.. begin-html-header

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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.
