Changes in Traitlets#


(Full Changelog)

Bugs fixed#

Maintenance and upkeep improvements#

Contributors to this release#

(GitHub contributors page for this release)

@azjps | @dependabot


(Full Changelog)

Maintenance and upkeep improvements#

Contributors to this release#

(GitHub contributors page for this release)

@blink1073 | @dependabot | @pre-commit-ci


(Full Changelog)

Bugs fixed#

  • FIX: map - to _ for environment variable prefix. #895 (@Carreau)

Maintenance and upkeep improvements#

Contributors to this release#

(GitHub contributors page for this release)

@blink1073 | @Carreau | @pre-commit-ci


(Full Changelog)

Enhancements made#

  • Add option to load traitlets values from environment. #856 (@Carreau)

Maintenance and upkeep improvements#

Contributors to this release#

(GitHub contributors page for this release)

@blink1073 | @Carreau | @pre-commit-ci


(Full Changelog)

Bugs fixed#

Maintenance and upkeep improvements#

Contributors to this release#

(GitHub contributors page for this release)

@blink1073 | @kylebarron


(Full Changelog)

Enhancements made#

Maintenance and upkeep improvements#

Documentation improvements#

Contributors to this release#

(GitHub contributors page for this release)

@blink1073 | @bollwyvl | @fleming79 | @pre-commit-ci | @shadchin


(Full Changelog)

Maintenance and upkeep improvements#

Contributors to this release#

(GitHub contributors page for this release)



(Full Changelog)

Maintenance and upkeep improvements#

Contributors to this release#

(GitHub contributors page for this release)



(Full Changelog)

Maintenance and upkeep improvements#

Contributors to this release#

(GitHub contributors page for this release)



(Full Changelog)

Maintenance and upkeep improvements#

Contributors to this release#

(GitHub contributors page for this release)



(Full Changelog)

Enhancements made#

Maintenance and upkeep improvements#

Documentation improvements#

  • rename MyClass to School #858 (@metaperl)

  • Even though returning a value from a validation method is ideal, by no means is proposal['value'] the only thing that can be returned #855 (@metaperl)

  • Document help parameter of TraitType.init(). #847 (@metaperl)

Other merged PRs#

  • Clean up license (#832)Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]> #832 (@dcsaba89)

Contributors to this release#

(GitHub contributors page for this release)

@blink1073 | @dcsaba89 | @dependabot | @maartenbreddels | @metaperl | @NickCrews | @pre-commit-ci | @rmorshea | @saulshanabrook | @scholer


(Full Changelog)

Enhancements made#

Maintenance and upkeep improvements#

  • Test that name and description can be set via constructor. #826 (@Carreau)

Contributors to this release#

(GitHub contributors page for this release)

@azjps | @blink1073 | @Carreau


(Full Changelog)

Bugs fixed#

Maintenance and upkeep improvements#

Documentation improvements#

Contributors to this release#

(GitHub contributors page for this release)

@blink1073 | @maartenbreddels | @pre-commit-ci | @rmorshea


(Full Changelog)

Enhancements made#

-Shell command-line tab-completion via argcomplete #811 (@azjps)

  • Define trait.__doc__ = for better API Docs #816 (@minrk)

Maintenance and upkeep improvements#

Documentation improvements#

  • Additional Application examples and docs #811 (@azjps)

Contributors to this release#

(GitHub contributors page for this release)

@azjps | @blink1073 | @minrk


(Full Changelog)

Bugs fixed#

Contributors to this release#

(GitHub contributors page for this release)



(Full Changelog)

Enhancements made#

  • Performance: decrease runtime overhead for constructing HasTraits (up to 20x faster) #777 (@maartenbreddels)

Maintenance and upkeep improvements#

Contributors to this release#

(GitHub contributors page for this release)

@azjps | @blink1073 | @maartenbreddels | @naterush | @pre-commit-ci


(Full Changelog)

Maintenance and upkeep improvements#

Contributors to this release#

(GitHub contributors page for this release)

@blink1073 | @dependabot | @maartenbreddels | @pre-commit-ci | @rmorshea


  • Clean up application typing

  • Update tests and docs to use non-deprecated functions

  • Clean up version handling

  • Prep for jupyter releaser

  • Format the changelog


  • Fix version_info

  • Make generated config files more lintable

  • Fix union trait from string

  • Add, and tidelift badge


  • Fix traitlet name in docstring

  • Re-support multiple-alias key for ArgParseConfigLoader


  • Make traitlets.__all__ explicit and validate in test.

