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JSONDecodeError, what it means and how to fix it

This error means json.loads (or a library call hiding it, like response.json()) received text that is not valid JSON, and the decoder failed at the stated line and column. The near-universal cause is that the text is not what the code assumed: an API returned an HTML error page or an empty body, or the data was a Python representation (single quotes, True, None) rather than real JSON.

What it looks like

import json
data = json.loads(response.text)   # the API actually sent an
                                   # HTML error page

Traceback (most recent call last):
  File "main.py", line 1, in <module>
json.decoder.JSONDecodeError: Expecting value:
line 1 column 1 (char 0)

# fixed: look at what actually arrived before parsing it
print(response.status_code, response.text[:200])

The fix, in order

1. Print the raw text before parsing it. response.text[:200] (or the file's first bytes) settles everything: an HTML page, an empty string, a login redirect, or JSON with a visible defect. Never debug JSON blind.
2. Check the status code and content type. APIs return HTML for 404s, 500s, and auth failures. A .json() call on a failed response produces this error, which sends debugging toward the parser when the real problem is the request. Branch on status first.
3. Distinguish real JSON from a Python dict's repr. Single-quoted strings, True/False/None, and trailing commas are Python syntax, not JSON. Data produced by print(dict) or str(dict) needs json.dumps at the source, or ast.literal_eval to recover, never json.loads.
4. Read char position for genuine JSON defects. “Expecting property name” points at a trailing comma or single quotes; “Unterminated string” at a truncated write. The position counts characters, so paste the text into a JSON validator to see it with context.
5. For files, check for truncation and double-writes. A file written by two processes, or read while another program was mid-write, parses until the seam and fails there; write to a temp name and rename atomically.

If it still happens

If the text prints as valid JSON but still fails, suspect invisible contamination: a byte-order mark at the start (decode with utf-8-sig) or log lines and warnings mixed into the stream before the JSON begins (common when a script's output is piped). The decoder fails at char 0 precisely because the JSON does not start at char 0.

Common questions

Related errors