Unexpected behaviour exhibited by Python's JSON module

Question

I am doing a simple wrapper program around a software tool, and this wrapper will, among other things, parse a JSON configuration file that will be coded by the user as a way for the user to specify certain configutations to the program.

Within the program, I have the following lines of code:

with open(os.path.join(args.config_bin_dir, 'basic_config.json'), 'r') as jsondata :
    print "JSON File contents:", jsondata.read()
    basic_config.update(json.load(jsondata))
    print "The basic_config after updating with JSON:", basic_config
jsondata.close()

The basic_config object is a dictionary object that gets constructed somewhere on top earlier up in the program, and over here I want to add the new dictionary key-value pairs obtained from parsing the user's JSON configuration file into the basic_config dictionary object.

When the program is run, I get the following error, a section of which is as follows:

File "path/to/python/script.py", line 42, in main
    basic_config.update(json.load(jsondata))
File "/usr/lib/python2.7/json/__init__.py", line 290, in load
  **kw)
File "/usr/lib/python2.7/json/__init__.py", line 338, 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

I'm quite puzzled by this error, but I noticed that it goes away so long as I remove the line print "JSON File contents:", jsondata.read() and run the code as either:

with open(os.path.join(args.config_bin_dir, 'basic_config.json'), 'r') as jsondata :
    # print "JSON File contents:", jsondata.read()
    basic_config.update(json.load(jsondata))
    print "The basic_config after updating with JSON:", basic_config
jsondata.close()

or

with open(os.path.join(args.config_bin_dir, 'basic_config.json'), 'r') as jsondata :
    # print "JSON File contents:", jsondata.read()
    basic_config.update(json.load(jsondata))
    # print "The basic_config after updating with JSON:", basic_config
jsondata.close()

I'll still get the same error if I comment out the line after the json.load method:

with open(os.path.join(args.config_bin_dir, 'basic_config.json'), 'r') as jsondata :
    print "JSON File contents:", jsondata.read()
    basic_config.update(json.load(jsondata))
    # print "The basic_config after updating with JSON:", basic_config
jsondata.close()

So I'm guessing that it has to do with calling the read method on the JSON file object before parsing that same file. Could anyone please explain to me why this is so, and whether I am mistaken about the cause of the error?

Thank you very much!

Answer 1

When you call read() on a file handle, its current read position is advanced to the end of the file. This mirrors the behavior of the underlying file descriptors. You can reset to the start of a file via jsondata.seek(0, 0).