  • Fix KeyError in Application.get_default_logging_config.


  • logging: Don’t attempt to close handlers unless they have been opened. Fixes ValueError: Unable to configure formatter 'console' traceback.


Traitlets 5.2 brings a couple of updates and changes to traitlets, and is recommended for all users. What follows is a non-exhaustive list of changes:

  • Application: add logging_config trait.

  • Disambiguate the use of the term value in the example.

  • Add mypy typing support.

  • DOC: Add log example.

  • Internal cleanup: linters, autoformatters, CI.


5.1.1 fixes compatibility issues with Python 3.10 (rc2).


Traitlets 5.1 brings a couple of updates and changes to traitlets, and is recommended for all users. What follow is a non-exhaustive list of changes:

  • Removal of the ipython_genutils dependency, this should remove any direct and indirect reliance on nose and prepare traitlets 5.1 for Python 3.10 and above compatibility, some test suite changes also accommodate Python 3.10 changes. If you package traitlets downstream, make sure to remove this dependency.

  • Removal of ipython_genutils may have change the default encoding detected for the command line argument parsing when not utf-8. We expect this to affect a small portion of older windows version. If you encounter issue let us know.

  • Stop recommendation of CUnicode in some circumstances as it’s deprecated.

  • Our test suite is now using GitHub action instead of travis CI. If you are packaging downstream using the git repository, you may have to exclude the .github folder now, and can remove exclusion of

  • It fixes a parsing issue for list of one single element on the CLI.

  • We reserve the right to remove official support for Python 3.7 in subsequent minor revisions.


  • Support deprecated literals for sets, tuples on the command-line: nbconvert --TagRemovePreprocessor.remove_cell_tags='{"tag"}'

  • Fix from_string_list for Tuples in general

  • Fix support for List(default_value=None, allow_none=True) and other Container traits

  • Fix help output for nested aliases and tuple traits


  • Support deprecated use of byte-literals for bytes on the command-line: ipython kernel --Session.key="b'abc'". The b prefix is no longer needed in traitlets 5.0, but is supported for backward-compatibility

  • Improve output of configuration errors, especially when help output would make it hard to find the helpful error message


  • Fix regression in handling --opt=None on the CLI for configurable traits with allow_none=True


  • Fix casting bytes to unicode


(This is an in-progress changelog, please let us know if something is missing/or could be phrased better)

Traitlets 5.0 is a new version of traitlets that accumulate changes over a period of more close to four years; A number of internal refactoring made the internal code structure cleaner and simpler, and greatly improved the diagnostic error messages as well has help and documentation generation.

We expect no code change needed for any consumer of the Python API (ipywidgets, and alike), though CLI argument parsing have seen a complete rewrite, so if you have an application that does use the parsing logic of traitlets you may see changes in behavior, and now have access to more features. There was also a cleanup of what was considered a part of the public API, certain previously exposed utility functions and types are no longer available. Please see an exhaustive list below.

::: seealso commandline docs for details about command-line parsing and the changes in 5.0.

Please let us know if you find issues with the new command-line parsing changes. :::

We also want to thanks in particular a number of regular contributor through the years that have patiently waited for their often large contribution to be available, if rough order of number of contribution:

  • Ryan Morshead - @rmorshea - For serving as a maintainer of the 4.x branch and providing a number of bug fix through the years.

  • Kostis Anagnostopoulos - @ankostis - Who push a major refactor of the CLI paring, as well as many help-generating function.

  • Benjamin Ragan-Kelley – @minrk – for reviewing and help fixing edge case in most of the above

  • Matthias Bussonnier – @carreau

  • Sylvain Corlay

  • Francisco de la Peña

  • Martin Renou

  • Yves Delley

  • Thomas Kluyver

  • hristian Clauss

  • maartenbreddels

  • Aliaksei Urbanski

  • Kevin Bates

  • David Brochart

As well as many of the passer-by, and less frequent contributors:

  • Tim Paine

  • Jake VanderPlas

  • Frédéric Chapoton

  • Dan Allan

  • Adam Chainz

  • William Krinsman

  • Travis DePrato

  • Todd

  • Thomas Aarholt

  • Lumir Balhar

  • Leonardo Uieda

  • Leo Gallucci

  • Kyle Kelley

  • Jeroen Demeyer

  • Jason Grout

  • Hans Moritz Günther

  • FredInChina

  • Conner Cowling

  • Carol Willing

  • Albert Zeyer

Major changes are:

  • Removal of Python 2 support,

  • Removal of Python 3.0-3.6 support

  • we now follow NEP 29, and are thus Python 3.7+ only.

