String format a JSON string gives KeyError

You need to double the outer braces; otherwise Python thinks { "File".. is a reference too:

output_format="{{ "File": "{filename}", "Success": {success}, "ErrorMessage": "{error_msg}", "LogIdentifier": "{log_identifier}" }}"

Result:

>>> print output_format.format(filename="My_file_name",
...                            success=True,
...                            error_msg='',
...                            log_identifier="123")
{ "File": "My_file_name", "Success": True, "ErrorMessage": "", "LogIdentifier": "123" }

If, indicentally, you are producing JSON output, you’d be better off using the json module:

>>> import json
>>> print json.dumps({'File': 'My_file_name',
...                   'Success': True,
...                   'ErrorMessage': '',
...                   'LogIdentifier': '123'})
{"LogIdentifier": "123", "ErrorMessage": "", "Success": true, "File": "My_file_name"}

Note the lowercase true in the output, as required by the JSON standard.