jinja2 variable inside a variable - dictionary

Question

I am passing a dictionary to a template.
dict road_len = {"US_NEWYORK_MAIN":24,
"US_BOSTON_WALL":18,
"FRANCE_PARIS_RUE":9,
"MEXICO_CABOS_STILL":8}
file_handle = output.txt
env.globals.update(country = "MEXICO")
env.globals.update(city = "CABOS")
env.globals.update(street = "STILL")
file_handle.write(env.get_template(template.txt).render(road_len=road_len)))
template.txt
This is a road length is: {{road_len["{{country}}_{{city}}_{{street}}"]}}
Expected output.txt
This is a road length is: 8
But this does not work as nested variable substitution are not allowed.

Answer 1

You never nest Jinja {{..}} markers. You're already inside a template context, so if you want to use the value of a variable you just use the variable. It helps if you're familiar with Python, because you can use most of Python's string formatting constructs.
So you could write:
This is a road length is: {{road_len["%s_%s_%s" % (country, city, street)]}}
Or:
This is a road length is: {{road_len[country + "_" + city + "_" + street]}}
Or:
This is a road length is: {{road_len["{}_{}_{}".format(country, city, street)]}}