  • remove six as a dependency

  • remove funcsig as a dependency.

  • no longer exposes the following functions / types:

    • ClassTypes

    • DefaultHandler

    • EventHandler

    • ForwardDeclaredMixin

    • ObserveHandler

    • SequenceTypes

    • ValidateHandler

    • add_article

    • class_of

    • getargspec

    • getmembers

    • is_trait

    • isidentifier

    • repr_type

Here is a list of most Pull requests that went into 5.0 and a short description.

  • #362 , #361 introduces:

    • help for aliases , aliases dict values can now be a tuple with (‘target’, ‘help string’)

    • subcommands can now be arbitrary callable and do not need to be subclass of Application

  • #306 Add compatibility with the trait package for Dictionaries and add the key_trait parameters allowing to restrict the type of the key of a mapping. The constructor parameters trait and traits are renamed to value_trait and per_key_traits.

  • #319 adds ability to introduce both shot and long version of aliases, allowing for short and long options - and --.

  • #322 rewrite command line argument parsing to use argparse, and allow more flexibility in assigning literals without quoting.

  • #332 Make it easier to redefined default values of parents classes.

  • #333 introduces a Callable trait.

  • #340 Old way of passing containers in the command line is now deprecated, and will emit warning on the command line.

  • #341 introduces --Application.show_config=True which will make by default any application show it configuration, all the files it loaded configuration from, and exit.

  • #349 unify ability to declare default values across traitlets with a singular method default method, and #525 adds a warning that Undefinedis deprecated.

  • #355 fix a random ordering issues in command lines flags.

  • #356 allow both self and cls in __new__ method for genericity.

  • #360 Simplify overwriting and extending the command line argument parser.

  • #371 introduces a FuzzyEnum trait that allow case insensitive and unique prefix matching.

  • #384 Ass a trait_values method to extra a mapping of trait and their values.

  • #393 Link now have a transform attribute (taking two functions inverse of each other), that affect how a value is mapped between a source and a target.

  • #394 Link now have a link method to re-link object after unlink has been called.

  • #402 rewrite handling of error messages for nested traits.

  • #405 all function that use to print help now have an equivalent that yields the help lines.

  • #413 traits now have a method trait_has_value, returning a boolean to know if a value has been assigned to a trait (excluding the default), in order to help avoiding circular validation at initialisation.

  • #416 Explicitly export traitlets in __all__ to avoid exposing implementation details.

  • #438 introduces .info_rst() to let traitlets overwrite the automatically generated rst documentation.

  • #458 Add a sphinx extension to automatically document options of Application instance in projects using traitlets.

  • #509 remove all base except: meaning traitlets will not catch a number of BaseException s anymore.

  • #515 Add a class decorator to enable tab completion of keyword arguments in signature.

  • #516 a Sentinel Traitlets was made public by mistake and is now deprecated.

  • #517 use parent Logger within logggin configurable when possible.

  • #522 Make loading config files idempotent and expose the list of loaded config files for long running services.

API changes#

This list is auto-generated by frappuccino, comparing with traitlets 4.3.3 API and edited for shortness:

