Question

i have encountered a very strange issue: i use json.dump to write a file and then use json.load to read the file.

The same code can run succeed on windows 7 but it can not do on mac os x 10.7
Below is the code:

class Result:
def __init__(self,name,result):
    self.name = name
    self.result = result

def __repr__(self):
    return 'Result  name : %s , result : %s' % (self.name,self.result)

class MyEncoder(json.JSONEncoder):
def default(self,obj):
    #convert object to a dict
    d = {'CaseResult':{}}
    d['CaseResult'][obj.name] = obj.result
    return d


def save(name,result):
    filename = 'basic.json'
    obj = Result(name,result)
    obj_json = MyEncoder().encode(obj)
    with open(filename, mode='ab+') as fp:
        json.dump(obj_json,fp)
        s=json.load(fp)

save('aaa','bbb')

in mac os it give an error “ValueError:NO JSON object could be decoded”
who can tell me why this happen and how can i resolve it

Answer 1

This problem is unrelated to being run on a Mac; this code should never work:

with open(filename, mode='ab+') as fp:
    json.dump(obj_json,fp)
    s=json.load(fp)

This is because after json.dump, your file pointer is at the end of the file. You must call fp.seek to reset it to the initial position, like this:

import os
with open(filename, mode='rb+') as fp:
    fp.seek(0, os.SEEK_END)
    pos = fp.tell()
    json.dump(obj_json,fp)
    fp.seek(pos)
    s=json.load(fp)