How to pass null to javascript from python using jinja

Question

I was doing it something like this in google app engine:

self.render('hello.html', value=null)

But obviously since python doesn't understand null it thinks it's an undefined variable and gives an error.

If I send it as a string (value="null") then javascript doesn't know that I mean null and not a string "null".

So what's the way to get around this?

And if it's any bit pertinent, I'm trying to pass the value null to a google scatter chart in javascript.

Answer 1

I'm assuming your are doing something like:

self.render('hello.html', value=[0.1, None, None, 0.5])

and having the template engine convert that to a string, so you end up with:

 [0.1, None, None, 0.5]

If so, you should have python convert it to javascript before passing to jinja:

import json
self.render('hello.html', value=json.dumps([0.1, None, None, 0.5]))

That'll give you:

[0.1, null, null, 0.5]

which is what I think you are after.

A even better solution would be to add a custom filter to jinja to handle javascript escaping. That would look something like:

import json
def jsonfilter(value):
    return json.dumps(value)

environment.filters['json'] = jsonfilter

Then in your template you could just do

{{value|json}}

Answer 2

assuming your template looks something like:

plotdata = [0.1, {{value}}, null, 0.5]

(depending on the template engine you are using), then:

self.render('hello.html', { "value": "null" })

should result in the rendered template producing:

plotdata = [0.1, null, null, 0.5]

If this is not working, please tell us the template engine you are using, the values you've tried sending, and the exact result of rendering..