Jsonschema

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- Add support for string format verification, by enabling use of the
``jsonschema.FormatChecker``. This is enabled by default, but can be disabled
with the ``--disable-format`` flag

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- Improved error output when the schema itself is invalid, either because it is
not JSON or because it does not validate under its relevant metaschema

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* Bugfixes

* Issue 17 - require path for error objects
* Issue 18 - multiple type validation for non-objects

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- Added the ``--default-filetype`` flag, which sets a default of JSON or YAML
loading to use when ``identify`` does not detect the filetype of an instance
file. Defaults to failure on extensionless files.
- Schemafiles are now passed through ``os.path.expanduser``, meaning that a
schema path of ``~/myschema.json`` will be expanded by check-jsonschema
itself (:issue:`9`)
- Performance enhancement for testing many files: only load the schema once
- Added ``--no-cache`` option to disable schema caching
- Change the default schema download cache directory from
``jsonschema_validate`` to ``check_jsonschema/downloads``.
e.g. ``~/.cache/jsonschema_validate`` is now
``~/.cache/check_jsonschema/downloads``.
Caches will now be in the following locations for different platforms
and environments:

- ``$XDG_CACHE_HOME/check_jsonschema/downloads`` (Linux/other, XDG cache dir)
- ``~/.cache/check_jsonschema/downloads`` (Linux/other, no XDG cache dir set)
- ``~/Library/Caches/check_jsonschema/downloads`` (macOS)
- ``%LOCALAPPDATA%\check_jsonschema\downloads`` (Windows, local app data set)
- ``%APPDATA%\check_jsonschema\downloads`` (Windows, no local app data set, but appdata set)

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- Update the azure-pipelines schema version to latest. Thanks to :user:`Borda`

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* Preliminary support for programmatic access to error details (Issue 5).
There are certainly some corner cases that don't do the right thing yet, but
this works mostly.

In order to make this happen (and also to clean things up a bit), a number
of deprecations are necessary:

* ``stop_on_error`` is deprecated in ``Validator.__init__``. Use
``Validator.iter_errors()`` instead.
* ``number_types`` and ``string_types`` are deprecated there as well.
Use ``types={"number" : ..., "string" : ...}`` instead.
* ``meta_validate`` is also deprecated, and instead is now accepted as
an argument to ``validate``, ``iter_errors`` and ``is_valid``.

* A bugfix or two