Pretty printing and compact output

Readable JSON helps humans and diffs; compact JSON saves bandwidth.

json.dumps and json.dump take formatting options:

indent=2 pretty-prints with nested indentation (an int or a string like "\t"). The default None produces a single line.
sort_keys=True orders object keys alphabetically, giving stable output for diffs, tests and caching.
separators=(",", ":") removes the spaces after commas and colons for the most compact form. The default is (", ", ": "), or (",", ": ") when indent is set.
ensure_ascii=False writes non-ASCII characters as-is. By default they are escaped ("é" becomes "\u00e9"), which is valid but unreadable. When writing files with ensure_ascii=False, make sure the file is opened with encoding="utf-8".

From the shell, python -m json.tool file.json validates and pretty-prints a file, and it reads stdin when no file is given:

curl -s https://api.example.com/x | python -m json.tool --sort-keys

Options include --indent, --compact, --sort-keys and --json-lines.

Formatting never changes the data, only whitespace and key order, so any JSON parser reads all variants identically.

Example

import json

order = {"id": 42, "customer": "Zoë", "items": [{"sku": "A1", "qty": 2}]}

print(json.dumps(order))
# {"id": 42, "customer": "Zo\u00eb", "items": [{"sku": "A1", "qty": 2}]}
print(json.dumps(order, ensure_ascii=False, separators=(",", ":")))
# {"id":42,"customer":"Zoë","items":[{"sku":"A1","qty":2}]}
print(json.dumps(order, indent=2, sort_keys=True, ensure_ascii=False))
# {
#   "customer": "Zoë",
#   "id": 42,
#   "items": [
#     {
#       "qty": 2,
#       "sku": "A1"
#     }
#   ]
# }

Remember

indent pretty-prints; separators=(',', ':') gives the most compact form
sort_keys=True makes output stable for diffs and tests
ensure_ascii=False keeps Unicode readable; pair it with UTF-8 files

Check yourself

Which option makes two equal dicts with different insertion orders produce identical JSON text?

Official docs: docs.python.org/3/library/json.html#json.dumps

Example

Remember

Check yourself

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