more pythonic way to format a JSON string from a list of tuples

There is a much better way to generate JSON strings: the json module.

import json
rs = json.dumps(dict(lst))

This takes advantage of the fact that dict() can take a sequence of key-value pairs (two-value tuples) and turn that into a mapping, which the json module directly translates to a JSON object structure.

Demonstration:

>>> import json
>>> lst = [("name", "value"), ("name2", "value2")]
>>> rs = json.dumps(dict(lst))
>>> print rs
{"name2": "value2", "name": "value"}

(lambda lst: json.dumps({item[0]:item[1] for item in lst}))([(1,2), (3,4)])