loading a datetime from a json-CodePudding

I am storing a datetime object to now when to re-stock, however when trying to load the datetime I get an error like: JSONDecodeError: Expecting value: line 1 column 1 (char 0) I hae no clue on what any of this means since I did not write the JSON lib requesting some help over and out.

Here is my current code

datetime.datetime.strptime(json.loads(File.read())["lastStocked"], "%Y-%m-%d %H:%M:%S.%f")

here is the json file

{
    "Reserve": 1000,
    "lastStocked": "2022-10-21 21:28:36.408608",
    "nextStocked": "2022-10-21 21:28:36.408672"
}

CodePudding user response：

File.read() is returning an empty string. That's why the error is indicating the first character of the first line.

You didn't post all of the relevant code, but I would guess you're calling File.read() twice. After the first read the file read pointer is at the end of the file so the second read returns nothing. Solutions include:

Copying the contents of the file to a variable and using that (only call File.read() once)
Call File.seek(0) before calling File.read() the second time
Close and re-open the file before calling File.read() the second time

Or the bug is somewhere else.

CodePudding user response：

Guessing at the missing code, I could reproduce the problem if the JSON was saved as UTF-8 w/ BOM encoding, which adds bytes EF BB BF (the UTF-8 encoding of the U FEFF byte order mark character) to the beginning of the file as a UTF-8 signature. The JSON encoder doesn't like that:

import json
import datetime

with open('test.json') as File:
    # simple way to debug...actually look at the data
    data = File.read()
    print(ascii(data))
    print(datetime.datetime.strptime(json.loads(data)["lastStocked"], "%Y-%m-%d %H:%M:%S.%f"))

Output below. Note the \xef\xbb\xbf printed at the start of the file:

'\xef\xbb\xbf{\n    "Reserve": 1000,\n    "lastStocked": "2022-10-21 21:28:36.408608",\n    "nextStocked": "2022-10-21 21:28:36.408672"\n}'
Traceback (most recent call last):
    ...
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

The following code will work, as utf-8-sig encoding will remove the BOM if present.

import json
import datetime

with open('test.json', encoding='utf-8-sig') as File:
    print(datetime.datetime.strptime(json.loads(File.read())["lastStocked"], "%Y-%m-%d %H:%M:%S.%f"))

or more simply with json.load instead:

import json
import datetime

with open('test.json', encoding='utf-8-sig') as File:
    print(datetime.datetime.strptime(json.load(File)["lastStocked"], "%Y-%m-%d %H:%M:%S.%f"))

Output:

2022-10-21 21:28:36.408608