The following items are new:
    + traitlets.Sentinel
    + traitlets.config.application.Application.emit_alias_help(self)
    + traitlets.config.application.Application.emit_description(self)
    + traitlets.config.application.Application.emit_examples(self)
    + traitlets.config.application.Application.emit_flag_help(self)
    + traitlets.config.application.Application.emit_help(self, classes=False)
    + traitlets.config.application.Application.emit_help_epilogue(self, classes)
    + traitlets.config.application.Application.emit_options_help(self)
    + traitlets.config.application.Application.emit_subcommands_help(self)
    + traitlets.config.application.Application.start_show_config(self)
    + traitlets.config.application.default_aliases
    + traitlets.config.application.default_flags
    + traitlets.config.default_aliases
    + traitlets.config.default_flags
    + traitlets.config.loader.DeferredConfig
    + traitlets.config.loader.DeferredConfig.get_value(self, trait)
    + traitlets.config.loader.DeferredConfigList
    + traitlets.config.loader.DeferredConfigList.get_value(self, trait)
    + traitlets.config.loader.DeferredConfigString
    + traitlets.config.loader.DeferredConfigString.get_value(self, trait)
    + traitlets.config.loader.LazyConfigValue.merge_into(self, other)
    + traitlets.config.loader.Undefined
    + traitlets.config.loader.class_trait_opt_pattern
    + traitlets.traitlets.BaseDescriptor.subclass_init(self, cls)
    + traitlets.traitlets.Bool.from_string(self, s)
    + traitlets.traitlets.Bytes.from_string(self, s)
    + traitlets.traitlets.Callable
    + traitlets.traitlets.Callable.validate(self, obj, value)
    + traitlets.traitlets.CaselessStrEnum.info_rst(self)
    + traitlets.traitlets.Complex.from_string(self, s)
    + traitlets.traitlets.Container.from_string(self, s)
    + traitlets.traitlets.Container.from_string_list(self, s_list)
    + traitlets.traitlets.Container.item_from_string(self, s)
    + traitlets.traitlets.Dict.from_string(self, s)
    + traitlets.traitlets.Dict.from_string_list(self, s_list)
    + traitlets.traitlets.Dict.item_from_string(self, s)
    + traitlets.traitlets.Enum.from_string(self, s)
    + traitlets.traitlets.Enum.info_rst(self)
    + traitlets.traitlets.Float.from_string(self, s)
    + traitlets.traitlets.FuzzyEnum
    + traitlets.traitlets.FuzzyEnum.info_rst(self)
    + traitlets.traitlets.FuzzyEnum.validate(self, obj, value)
    + traitlets.traitlets.HasTraits.trait_defaults(self, *names, **metadata)
    + traitlets.traitlets.HasTraits.trait_has_value(self, name)
    + traitlets.traitlets.HasTraits.trait_values(self, **metadata)
    + traitlets.traitlets.Instance.from_string(self, s)
    + traitlets.traitlets.Int.from_string(self, s)
    + traitlets.traitlets.ObjectName.from_string(self, s)
    + traitlets.traitlets.TCPAddress.from_string(self, s)
    + traitlets.traitlets.TraitType.default(self, obj='None')
    + traitlets.traitlets.TraitType.from_string(self, s)
    + traitlets.traitlets.Unicode.from_string(self, s)
    + traitlets.traitlets.Union.default(self, obj='None')
    + traitlets.traitlets.UseEnum.info_rst(self)
    + traitlets.utils.cast_unicode(s, encoding='None')
    + traitlets.utils.decorators
    + traitlets.utils.decorators.Undefined
    + traitlets.utils.decorators.signature_has_traits(cls)
    + traitlets.utils.descriptions
    + traitlets.utils.descriptions.add_article(name, definite=False, capital=False)
    + traitlets.utils.descriptions.class_of(value)
    + traitlets.utils.descriptions.describe(article, value, name='None', verbose=False, capital=False)
    + traitlets.utils.descriptions.repr_type(obj)

The following items have been removed (or moved to superclass):
    - traitlets.ClassTypes
    - traitlets.SequenceTypes
    - traitlets.config.absolute_import
    - traitlets.config.application.print_function
    - traitlets.config.configurable.absolute_import
    - traitlets.config.configurable.print_function
    - traitlets.config.loader.KeyValueConfigLoader.clear
    - traitlets.config.loader.KeyValueConfigLoader.load_config
    - traitlets.config.loader.flag_pattern
    - traitlets.config.loader.kv_pattern
    - traitlets.config.print_function
    - traitlets.traitlets.ClassBasedTraitType.error
    - traitlets.traitlets.Container.element_error
    - traitlets.traitlets.List.validate
    - traitlets.traitlets.TraitType.instance_init
    - traitlets.traitlets.Union.make_dynamic_default
    - traitlets.traitlets.add_article
    - traitlets.traitlets.class_of
    - traitlets.traitlets.repr_type
    - traitlets.utils.getargspec.PY3
    - traitlets.utils.importstring.string_types
    - traitlets.warn_explicit

The following signatures differ between versions:

