do both json.load() and json.dump() at once in Python

Question

I am trying to do something like this which uses reading, appending and writing at the same time.

with open("data.json", mode="a+") as file:
            # 1.Reading old data
            data = json.load(file)  
            # 2. Updating old data with new data
            data.update(new_dict)
            # 3.Writing into json file
            json.dump(data,file,indent=4)

But it shows json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

Answer 1

First you need to open the file in mode="r+" . Update the old data with new, then seek(0) to the beginning of the file, write your updated json data, and then truncate the rest:

with open("data.json", mode="r+") as file:
    file.seek(0, 2)
    if file.tell():
        file.seek(0)
        data = json.load(file)  
        data.update(new_dict)
    else:
        data = new_dict
    file.seek(0)
    json.dump(data, file, indent=4)
    file.truncate()

Reason it doesn't work with a+ mode is that it will always write at the end of the file, irrespective of seek(0) . So your updated json data just gets appended to the older one like a normal text data, but since its not a valid json syntax, it causes JSON Decode error.

Check here for more detailed info on how the different open modes work.