Example:

import json
j = json.loads(
"""
{
    "phoneNumber": [
        {
            "type": "home",
            "number": "212 555-1234"
        },
}

""")
print(j)

Output>>
Traceback (most recent call last):
  File "<stdin>", line 13, in <module>
  File "/usr/lib/python2.7/json/__init__.py", line 326, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python2.7/json/decoder.py", line 366, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python2.7/json/decoder.py", line 384, in raw_decode
    raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded


My comment:
It is annoying in case of manual json editing where you can't know where jeson typeing error is.

For example i foun out, that just extra comma crate such error

j = json.loads("""{ "" :[{},]}""")

The errors are being handled correctly, but it would indeed be nice for them to provide more information about the specific error.  That's an enhancement, though.

Since AFAIK json.loads runs through every simil-str object, I think that a repr() is fine here. What do you think?

A repr of what?

> A repr of what?
Of the variable "s" in Lib/json/decoder.py#l358

So you would see a copy of the string being decoded in the error message?  That string might be very long, and would be of doubtful utility.  What I had in mind was at least printing the character position of the parse failure.  That may also be of limited utility, though, since in many cases the parse failure will happen at the end of the string.  This is a problem for all parsers, and the solution is often non-trivial, depending on the parser.  

Someone who wants to see this happen will need to look at the parser and see what information it has at the point of parse failure that might be useful and practical to expose.

Here is a patch that makes error message more informative. I also changed message "Expecting object" to "Expecting value" as more conformed to JSON terminology.

Patch updated. Conformed other error messages. Now expected exceptions raised for all of the following strings:

''
'['
'[42'
'[42,'
'{'
'{"'
'{"spam'
'{"spam"'
'{"spam":'
'{"spam":42'
'{"spam":42,'

Patch updated. Tests added. One discrepancy between Python an C versions fixed.

In fact, we can get rid of StopIteration and directly raise ValueError. I don't know what are the functions of StopIteration here, so preserve it. Perhaps the author could explain the details.

Patch updated. Fixed a debug artifact and a deprecation warning.

Patch updated.  Added new test cases as Ezio want..

New changeset 17b4bf9e9f43 by Ezio Melotti in branch 'default':
#16009: JSON error messages now provide more information.  Patch by Serhiy Storchaka.
http://hg.python.org/cpython/rev/17b4bf9e9f43

Fixed, thanks for the patch!