import json
class safe_open:
    u"""backup file then open for write, restore old version in the case of unhandled exceptions"""
    def __init__(self, name,fun,*args):
        self.name=name
        self.bak=self.name+".bak"
        self.fun=fun
        self.args=args
    def __enter__(self):
        if os.path.exists(self.bak):
            os.remove(self.bak)
        if os.path.exists(self.name):
            os.rename(self.name,self.bak)
        self.f=self.fun(self.name,*self.args)
        return self.f
    def __exit__(self, exc_type, exc_value, traceback):
        self.f.close()
        if not exc_type is None:
            if os.path.exists(self.bak):
                os.remove(self.name)
                os.rename(self.bak,self.name)
with safe_open("aaa.json",open,"wb") as f:
    json.dump(safe_open,f)

if not exc_type is None: --> if exc_type is not None:

>>> import json
>>> import io
>>> 
>>> s = io.StringIO('start')
>>> s.read()
'start'
>>> json.dump([1, 2, 3], s)
>>> json.dump([1, 2, 3], s)
>>> s.seek(0)
0
>>> s.read()
'start[1, 2, 3][1, 2, 3]'
>>>