First release candidate for Traits 6.0.0

Release notes

Traits 6.0 is a major update to the Traits package, with a number of
backward incompatible changes from its predecessor. Notable changes:

• Python 2.7 is no longer supported; Traits 6.0 requires Python 3.5 or later.
• Trait types related to Python 2 (for example Unicode and Long) have
  been deprecated in favour of their Python 3 equivalents (for example Str
  and Int).
• Many little-used historical features of Traits have been deprecated, and
  are scheduled for removal in Traits 7.0.
• Some historical features of Traits that had no evidence of external usage
  were removed in Traits 6.0.
• Introspection of CTrait and HasTraits objects is greatly improved.
  All of the internal state that was previously hidden within the C extension
  is now accessible from Python.
• The Traits codebase has undergone some significant reorganizations,
  reformattings and style cleanups to make it easier to work with, and
  to improve the separation between Traits and TraitsUI.
• This release was focused mainly on cleanup and bugfixing. Nevertheless,
  it contains a sprinkling of new features. There's a new Datetime
  trait type. The Enum trait type now supports Python enumerations.
  The File trait type supports path-like objects.

More than 150 PRs went into this release. The following people contributed
code changes for this release:

• Kit Yan Choi
• Mark Dickinson
• Kevin Duff
• Robert Kern
• Midhun Madhusoodanan
• Shoeb Mohammed
• Sai Rahul Poruri
• Corran Webster
• John Wiggins

Porting guide

For the most part, existing code that works with Traits 5.2.0 should
continue to work with Traits 6.0.0 without changes. However, there
are some potentially breaking changes in Traits 6.0.0, and we recommend
applying caution when upgrading.

Here's a guide to dealing with some of the potentially breaking changes.

• The Unicode and CUnicode trait types are now simply synonyms for
  Str and CStr. Unicode and CUnicode are considered deprecated.
  For now, no deprecation warning is issued on use of these deprecated trait
  types, but in Traits 6.1.0 and later, warnings may be issued, and in Traits
  7.0.0 these trait types may be removed. It's recommended that users update
  all uses of Unicode to Str and CUnicode to CStr to avoid
  warnings or errors in the future.

• Similarly, Long and CLong are now synonyms for Int and CInt.
  The same recommendations apply as for the Unicode / Str trait types.

• Uses of NO_COMPARE, OBJECT_IDENTITY_COMPARE and RICH_COMPARE
  should be replaced with the appropriate ComparisonMode enumeration
  members.

• The validation for a Instance(ISomeInterface) trait type has changed,
  where ISomeInterface is a subclass of Interface. Previously, an
  assignment to such a trait validated the type of the assigned value against
  the interface, method by method. Now an isinstance check is performed
  against the interface instead. Make sure that classes implementing a given
  interface have the appropriate provides decorator.

  One notable side-effect of the above change is that plain mock.Mock
  instances can no longer be assigned to Instance(ISomeInterface) traits.
  To get around this, use spec=ISomeInterface when creating your mock
  object.

  This change does not affect Instance traits for non-interface classes.

• The format of TraitListEvents has changed: for list events generated from
  a slice set or slice delete operation where that slice had a step other
  than 1, the added and removed fields of the event had an extra
  level of list wrapping (for example, added might be [[1, 2, 3]]
  instead of [1, 2, 3]). In Traits 6.0, this extra wrapping has been
  removed. There may be existing code that special-cased the extra wrapping.

• Many classes and functions have moved around within the Traits codebase.
  If you have code that imports directly from Traits modules and subpackages
  instead of from traits.api or the other subpackage api modules, some
  of those imports may fail. To avoid potential for ImportErrors, you
  should import from traits.api whenever possible. If you find yourself
  needing some piece of Traits functionality that isn't exposed in
  traits.api, and you think it should be, please open an issue on the
  Traits bug tracker.

Features

• Add new Datetime trait type. (#737, #814, #813, #815, #848)

• Support Python Enums as value sets for the Enum trait. (#685, #828, #855)

• Add Subclass alias for the Type trait type. (#739)

• Add path-like support for the File trait. (#736)

• Add new ComparisonMode enumeration type to replace the old
  NO_COMPARE, OBJECT_IDENTITY_COMPARE and RICH_COMPARE
  constants. The old constants are deprecated. (#830, #719, #680)

• Add fast validation for Callable trait type; introduce
  new BaseCallable trait type for subclassing purposes.
  (#798, #795, #767)

• Add CTrait.comparison_mode property to allow inspection and
  modification of a trait's comparison mode. (#758, #735)

• Add as_ctrait converter function to traits.api. This function
  converts a trait-like object or type to a CTrait, raising TypeError
  for objects that can't be interpreted as a CTrait. It's intended
  for use by users who want to create their own parameterised trait
  types.

