Python Bokeh tooltip text formatting

Question

The tooltip example presented in the reference guide show the following examples of formatting:

hover.tooltips = [
    ("index", "$index"),
    ("(x,y)", "($x, $y)"),
    ("radius", "@radius"),
    ("fill color", "$color[hex, swatch]:fill_color"),
    ("foo", "@foo"),
    ("bar", "@bar"),
    ("baz", "@baz{safe}"),
    ("total", "@total{$0,0.00}"

The 3 examples {safe}, {$0,0.00} and "$color[hex, swatch]:fill_color" are not clear: where can i find some documentation on them?

Basically I would like to understand what is possible and what isn't.

At the moment (for instance) I have 1 input that us a very long string (its a newspaper article) that I would like to format so it only shows the x first characters.

Other example I have a field @datetime that is retrieving its value from a datetime index. At the moment the tooltip displays that value as a int64 character. How to use a format tool such as Timestamp.strftime("%d-%m-%Y") so that it shows the date time in human readable format?

But I would like to have a clearer overview of what is possible/how that aspect of bokeh works

Answer 1

Since this answer was originally posted, new work has gone into Bokeh to make things simpler. A datetime field can be formatted as a datetime directly by the hover tool, by specifying a formatter, e.g.:

HoverTool(tooltips=[('label', '@datetime{%F}')],
          formatters={'datetime': 'datetime'})

It is no longer necessary to pre-format date fields in the data source as below (although it certainly still works). For more information see Formatting Tooltip Fields

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OLD ANSWER:

This is still an open issue for the project:

https://github.com/bokeh/bokeh/issues/1239

However, given some other recent work, this should now be fairly easy to implement this feature in a natural way. I have scheduled the task for the next 0.12.6 milestone.

Also, although Bokeh has extensive and rich documentation, there are still occasional gaps. This happens to be one of them, unfortunately. I note that there is an open issue to improve this:

https://github.com/bokeh/bokeh/issues/2595

I've updated it to make sure it is also included in the 0.12.6 milestone.

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In the mean time, your best option is to pre-format the data as you want it to appear in the tooltip in Python. Then add a column to your data source that has the formatted version, the configure the hover tool to display this column:

source.data['formatted_date'] = my_pretty_print(source.date['date'])

hover.tooltips = [ ("date", "@formatted_date") ]