the Ruby world by Sam Saffron.

In 2024, PyMiniRacer was revived, and adopted by Ben Creech. Upon discussion with the original Sqreen authors, we decided to re-launch PyMiniRacer as a fork under and