Floats are serialized with unnecessary decimal place

[nwoltman]

With this struct definition:

#[derive(Serialize)]
struct MyStruct {
    a: f64,
    b: f64,
}

Instantiated like this:

MyStruct {
    a: 1.0,
    b: 1.2
}

serde_json serializes the struct like this:

{
  "a": 1.0,
  "b": 1.2
}

But I'd expect the serialized output to be:

{
  "a": 1,
  "b": 1.2
}

Even though the JSON that serde produces is technically valid, it's different from how most other languages serialize floats, and it's even different from how rust's built-in formatter formats floats: format!("{}", 1.0f64) returns 1.

[ssokolow]

On the other hand, it does provide a hint to languages without implicit integer→float conversion that the value should be deserialized as a float, not an integer. Whether that's useful in a real-world context or a footgun is anybody's guess though.

(The question is how likely such a program is to encounter a whole number during testing rather than production.)

[nwoltman]

it does provide a hint to languages without implicit integer→float conversion that the value should be deserialized as a float, not an integer.

Perhaps that is useful. Maybe there could be an option to choose how the JSON gets formatted?

[dtolnay]

This is intentional, but can be customized by providing a Formatter impl with the behavior you want.
https://docs.rs/serde_json/1.0.56/serde_json/struct.Serializer.html#method.with_formatter

[JaydenElliott]

For anyone with this problem but only want to change the behavior of one struct field, the following may be a good alternative to writing your own formatter:

#[derive(Serialize)]
struct Example {
  #[serde(serialize_with = "custom_func")]
  x: f32
}

[tr3ysmith]

I guess my bigger question to this, is why is this intentional? This isn't common with web based applications. I'm running into an issue right now where I have a rust application talking to a web application and they're trying to compare their JSON payloads byte wise, and this is causing indifferences.

@dtolnay

[fawdlstty]

I have a question that How to serialize an serde_json::Value object and specify the number of decimal places for all floating-point types inside it?

[nyurik]

What's even more concerning is that my struct with f64 floats whose value is 180.0 sometimes serializes to text with an extra .0, and sometimes without it - depending if it runs in GitHub action or on my linux laptop. This makes integration testing much harder as the values are less predictable.