json.tool is very cute and handy for making json readable.

But rewriting a file in place requires tools like sponge (on POSIX) or a tmpfile, because 

$ python -m json.tool foo.json foo.json

results in an empty foo.json.

I propose soaking up the infile before opening the outfile for writing, to prevent that. Much like sort -o does, but without the explicit flag.
The patch I have prepared changes no behaviours, other than preventing an empty file... (still I see this as an enhancement and not a bug fix)

This is arguably a bugfix since truncating the `infile` cannot be construed as a useful feature in any sense. Thus I'm inclined to backport this to 3.10.1 and 3.9.9 too.

New changeset 815dad42d53fc40a6dc057e067f4a8a885c3b858 by Chris Wesseling in branch 'main':
bpo-45644:  Make json.tool read infile before writing to outfile (GH-29273)
https://github.com/python/cpython/commit/815dad42d53fc40a6dc057e067f4a8a885c3b858

New changeset 6603f6b5bdf0ed164bad4eb432619da2e95fb518 by Miss Islington (bot) in branch '3.10':
bpo-45644:  Make json.tool read infile before writing to outfile (GH-29273) (GH-29445)
https://github.com/python/cpython/commit/6603f6b5bdf0ed164bad4eb432619da2e95fb518

New changeset a932631890d148444e5f9b09f4b57305475d6386 by Miss Islington (bot) in branch '3.9':
bpo-45644:  Make json.tool read infile before writing to outfile (GH-29273) (GH-29446)
https://github.com/python/cpython/commit/a932631890d148444e5f9b09f4b57305475d6386

Thanks, Chris! ✨ 🍰 ✨

The proposed path does not work for `python -m json.tool --json-lines test.json test.json`. There is also https://github.com/python/cpython/pull/7865 that aims to fix this bug but I did not have the time to get back to it.

@Remi

I left the current behaviour for --json-lines untouched, on purpose.
My reasoning was that the json-lines format is often seen in JSON streaming, and I didn't want to break the case where the input is an endless stream from stdin or a named pipe. And to keep the patch simple with minimal change to the exposed interface.

Simplest fix for the case in this current code would be: iff infile equals outfile (minding their types), call list(objs) on the generator[1] to materialise it in one go. The only case where that would break would be when infile == outfile and is a named pipe, but I can't imagine why I would want to both read and write to the same FIFO, other than comedic effect.

  [1] https://github.com/python/cpython/blob/122ca4d73faba279a579aa2011fa34661ce537a2/Lib/json/tool.py#L65