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📣 Announcing Dash Validation

Hello Dash Community :wave: –

I’m excited to announce the new Dash Validation feature, which will be a major help in debugging Dash applications!

You can install Dash Validation with the following versions:

pip install dash==0.29.0rc6
pip install dash-renderer==0.15.0rc1
pip install dash-core-components==0.31.0rc2
pip install dash-html-components==0.14.0rc2

Validation is enabled by default with these versions. To turn it off, simply set app.config.disable_component_validation = True.

All Dash component properties have a strict schema as defined in the PropTypes of the React Component definition.

Currently, when Dash components are initialized or when properties of Dash components are updated via callbacks, the property values are not validated against this schema. This can sometimes lead to difficult-to-debug errors being thrown in the front end, e.g. 1, 2. With this update, Dash will strictly validate against this schema and throw an informative error any time a value does not validate.

With Validation enabled, Dash will throw an error every time:

  • You initialize a component with a property that does not match the property schema.
    • For example, trying to set html.Div(children=[[]]). The children property does not allow for nested lists.
  • A callback to a property returns a value that does not match the property schema.
    • For example, trying to set Input.disabled to the string "hello world". disabled should be a Boolean.

Would be great to hear any feedback / issues here, or on page for this PR

Here are some example error messages that would be thrown:

Updating RadioItems.options with a typo in the label key.

CallbackOutputValidationError: 


A Dash Callback produced an invalid value!

Dash tried to update the `options` prop of the
`RadioItems` with id `output3` by calling the
`crash_it3` function with `(1)` as arguments.

This function call returned `[{'value': {'not okay': True}, 'labl': 'not okay'}]`, which did not pass
validation tests for the `RadioItems` component.

The expected schema for the `options` prop of the
`RadioItems` component is:

***************************************************************
{'schema': {'allow_unknown': False,
            'nullable': False,
            'schema': {'disabled': {'type': 'boolean'},
                       'label': {'type': 'string'},
                       'value': {'type': 'string'}},
            'type': 'dict'},
 'type': 'list'}
***************************************************************

The errors in validation are as follows:

* options
 * 0
  * value	<- must be of string type
  * labl	<- unknown field

Updating Graph.figure with an invalid Plotly figure

CallbackOutputValidationError: 


A Dash Callback produced an invalid value!

Dash tried to update the `figure` prop of the
`Graph` with id `output2` by calling the
`crash_it2` function with `(1)` as arguments.

This function call returned `{'layout': {}, 'data': {'y': [1, 2, 3], 'x': [1, 2, 3], 'type': 'scatter'}}`, which did not pass
validation tests for the `Graph` component.

The expected schema for the `figure` prop of the
`Graph` component is:

***************************************************************
{'validator': 'plotly_figure'}
***************************************************************

The errors in validation are as follows:

* figure	<- Invalid Plotly Figure:


    Invalid value of type '__builtin__.dict' received for the 'data' property of 
        Received value: {'y': [1, 2, 3], 'x': [1, 2, 3], 'type': 'scatter'}

    The 'data' property is a tuple of trace instances
    that may be specified as:
      - A list or tuple of trace instances
        (e.g. [Scatter(...), Bar(...)])
      - A list or tuple of dicts of string/value properties where:
        - The 'type' property specifies the trace type
            One of: ['mesh3d', 'splom', 'scattercarpet',
                     'scattergl', 'scatterternary', 'pie',
                     'surface', 'histogram', 'ohlc', 'heatmapgl',
                     'cone', 'scatterpolar', 'table',
                     'scatterpolargl', 'histogram2d', 'contour',
                     'carpet', 'box', 'violin', 'bar',
                     'contourcarpet', 'area', 'choropleth',
                     'candlestick', 'streamtube', 'parcoords',
                     'heatmap', 'barpolar', 'scattermapbox',
                     'scatter3d', 'pointcloud',
                     'histogram2dcontour', 'scatter', 'scattergeo',
                     'sankey']

        - All remaining properties are passed to the constructor of
          the specified trace type

        (e.g. [{'type': 'scatter', ...}, {'type': 'bar, ...}])

Initializing RadioItems.options with an invalid array

ComponentInitializationValidationError: 


A Dash Component was initialized with invalid properties!

Dash tried to create a `RadioItems` component with the
following arguments, which caused a validation failure:

***************************************************************
{'id': 'output3', 'options': [{'label': 'okay', 'value': {}}]}
***************************************************************

The expected schema for the `RadioItems` component is:

***************************************************************
{'className': {'type': 'string'},
 'dashEvents': {'allowed': ['change'], 'type': ('string', 'number')},
 'fireEvent': {},
 'id': {'type': 'string'},
 'inputClassName': {'type': 'string'},
 'inputStyle': {'type': 'dict'},
 'labelClassName': {'type': 'string'},
 'labelStyle': {'type': 'dict'},
 'options': {'schema': {'allow_unknown': False,
                        'nullable': False,
                        'schema': {'disabled': {'type': 'boolean'},
                                   'label': {'type': 'string'},
                                   'value': {'type': 'string'}},
                        'type': 'dict'},
             'type': 'list'},
 'setProps': {},
 'style': {'type': 'dict'},
 'value': {'type': 'string'}}
***************************************************************

The errors in validation are as follows:


* options
 * 0
  * value	<- must be of string type
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