  The as_ctrait feature comes with, and relies upon, a new informal
  interface: objects that can be converted to something of type CTrait can
  provide an zero-argument as_ctrait method that returns a new CTrait.
  Types can provide an instantiate_and_get_ctrait method, which when
  called with no arguments provides a new CTrait for that type.
  (#783, #794)

• Add a new HasTraits._class_traits method for introspection of an
  object's class traits. This parallels the existing
  HasTraits._instance_traits method. This method is intended for use in
  debugging. It's not recommended for users to modify the returned dictionary.
  (#702)

• Add CTrait.set_default_value method for setting information about the
  default of a CTrait. This provides an alternative to the previous method
  of using CTrait.default_value. The use of CTrait.default_value to set
  (rather than get) default information is deprecated. (#620)

• Add new methods HasTraits._trait_notifications_enabled,
  HasTraits._trait_notifications_vetoed to allow introspection of the
  notifications states set by the existing methods
  HasTraits._trait_change_notify and HasTraits._trait_veto_notify.
  (#704)

• Add TraitKind, ValidateTrait and DefaultValue Python enumeration
  types to replace previous uses of magic integers within the Traits codebase.
  (#680, #857)

• The various CTrait internal flags are now exposed to Python as
  properties: CTrait.is_property (read-only), CTrait.modify_delegate,
  CTrait.setattr_original_value, CTrait.post_setattr_original_value,
  CTrait.is_mapped, and CTrait.comparison_mode. (#666, #693)

Changes

• When pickling a CTrait, the py_post_setattr and py_validate
  fields are pickled directly. Previously, callables for those fields were
  replaced with a -1 sentinel on pickling. (#780)
• A TraitListEvent is no longer emitted for a slice deletion which
  doesn't change the contents of the list. (For example, del obj.mylist[2:]
  on a list that only has 2 elements.) (#740)
• The added and removed attributes on a TraitListEvent are now
  always lists containing the added or removed elements. Previously, those
  lists were nested inside another list in some cases. (#771)
• Change Instance(ISomeInterface) to use an isinstance check on
  trait set instead of using the dynamic interface checker. (#630)
• Create an new AbstractViewElement abstract base class, and register
  the TraitsUI ViewElement as implementing it. This paves the way for
  removal of Traits UI imports from Traits. (#617)
• ViewElements are now computed lazily, instead of at HasTraits
  subclass creation time. This removes a traitsui import from
  the trait.has_traits module. (#614)
• The traits.util.clean_filename utility now uses a different algorithm,
  and should do a better job with accented and Unicode text. (#589)
• Floating-point and integer checks are now more consistent between classes.
  In particular, BaseInt validation now matches Int validation, and
  Range type checks now match those used in Int and Float. (#588)
• An exception other than TraitError raised during validation of a
  compound trait will now be propagated. Previously, that exception would
  be swallowed. (#581)
• Traits no longer has a runtime dependency on the six package. (#638)
• Use pickle protocol 3 instead of pickle protocol 1 when writing pickled
  object state to a file in configure_traits. (#796)
• In traits.testing.optional_dependencies, make sure traitsui.api is
  available whenever traitsui is. (#616)
• TraitInstance now inherits directly from TraitHandler instead of
  (the now removed) ThisClass. (#761)

Fixes

• Fix a use of the unsupported ValidateTrait.int_range. (#805)
• Remove unnecessary copy method override from TraitSetObject. (#759)
• Fix TraitListObject.clear to issue the appropriate items event. (#732)
• Fix confusing error message when [None] passed into
  List(This(allow_none=False)). (#734)
• Fix name-mangling of double-underscore private methods in classes whose
  name begins with an underscore. (#724)
• Fix bytes_editor and password_editor bugs, and add tests for
  all editor factories. (#660)
• Fix coercion fast validation type to do an exact type check instead of
  an instance check. This ensures that instances of subclasses of the
  target type are properly converted to the target type. For example,
  if True is assigned to a trait of type CInt, the resulting
  value is now 1. Previously, it was True. (#647)
• Fix BaseRange to accept the same values as Range. (#583)
• Fix integer Range to accept integer-like objects. (#582)
• Fix floating-point Range to accept float-like values. (#579)
• Fix a missing import in the adaptation benchmark script. (#575)
• Fix issues with the filename argument to configure_traits. (#572)
• Fix a possible segfault from careless field re-assignments in
  ctraits.c. (#844)

