Single quotes in JSON: "Expected property name or '}'"

JSON only accepts double quotes around keys and strings. 'Single quotes' are fine in JavaScript and Python code, so JSON written by hand, or a Python dict printed with str() or print(), fails at the first quote.

What each runtime prints

We ran these inputs through JSON.parse in each browser and Node version, and through json.loads in Python. These are the messages exactly as printed.

{'name': 'Ada'}

{'ok': True, 'value': None}

• Safari is the only one that names the problem: "Single quotes (') are not allowed in JSON".
• Every parser that gives a position stops at column 2, the first ', so a Python dict with True and None fails with exactly the same message as plain single quotes. None of them get as far as True.
• Python says "Expecting property name enclosed in double quotes", which is the right hint.

Common causes

1. 1

   A Python dict printed instead of serialized

   str(data), print(data), f"{data}" and logging a dict all give Python's own format: 'single quotes', True, False and None. It looks like JSON but is not.

2. 2

   A JavaScript object literal pasted as JSON

   Code like {name: 'Ada'} copied into a .json file, a request body or a test fixture.

3. 3

   Shell quoting

   Writing JSON inside a shell command and swapping the quote styles to make the shell happy, as in curl -d "{'a': 1}".

How to fix it

Serialize the data with the JSON library instead of printing it.

JavaScript

const body = JSON.stringify({ name: 'Ada' }); // '{"name":"Ada"}'

Python

import json

body = json.dumps({"ok": True, "value": None})  # '{"ok": true, "value": null}'

• If you already have text in Python's format and cannot regenerate it, ast.literal_eval() reads it back into a dict safely (it only accepts literals, never code). Then json.dumps() it.
• In a shell, keep the JSON in single quotes outside and double quotes inside: curl -d '{"a": 1}'.

Related

• Trailing comma in JSON: "Expected double-quoted property name"
• "[object Object]" is not valid JSON
• JSON Repair
• JSON Formatter