Python

JSONDecodeError: What It Means and How to Fix It

Error reference

JSONDecodeError

Language

Python

Severity

Medium

When it happens

json.JSONDecodeError is raised when json.loads() or json.load() receives a string that is not valid JSON.

Potential fixes

Always validate the raw response before parsing: check that it starts with { or [. Use try/except json.JSONDecodeError to handle malformed input gracefully. Log the raw string when a decode error occurs so you can inspect what was actually received.

Related errors

ValueError UnicodeDecodeError

Deep dive

json.JSONDecodeError is raised when json.loads() or json.load() receives a string that is not valid JSON. It is a subclass of ValueError and includes the position of the syntax error in the input, making it easier to debug malformed payloads.

Why it happens

•Using single quotes instead of double quotes (JSON requires double quotes).
•Trailing commas in objects or arrays, which are not permitted in JSON.
•Receiving an HTML error page or empty string where JSON is expected.
•A truncated response from a network request that was not fully received.

Potential fixes

Always validate the raw response before parsing: check that it starts with { or [. Use try/except json.JSONDecodeError to handle malformed input gracefully. Log the raw string when a decode error occurs so you can inspect what was actually received.

Examples

Python

Code that triggers the error

import json

data = json.loads("{'key': 'value'}")  # single quotes are not valid JSON

Error output

json.decoder.JSONDecodeError: Expecting property name enclosed in double quotes: line 1 column 2 (char 1)

Fixed code

import json

raw = '{"key": "value"}'  # double quotes
try:
    data = json.loads(raw)
    print(data["key"])
except json.JSONDecodeError as e:
    print(f"Invalid JSON at position {e.pos}: {e.msg}")

Practice in English

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"I hit an error...

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