I have an application that is sending a JSON object (formatted with Prototype) to an ASP server. On the server, the Python 2.6 "json" module tries to loads() the JSON, but it's choking on some combination of backslashes. Observe:

>>> s
'{"FileExists": true, "Version": "", "Path": "\\\\host\\dir\\file.exe"}'

>>> tmp = json.loads(s)
Traceback (most recent call last):
  File "<interactive input>", line 1, in <module>
  {... blah blah blah...}
  File "C:\Python26\lib\json\", line 155, in JSONString
    return scanstring(match.string, match.end(), encoding, strict)
  ValueError: Invalid \escape: line 1 column 58 (char 58)

>>> s[55:60]

So column 58 is the escaped-backslash. I thought this WAS properly escaped! UNC is \\host\dir\file.exe, so I just doubled up on slashes. But apparently this is no good. Can someone assist? As a last resort I'm considering converting the \ to / and then back again, but this seems like a real hack to me.

Thanks in advance!

+3  A: 

The correct json is:

r'{"FileExists": true, "Version": "", "Path": "\\\\host\\dir\\file.exe"}'

Note the letter r if you omit it you need to escape \ for Python too.

>>> import json
>>> d = json.loads(s)
>>> d.keys()
[u'FileExists', u'Path', u'Version']
>>> d.values()
[True, u'\\\\host\\dir\\file.exe', u'']

Note the difference:

>>> repr(d[u'Path'])
>>> str(d[u'Path'])
>>> print d[u'Path']

Python REPL prints by default the repr(obj) for an object obj:

>>> class A:
...   __str__ = lambda self: "str"
...   __repr__  = lambda self: "repr"
>>> A()
>>> print A()

Therefore your original s string is not properly escaped for JSON. It contains unescaped '\d' and '\f'. print s must show '\\d' otherwise it is not correct JSON.

NOTE: JSON string is a collection of zero or more Unicode characters, wrapped in double quotes, using backslash escapes ( I've skipped encoding issues (namely, transformation from byte strings to unicode and vice versa) in the above examples.

J.F. Sebastian
:)>>> s = r'{"FileExists": true, "Version": "", "Path": "\\\\host\\dir\\file.exe"}'>>> json.loads(s){u'FileExists': True, u'Path': u'\\\\host\\dir\\file.exe', u'Version': u''}
So what is r actually doing? How can I apply it to a string that's already stored as, say, "foo". Is it some kind of encoding?
`'s ='` breaks formatting.
J.F. Sebastian
@Chris: `r''` is convenient to write Windows paths and regexps (you don't need to escape backslash in such literal strings). `r''` only matters how you write literals. It has no meaning for string objects.
J.F. Sebastian
Ok. What was happening was that I was making up this test data and using them in the python shell for "rehearsing" what I wanted my application to do. I guess I was overescaping, because once I said, "eff this, let's just try it live", it worked!Thanks for the comments that led me to a better understanding.
+1  A: 
>>> s
'{"FileExists": true, "Version": "", "Path": "\\\\host\\dir\\file.exe"}'
>>> print s
{"FileExists": true, "Version": "", "Path": "\\host\dir\file.exe"}

You've not actually escaped the string, so it's trying to parse invalid escape codes like \d or \f. Consider using a well-tested JSON encoder, such as json2.js.

John Millikin