How do you declare python variables within flask templates?

Try this:

{% set x = [0,1,2,3,4,5] %}

See Jinja docs.

Flask uses Jinja as its default templating engine.

The templating language is python-esque, but is not python. This is different from something like a phtml file, which is php interspersed with html.

Check the jinja documentation for more of what you can do, but here's how you set a variable within a template:

{% set x = [0,1,2,3,4,5] %}

http://jinja.pocoo.org/docs/2.9/templates/#assignments