Description

dougthor42

opened on Jun 7, 2022

Issue body actions

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

1. Create the following json file:

   {"foo": 4.4257052820783003}

2. 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 ☹️.

Reacted by Simon Kocurek and jvoravong

Activity

1. asottile commented on Jun 7, 2022

   

   asottile

   on Jun 7, 2022

   Member

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   yeah I suspect this isn't really fixable due to json -- but feel free to take a stab at it
2. liortct commented on Aug 17, 2022

   

   liortct

   on Aug 17, 2022

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   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.
3. SilverTux commented on Oct 6, 2022

   

   SilverTux

   on Oct 6, 2022

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   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
4. dougthor42 commented on Oct 7, 2022

   

   dougthor42

   on Oct 7, 2022

   ContributorAuthor

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   @asottile How would you feel about a custom json encoder/decoder that handles floats using decimal.Decimal?

   json.loads also has a parse_float arg that might be usable instead or in addition.
5. riccardobucco commented on Oct 10, 2022

   

   riccardobucco

   on Oct 10, 2022

   More actions

   Python has no built-in arbitrary-precision floats. Here is an example:

   >>> float(4.4257052820783003)
   4.4257052820783
   

   So it doesn't matter what you use, you can't have a float object with arbitrary precision.
6. t-b

   added 2 commits that reference this issue on Feb 26, 2026

   pre-commit: Add JSON pretty printer

   f69fc51

   pre-commit: Add JSON pretty printer

   14eeb22
7. t-b

   added a commit that references this issue on Mar 13, 2026

   pre-commit: Add JSON pretty printer

   cab6c62
8. t-b

   added 2 commits that reference this issue on Mar 23, 2026

   pre-commit: Add JSON pretty printer

   5a9aed2

   pre-commit: Add JSON pretty printer

   d81263d

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