You can’t run arbitrary Python code in jinja; it doesn’t work like JSP in that regard (it just looks similar). All the things in jinja are custom syntax.
For your purpose, it would make most sense to define a custom filter, so you could for example do the following:
The grass is {{ variable1 | splitpart(0, ',') }} and the boat is {{ splitpart(1, ',') }}
Or just:
The grass is {{ variable1 | splitpart(0) }} and the boat is {{ splitpart(1) }}
The filter function could then look like this:
def splitpart (value, index, char = ','):
return value.split(char)[index]
An alternative, which might make even more sense, would be to split it in the controller and pass the splitted list to the view.