Prevent initial call

[alexcjohnson]

Closes #1225
Supersedes #1123

Adds the ability to control whether or not callbacks fire when the page or layout chunk first loads. This can simplify a lot of code like if n_clicks is None: raise PreventUpdate, and it can improve loading performance, particularly for pattern-matching callbacks, where a page can easily generate hundreds of callback invocations, since there is no longer even a server call for these callbacks.

This option is off by default, but can either be turned on for each callback independently:

@app.callback(outputs, inputs, prevent_initial_call=True)
def cb(...):
    ...

app.clientside_callback(func, outputs, inputs, prevent_initial_call=True)

or app-wide, in which case it can be turned off for individual callbacks:

app = dash.Dash(prevent_initial_callbacks=True)

@app.callback(outputs, inputs, prevent_initial_call=False)
def cb(...)
    ...

• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR
• I have added entry in the CHANGELOG.md
• Docs follow-up issue: https://github.com/plotly/dash-docs/issues/874

[Marc-Andre-Rivet]

💃

[alexcjohnson]

I'm not sure what I was thinking here with clearing out changedPropIds, but in this case (we've found a callback by its inputs due to a layout change, and just determined that all its outputs are in the same layout chunk) we know the callback is already included in the queue via its outputs so the best thing to do is simply not re-include it at all. This extra addCallback was harmless before prevent_initial_call, just a slight bit of extra work; but now that we have prevent_initial_call it's possible this callback would not be added based on its outputs.

If this condition isn't met, there are outputs outside the new layout chunk and it's not in fact an initial call anyway - so it's appropriate to add the callback and that's what the new logic does.

[alexcjohnson]

pytest does a nice job logging dictionary test parametrizations (flavor0, flavor1 etc):

[maartenbreddels]

Initial impression is really good! Together with #832 we can avoid any unneeded calls, and have a very fast page load.