Lambdas in tranform field?

Community Support
1

Hello,
I have uses a CustomJSTransform successfully to produce on-the-fly the values for the “size” of a scatter plot. Basically I am doing for size the same as what is done for y with the normalize used in this example.

However it seems a bit of an overkill to call in JS to just rescale some numbers. So I tried something like this with a lambda function.

p.scatter('x','y',
          , size={"field":'rad', "transform": lambda  x: x/10 }\
          , source=visite\
          , fill_alpha=0.15\
          ,fill_color={"field" :'fill_value',  "transform" : color_mapper}\
         , line_color='red')

Of course I got a Value Error because that lambda is not supported.

ValueError: expected an element of either String, Dict(Enum('expr', 'field', 'value', 'transform', 'units'), Either(String, Instance(Transform), Instance(Expression), Float)) or Float, got {'field': 'rad', 'transform': <function <lambda> at 0x7fd335d93b80>}

Am I just missing the syntax to use a lambda in the “transform” field or maybe I am the only one to feel the need for that?

2

Hi @foice, one of the confusing aspects of Bokeh for many users is that, although they use it by writing Python code, almost all the actual is done JavaScript code, in a browser. Transforms cannot be Python lambdas, because transform expression are only executed in a browser, i.e. they can only be JavaScript code.

There are a number of pre-made transforms that Bokeh provides, but if you need something custom, you can also use the CustomJSTransform to supply a snippet of JavaScript code to execute for the transform. There is an example in that link that is actually very similar to what you are trying to do above, so I won’t repeat it here.

3

Thanks for the explanation on the actual JS backend.

I think you are referring to the normalize JS example in the link, which is what I had done already. Or are you pointing me to another example more fit for my request?

4

No, the same one. There are very many support questions and only one of me, so I tend to read them very quickly and go straight to the code if there is code. I missed your first paragraph.

5

I guess I will add: we used to support something like this with a Python to JS transpiler but we deliberately removed it. It couldn’t work in all situations, blurred the runtime boundary in a way that confused people to think real Python code would be getting run, often required “weird” or unintuitive “Python” code as input, and in general was just a big source of support questions.

6

What you say makes a lot of sense.