Fix: json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

Your Python program stops with:

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

The parser expected a JSON value (an object, array, string, number, true, false or null) at the very first character and did not find one. The message describes the input, not your code, so the first step is always to look at the input.

Step 1: print what you actually received

print(repr(text[:200]))   # repr shows empty strings, whitespace and hidden characters

Cause 1: the input is empty

json.loads("") raises exactly this error. Typical sources are an empty file, a file still being written by another process, or an HTTP response with no body, such as status 204 No Content.

Cause 2: you got HTML or plain text instead of JSON

If the output starts with < or <!doctype html>, the server sent a web page, often a login page, an error page or a rate-limit notice. Check the status code and the Content-Type header before parsing:

import json
import urllib.request

with urllib.request.urlopen("https://api.example.com/items") as resp:
    body = resp.read().decode("utf-8")
    ctype = resp.headers.get("Content-Type", "")

if "json" not in ctype:
    raise RuntimeError(f"expected JSON, got {ctype!r}: {body[:200]!r}")
data = json.loads(body)

urllib raises HTTPError for 4xx and 5xx responses. With the third-party requests library, call response.raise_for_status() before response.json(), so an error page fails with a clear message instead of a parse error.

Cause 3: the file was already read

A file object has a read position. After f.read(), the position is at the end, and a following json.load(f) sees an empty string:

with open("data.json", encoding="utf-8") as f:
    raw = f.read()
    data = json.load(f)      # fails: nothing left to read

with open("data.json", encoding="utf-8") as f:
    raw = f.read()
data = json.loads(raw)       # parse the string you already have

Related messages

• Unexpected UTF-8 BOM (decode using utf-8-sig): the file starts with a byte order mark. Open it with encoding="utf-8-sig".
• Extra data: line 2 column 1: the file holds one JSON object per line (JSON Lines). Parse each line separately.
• Expecting property name enclosed in double quotes: single quotes (often a Python dict written with str()), or a trailing comma. Write files with json.dump().

A loader with useful errors

This helper handles the BOM, rejects empty files, and shows the text around the problem:

import json
from pathlib import Path

def load_json_file(path):
    text = Path(path).read_text(encoding="utf-8-sig")
    if not text.strip():
        raise ValueError(f"{path} is empty")
    try:
        return json.loads(text)
    except json.JSONDecodeError as err:
        snippet = text[max(err.pos - 40, 0):err.pos + 40]
        raise ValueError(f"{path}: {err.msg} at line {err.lineno}, column {err.colno}: {snippet!r}") from None

def load_json_lines(path):
    with open(path, encoding="utf-8") as f:
        return [json.loads(line) for line in f if line.strip()]

If you control the code that produces the data, the real fix is there: always write with json.dump, write UTF-8, and return a proper status code with a JSON body for errors.