    - traitlets.config.application.Application.generate_config_file(self)
    + traitlets.config.application.Application.generate_config_file(self, classes='None')

    - traitlets.config.application.catch_config_error(method, app, *args, **kwargs)
    + traitlets.config.application.catch_config_error(method)

    - traitlets.config.configurable.Configurable.class_config_section()
    + traitlets.config.configurable.Configurable.class_config_section(classes='None')

    - traitlets.config.configurable.Configurable.class_get_trait_help(trait, inst='None')
    + traitlets.config.configurable.Configurable.class_get_trait_help(trait, inst='None', helptext='None')

    - traitlets.config.loader.ArgParseConfigLoader.load_config(self, argv='None', aliases='None', flags='None')
    + traitlets.config.loader.ArgParseConfigLoader.load_config(self, argv='None', aliases='None', flags='<deprecated>', classes='None')

    - traitlets.traitlets.Dict.element_error(self, obj, element, validator)
    + traitlets.traitlets.Dict.element_error(self, obj, element, validator, side='Values')

    - traitlets.traitlets.HasDescriptors.setup_instance(self, *args, **kwargs)
    + traitlets.traitlets.HasDescriptors.setup_instance(*args, **kwargs)

    - traitlets.traitlets.HasTraits.setup_instance(self, *args, **kwargs)
    + traitlets.traitlets.HasTraits.setup_instance(*args, **kwargs)

    - traitlets.traitlets.TraitType.error(self, obj, value)
    + traitlets.traitlets.TraitType.error(self, obj, value, error='None', info='None')



4.3.2 on GitHub

4.3.2 is a tiny release, relaxing some of the deprecations introduced in 4.3.1:

  • using _traitname_default() without the @default decorator is no longer deprecated.

  • Passing config=True in traitlets constructors is no longer deprecated.


4.3.1 on GitHub

  • Compatibility fix for Python 3.6a1

  • Fix bug in Application.classes getting extra entries when multiple Applications are instantiated in the same process.


4.3.0 on GitHub

  • Improve the generated config file output.

  • Allow TRAITLETS_APPLICATION_RAISE_CONFIG_FILE_ERROR env to override Application.raise_config_file_errors, so that config file errors can result in exiting immediately.

  • Avoid using root logger. If no application logger is registered, the 'traitlets' logger will be used instead of the root logger.

  • Change/Validation arguments are now Bunch objects, allowing attribute-access, in addition to dictionary access.

  • Reduce number of common deprecation messages in certain cases.

  • Ensure command-line options always have higher priority than config files.

  • Add bounds on numeric traits.

  • Improves various error messages.


4.2.2 - 2016-07-01#

4.2.2 on GitHub

Partially revert a change in 4.1 that prevented IPython’s command-line options from taking priority over config files.

4.2.1 - 2016-03-14#

4.2.1 on GitHub

Demotes warning about unused arguments in HasTraits.__init__ introduced in 4.2.0 to DeprecationWarning.

4.2.0 - 2016-03-14#

4.2 on GitHub

  • JSONFileConfigLoader can be used as a context manager for updating configuration.

  • If a value in config does not map onto a configurable trait, a message is displayed that the value will have no effect.

  • Unused arguments are passed to super() in HasTraits.__init__, improving support for multiple inheritance.

  • Various bugfixes and improvements in the new API introduced in 4.1.

  • Application subclasses may specify raise_config_file_errors = True to exit on failure to load config files, instead of the default of logging the failures.

4.1 - 2016-01-15#

4.1 on GitHub

Traitlets 4.1 introduces a totally new decorator-based API for configuring traitlets. Highlights:

  • Decorators are used, rather than magic method names, for registering trait-related methods. See using_traitlets and migration for more info.

  • Deprecate Trait(config=True) in favor of Trait().tag(config=True). In general, metadata is added via tag instead of the constructor.

Other changes:

  • Trait attributes initialized with read_only=True can only be set with the set_trait method. Attempts to directly modify a read-only trait attribute raises a TraitError.

  • The directional link now takes an optional transform attribute allowing the modification of the value.

  • Various fixes and improvements to config-file generation (fixed ordering, Undefined showing up, etc.)

  • Warn on unrecognized traits that aren’t configurable, to avoid silently ignoring mistyped config.

4.0 - 2015-06-19#

4.0 on GitHub

First release of traitlets as a standalone package.