Using the wrong sort of quotes in json gives unhelpful error messages:

>>> json.loads("{'test':'test'}")
Traceback (most recent call last):
  ...
ValueError: Expecting property name: line 1 column 1 (char 1)

Unless you know that strings in JSON must be delimited with 
double-quotes and not single (a very surprising fact to those used to 
Python) this error message is perplexing. I suggest something like:

Single-quoted strings are invalid property names: line 1 column 1 
(char 1)

or 

Parse error, invalid char: line 1, column 1 (char 1)