Description
pretty-format-json
modifies floating point numbers that have too many digits of precision.
Background
I have to work with JSON files that may be generated by non-python programs. These files, for whatever reason, have numbers that have up to 16 digits after the decimal place.
(It's absurd, really. The people taking these measurements are somehow able to measure 0.1μHz on a 10GHz scale?? Yeah, they're saving values like 5.9257052820783001 GHz
. Someone needs to teach them about significant figures... but that's beside the point. The point is I have to deal with this data 😒)
Steps to Reproduce
- Create the following json file:
{"foo": 4.4257052820783003}
- Run pretty-format-json on it.
Expected Output:
{
"foo": 4.4257052820783003
}
Actual Output:
{
"foo": 4.4257052820783
}
The diff from expected is:
{
- "foo": 4.4257052820783003
+ "foo": 4.4257052820783
}
Version Info
- pre-commit: 2.19.0
- pre-commit-hooks: v4.2.0
- Python: 3.8.8
- OS: Debian 11
Discussion
This might be something that has to be fixed within the python builtin json
package. A custom JSON encoder/decoder that wraps things using decimal.Decimal
might work too.
I've created a test case for this. See my high-precision-numbers branch or the diff.
I'll see if I have time to actually fix this, but I don't expect to
Activity
asottile commentedon Jun 7, 2022
yeah I suspect this isn't really fixable due to
json
-- but feel free to take a stab at itliortct commentedon Aug 17, 2022
As @asottile said the problem is in

json
standard library.Therefore, this issue isn't fixable in the pre-commit hook and I advice to close this issue.
SilverTux commentedon Oct 6, 2022
Hi, I have a PR #818 to solve this issue. What do you think about it, can you check it please? @dougthor42 @asottile
I've already included the tests for this from @dougthor42's diff
dougthor42 commentedon Oct 7, 2022
@asottile How would you feel about a custom json encoder/decoder that handles floats using
decimal.Decimal
?json.loads
also has aparse_float
arg that might be usable instead or in addition.riccardobucco commentedon Oct 10, 2022
Python has no built-in arbitrary-precision floats. Here is an example:
So it doesn't matter what you use, you can't have a
float
object with arbitrary precision.