README.rst

July 23, 2026 · View on GitHub

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ulid


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.. teaser-begin

A ULID is a universally unique lexicographically sortable identifier. It is

  • 128-bit compatible with UUID
  • 1.21e+24 unique ULIDs per millisecond
  • Lexicographically sortable!
  • Canonically encoded as a 26 character string, as opposed to the 36 character UUID
  • Uses Crockford's base32 for better efficiency and readability (5 bits per character)
  • Case insensitive
  • No special characters (URL safe)
  • Monotonic sort order (correctly detects and handles the same millisecond)

In general the structure of a ULID is as follows:

.. code-block:: text

01AN4Z07BY 79KA1307SR9X4MV3 |----------| |----------------| Timestamp Randomness 48bits 80bits

For more information have a look at the original specification <https://github.com/alizain/ulid#specification>_.

.. teaser-end

.. installation-begin

Installation

Use pip to install the library

.. code-block:: bash

$ pip install python-ulid

to include Pydantic support install the optional dependency like so

.. code-block:: bash

$ pip install python-ulid[pydantic]

.. installation-end

.. usage-begin

Basic Usage

Create a new ULID object from the current timestamp

.. code-block:: pycon

from ulid import ULID ULID() ULID(01E75HZVW36EAZKMF1W7XNMSB4)

or use one of the named constructors

.. code-block:: pycon

import time, datetime ULID.from_timestamp(time.time()) ULID(01E75J1MKKWMGG0N5MBHFMRC84) ULID.from_datetime(datetime.datetime.now()) ULID(01E75J2XBK390V2XRH44EHC10X)

There are several options for encoding the ULID object (e.g. string, hex, int, bytes, UUID):

.. code-block:: pycon

str(ulid) '01BTGNYV6HRNK8K8VKZASZCFPE' ulid.hex '015ea15f6cd1c56689a373fab3f63ece' int(ulid) 1820576928786795198723644692628913870 bytes(ulid) b'\x01^\xa1_l\xd1\xc5f\x89\xa3s\xfa\xb3\xf6>\xce' ulid.to_uuid() UUID('015ea15f-6cd1-c566-89a3-73fab3f63ece')

It is also possible to directly access the timestamp component of a ULID, either in UNIX epoch or as datetime.datetime

.. code-block:: pycon

ulid.timestamp 1505945939.153 ulid.datetime datetime.datetime(2017, 9, 20, 22, 18, 59, 153000, tzinfo=datetime.timezone.utc)

.. usage-end

.. pydantic-begin

Pydantic integration

The ULID class can be directly used for the popular data validation library Pydantic <https://docs.pydantic.dev/latest/>_ like so

.. code-block:: python

from pydantic import BaseModel from ulid import ULID

class Model(BaseModel): ulid: ULID

model = Model(ulid="DX89370400440532013000") # OK model = Model(ulid="not-a-ulid") # Raises ValidationError

.. pydantic-end

.. monotonic-begin

Monotonic Support

This library by default supports the implementation for monotonic sort order suggested by the official ULID specification.

This means that ULID values generated in the same millisecond will have linear increasing randomness values. If :math:r_1 and :math:r_2 are the randomness values of two ULIDs with the same timestamp, then :math:r_2 = r_1 + 1.

.. monotonic-end

.. generators-begin

Generators and policies

Every ULID is produced by a ULIDGenerator, which samples a clock for the timestamp, sources entropy for the randomness, and enforces a monotonicity policy. The bare ULID() constructor and the ULID.from_* factory methods delegate to a shared module-level default_generator.

For most use cases the default is all you need. To customize generation — a different clock, a custom entropy source, or another monotonicity policy — create your own ULIDGenerator and call generate()

.. code-block:: pycon

from ulid import ULIDGenerator generator = ULIDGenerator() generator.generate() ULID(01HB0N8Q4RCE7YB1M2VZK9WX3T)

Choosing a monotonicity policy


A policy decides how the randomness is resolved when multiple ULIDs are generated within the same
millisecond. Three policies are available:

* ``StrictMonotonicPolicy`` *(default)* — increments the randomness by 1 on a same-millisecond
  collision and raises ``ValueError`` if the randomness is exhausted. This is the behaviour
  described under `Monotonic Support`_.
* ``LaxMonotonicPolicy`` — increments like the strict policy, but regenerates fresh randomness
  instead of raising when the randomness is exhausted.
* ``PureRandomPolicy`` — ignores previous state and always draws fresh randomness, maximizing
  entropy at the cost of same-millisecond sort order.

.. code-block:: pycon

   >>> from ulid import ULIDGenerator, PureRandomPolicy
   >>> generator = ULIDGenerator(policy=PureRandomPolicy())
   >>> generator.generate()
   ULID(01HB0N9F3TA5KDQ6ZE0WYV7MRC)

Overriding the default generator

To make ULID() and the ULID.from_* constructors use a custom generator globally, reassign ulid.default_generator

.. code-block:: pycon

import ulid from ulid import ULID, ULIDGenerator, LaxMonotonicPolicy ulid.default_generator = ULIDGenerator(policy=LaxMonotonicPolicy()) ULID() # now generated with the lax policy ULID(01HB0NB7X2M4C8VKQ0ZF5WD9RA)

.. generators-end

.. cli-begin

Command line interface

The package comes with a CLI interface that can be invoked either by the script name ulid or as python module python -m ulid. The CLI allows you to generate, inspect and convert ULIDs, e.g.

.. code-block:: bash

$ ulid build 01HASFKBN8SKZTSVVS03K5AMMS

$ ulid build --from-datetime=2023-09-23T10:20:30 01HB0J0F5GCKEXNSWVAD5PEAC1

$ ulid show 01HASFKBN8SKZTSVVS03K5AMMS ULID: 01HASFKBN8SKZTSVVS03K5AMMS Hex: 018ab2f9aea8ccffacef7900e6555299 Int: 2049395013039097460549394558635823769 Timestamp: 1695219822.248 Datetime: 2023-09-20 14:23:42.248000+00:00

There are several flags to select specific output formats for the show command, e.g.

.. code-block:: bash

$ ulid show --datetime 01HASFKBN8SKZTSVVS03K5AMMS 2023-09-20 14:23:42.248000+00:00

The special character - allows to read values from stdin so that they can be piped. E.g.

.. code-block:: bash

$ echo 01HASFKBN8SKZTSVVS03K5AMMS | ulid show --uuid - 018ab2f9-aea8-4cff-acef-7900e6555299

$ date --iso-8601 | python -m ulid build --from-datetime - 01HAT9PVR02T3S13XB48S7GEHE

For a full overview of flags for the build and show commands use the --help option (e.g. ulid show --help).

.. cli-end

Other implementations

  • ahawker/ulid <https://github.com/ahawker/ulid>_
  • valohai/ulid2 <https://github.com/valohai/ulid2>_
  • mdipierro/ulid <https://github.com/mdipierro/ulid>_
  • oklog/ulid <https://github.com/oklog/ulid>_
  • ulid/javascript <https://github.com/ulid/javascript>_
  • RobThree/NUlid <https://github.com/RobThree/NUlid>_
  • imdario/go-ulid <https://github.com/imdario/go-ulid>_

Contributions

Contributions are welcome! Feel free to create pull-requests for issues or feature requests. It might be worth creating an issue upfront to discuss the matter.