Question

I have a dictionary in Python that I would like to serialize in JSON and convert to a proper C string so that it contains a valid JSON string that corresponds to my input dictionary. I’m using the result to autogenerate a line in a C source file. Got it? Here’s an example:

>>> import json
>>> mydict = {'a':1, 'b': 'a string with "quotes" and \t and \\backslashes'}
>>> json.dumps(mydict)
'{"a": 1, "b": "a string with \\"quotes\\" and \\t and \\\\backslashes"}'
>>> print(json.dumps(mydict))
{"a": 1, "b": "a string with \"quotes\" and \t and \\backslashes"}

What I need to generate is the following C string:

"{\"a\": 1, \"b\": \"a string with \\\"quotes\\\" and \\t and \\\\backslashes\"}"

In other words, I need to escape the backslash and double-quote on the result of calling json.dumps(mydict). At least I think I do…. Will the following work? Or am I missing an obvious corner case?

>>> s = '"'+json.dumps(mydict).replace('\\','\\\\').replace('"','\\"')+'"'
>>> print s
"{\"a\": 1, \"b\": \"a string with \\\"quotes\\\" and \\t and \\\\backslashes\"}"

Answer 1

A C string starts with a quote and ends with a quote, has no embedded nulls, has all embedded quotes escaped with backslash, and all embedded backslash literals are doubled.

So take your string, double the backslashes and escape the quotes with a backslash. I think your code is exactly what you need:

s = '"' + json.dumps(mydict).replace('\\', r'\\').replace('"', r'\"') + '"'

Alternatively, you could go for this slightly less robust version:

def c_string(s):
    all_chars = (chr(x) for x in range(256))
    trans_table = dict((c, c) for c in all_chars)
    trans_table.update({'"': r'\"', '\\': r'\\'})
    return "".join(trans_table[c] for c in s)

def dwarf_string(d):
    import json
    return '"' + c_string(json.dumps(d)) + '"'

I’d love to use string.maketrans() but a translation table can map a character to at most a single character.