>>> jinja2.Template('{% set a = -1 %}{% for x in range(5) %}[{{ a }}:{% set a = x %}{{ a }}] {% endfor %}{{ a }}').render()
u'[:0] [:1] [:2] [:3] [:4] -1'

>>> jinja2.Template('{% set a = -1 %}{% for x in range(5) %}[{{ a }}:{% set a = x %}{{ a }}] {% endfor %}{{ a }}').render()
u'[-1:0] [0:1] [1:2] [2:3] [3:4] -1'