Dash

Added
- [319](https://github.com/plotly/dash/pull/309) Add a modified time query string to assets included in the index in order to bust the cache.

Added
- [316](https://github.com/plotly/dash/pull/316) Add `ie-compat` meta tag to the index by default.
- [305](https://github.com/plotly/dash/pull/305) Add `external_script` and `external_css` keywords to dash `__init__`.
- Dash components are now generated at build-time and then imported rather than generated when a module is imported. This should reduce the time it takes to import Dash component libraries, and makes Dash compatible with IDEs.

Fixed
- [273](https://github.com/plotly/dash/pull/273) Raise a more informative error if a non-JSON-serializable value is returned from a callback.

Added
- [286](https://github.com/plotly/dash/pull/286) Asset files & index customization.
- [294](https://github.com/plotly/dash/pull/294) Raise an error if there is no layout present when the server is run.
- [renderer55](https://github.com/plotly/dash-renderer/pull/55) Add `_dash-error` class to the "Error loading layout" and "Error loading dependencies" messages.

Fixed
- Attempting to render a `Boolean` value to the page no longer crashes the app.
- [renderer57](https://github.com/plotly/dash-renderer/issues/57) If a callback references an `id` which does not exist in the DOM tree at the time it is executed, throw a more informative front-end exception.
- [renderer54](https://github.com/plotly/dash-renderer/pull/54) Previously, if a component called `updateProps` with multiple properties, Dash would fire the callback multiple times (once for each property). Now the callback only fires once.

Added
- [237](https://github.com/plotly/dash/pull/237) Support `aria-*` and `data-*` attributes in all dash html components. These new keywords can be added using a dictionary expansion, e.g. `html.Div(id="my-div", **{"data-toggle": "toggled", "aria-toggled": "true"})`
- [renderer45](https://github.com/plotly/dash-renderer/pull/45) Allow user to choose between React versions '15.4.2' and '16.2.0':
python
import dash_renderer

Set the react version before setting up the Dash application
dash_renderer._set_react_version('16.2.0')

app = dash.Dash(...)


Fixed
- [renderer50](https://github.com/plotly/dash-renderer/pull/50) Update MANIFEST.in to include `react` and `react-dom` bundles for development mode

Added
- [207](https://github.com/plotly/dash/pull/207) Support React components using [Flow](https://flow.org/en/docs/react/) types. `component_loader` now has the following behavior to create docstrings as determined in discussion in [#187](https://github.com/plotly/dash/issues/187):
1. If a Dash component has `PropTypes`-generated typing, the docstring uses the `PropTypes`, _regardless of whether the component also has Flow types (current behavior)._
2. Otherwise if a Dash component has Flow types but _not `PropTypes`_, the docstring now uses the objects generated by `react-docgen` from the Flow types.

Fixed
- [renderer42](https://github.com/plotly/dash-renderer/pull/42) Fix [renderer#41](https://github.com/plotly/dash-renderer/issues/41) and [renderer#44](https://github.com/plotly/dash-renderer/issues/44).
- In some cases, during initialization, callbacks may fired multiple times instead of just once. This only happens in certain scenarios where outputs have overlapping inputs and those inputs are leaves (they don't have any inputs of their own).
- If an output component is returned from a callback and its inputs were _not_ returned from the same input (i.e. they were already visible), then the callback to update the output would not fire. This has now been fixed. A common scenario where this app structure exists is within a Tabbed app, where there are global controls that update each tab's contents and the tab's callback just displays new output containers.