Deprecations

• The NO_COMPARE, OBJECT_IDENTITY_COMPARE and RICH_COMPARE
  constants are deprecated. Use the corresponding members of the
  ComparisonMode enumeration instead. (#719)
• The Unicode, CUnicode, BaseUnicode and BaseCUnicode trait
  types are deprecated. Use Str, CStr, BaseStr and BaseCStr
  instead. (#648)
• The Long, CLong, BaseLong and BaseCLong trait types are
  deprecated. Use Int, CInt, BaseInt and BaseCInt instead.
  (#645, #573)
• The AdaptedTo trait type is deprecated. Use Supports instead. (#760)
• The following trait type aliases are deprecated. See the documentation for
  recommended replacments. false, true, undefined, ListInt,
  ListFloat, ListStr, ListUnicode, ListComplex, ListBool,
  ListFunction, ListMethod, ListThis, DictStrAny,
  DictStrStr, DictStrInt, DictStrFloat, DictStrBool,
  DictStrList. (#627)
• Use of the filename argument to configure_traits (for storing
  state to or restoring state from pickle files) is deprecated. (#792)
• The TraitTuple, TraitList and TraitDict trait handlers
  are deprecated. Use the Tuple, List and Dict trait types instead.
  (#770)
• Use of CTrait.default_value for setting default value information is
  deprecated. Use CTrait.set_default_value instead. (#620)
• Use of the rich_compare trait metadata is deprecated. Use the
  comparison_mode metadata instead. (#598)

Removals

• Python 2 compatibility support code has been removed. (#638, #644)
• Traits categories have been removed. (#568)
• The following trait handlers have been removed: ThisClass,
  TraitClass, TraitExpression, TraitCallable, TraitString,
  TraitRange, TraitWeakRef. (#782, #711, #699, #698, #625, #593, #587,
  #640)
• CTrait.rich_compare has been removed. (#598)
• The cTrait.cast method has been removed. (#663)
• The magical TraitValue and associated machinery have been removed. (#658)
• The Generic trait type has been removed. (#657)
• The UStr trait type and HasUniqueStrings class have been removed.
  (#654)
• The str_find and str_rfind helper functions have been removed. (#633)
• The global _trait_notification_handler has been removed. (#619)
• BaseTraitHandler.repr has been removed. (#599)
• HasTraits.trait_monitor was undocumented, untested, and broken, and
  has been removed. (#570)
• The TraitInstance trait handler (not to be confused
  with the Instance trait type) no longer supports adaptation. (#641)
• The DynamicView and HasDynamicViews classes have been removed
  from Traits and moved to TraitsUI instead. (#609)
• DictStrLong has been removed. (#573)

Test suite

• Fix various tests to be repeatable. (#802, #729)
• Fix deprecation warnings in the test suite output. (#820, #804, #716)
• Add machinery for testing unpickling of historical pickles. (#787)
• Remove print statements from test suite. (#752, #768)
• Fix a test to clean up the threads it creates. (#731)
• Add tests for extended trait change issues #537 and #538 (#543)
• Other minor test fixes. (#700, #821)

Documentation

• Improve documentation of trait container objects. (#810)
• Improve documentation for the traits.ctraits module. (#826, #824,
  #659, #653, #829, #836)
• Fix badly formatted TraitHandler documentation. (#817)
• Fix and improve badly formatted trait types documentation. (#843)
• Fix broken module links in section titles in API documentation. (#823)
• Additional class docstring fixes. (#854)
• Add changelog to built documentation, and absorb old changelog into
  the new one. (#800, #799)
• Remove deprecated traits from the user manual. (#656)
• Fix various Sphinx warnings (#717)
• Use SVG badges in README (#567)

Build and continuous integration

• Enable C asserts in Travis CI runs. (#791)
• Abort CI on compiler warnings in Travis CI runs. (#769)
• Run a flake8 check in both Travis CI and Appveyor runs. (#753, #762)
• Checking copyright statements in Python files as part of CI runs. (#749)
• Turn warnings into errors when building documentation in CI. (#744)
• Add gnureadline as a development dependency on macOS and Linux. (#607)
• Add an etstool.py option to run tests quietly. (#606)
• Enable the coverage extension for the documentation build. (#807)
• Remove mocking in documentation configuration, and fix a deprecated
  configuration option. (#696)

Maintenance and code organization

This release includes a lot of refactoring and many minor improvements
that will primarily benefit those working with the Traits codebase. These
changes should not affect user-visible functionality. Here's a summary
of the more significant changes.

• A major refactor has removed most of the circular dependencies between
  modules. (#730)
• The codebase is now mostly flake8 clean. (#786, #753, #747, #748, #746,
  #595)
• Copyright headers have been made consistent for all Python files. (#754)
• ctraits.c has been run through clang-tidy and clang-format in
  order to bring it closer to PEP 7 style. (#715)
• Editor factories have been moved into a new traits.editor_factories
  module, to help compartmentalize code dependencies on TraitsUI. (#661)
• Trait container object classes (TraitDictObject, TraitListObject,
  TraitSetObject) have each been moved into their own module, along
  with their associated event type. (#677)
• Miscellaneous other minor fixes, refactorings and cleanups.
  (#785, #777, #750, #726, #714, #712, #708, #701, #682, #665, #651,
  #652, #639, #636, #634, #626, #632, #611, #613, #612, #605, #603,
  #600, #597, #586, #585, #584, #580, #577, #578, #564, #806)