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Zero-sized arrays in zarrs #352
Description
Thank you for zarrs, it is a great library!
I ran into a small issue, where zarrs fails to create arrays where one dimension has size zero:
use anyhow::Result;
use std::sync::Arc;
use zarrs::array::{ArrayBuilder, FillValue, data_type};
use zarrs::storage::store::MemoryStore;
fn main() -> Result<()> {
let store = Arc::new(MemoryStore::new());
// Create an array without zero dimension -- this should work fine.
ArrayBuilder::new(
vec![10, 20, 1],
vec![1, 5, 1],
data_type::float64(),
FillValue::from(0.0f64),
)
.build(store.clone(), "/float_zero")?
.store_metadata()?;
println!("Array without zero dimension created successfully.");
ArrayBuilder::new(
vec![10, 20, 0],
vec![1, 5, 0],
data_type::float64(),
FillValue::from(0.0f64),
)
.build(store.clone(), "/float_zero")?
.store_metadata()?;
println!("Array with zero dimension created successfully.");
Ok(())
}The output is
Array without zero dimension created successfully.
Error: configuration is unsupported: invalid value: integer `0`, expected a nonzero u64
This works fine in zarr:
import zarr
store = zarr.storage.MemoryStore()
array = zarr.create_array(store, name="zero_size", shape=(10, 20, 0), chunks=(1, 5, 0), dtype="float64")zero-sized arrays come up for me surprisingly often, as special cases of more general code.
Activity
LDeakin commentedon Feb 14, 2026
Much appreciated!
Just wondering what do you actually want to do this these arrays?
zarr-pythonis going to give youzarr.errors.BoundsCheckErrorif you try to do anything with them. Or aZeroDivisionError: division by zerootherwise.I'd note that the Zarr V3 specification says this
Is that what you actually want, a scalar?
Alternatively, if you really just want an Array with a shape of zero on any dimension,
zarrsdoes support that. However, theregulargrid is not well defined with zero-sized chunk dimensions. I'm hesitant to change that, because it breaks many of the expectations throughout the library if a chunk were to have a shape of zero.LDeakin commentedon Feb 14, 2026
@d-v-b any thoughts on if
chunks=(1, 5, 0)should just be rejected byzarr-python?aseyboldt commentedon Feb 16, 2026
Ah sorry, I get it now, this was my mistake...
I really do want an array with one axis being zero, and it works just fine if I set a non-zero chunk size. I had tried that as well, but it then failed (correctly, I think) when I then tried to write an empty chunk. I just should not write empty chunks...
Thanks for checking though!
d-v-b commentedon Feb 16, 2026
I think the regular chunk grid spec allows it? At least I can't see where it's explicitly disallowed. We do allow this in zarr python, but I'm guessing that's a side-effect of design rather than trying to be comprehensive. I could be wrong though.
LDeakin commentedon Feb 16, 2026
The division and modulo operations are undefined with zeros
d-v-b commentedon Feb 16, 2026
so they are! in that case zarr-python is out of spec and this needs to be fixed. I assume there are some folks who rely on this feature, so we might end up with a conversation in the spec repo...
d-v-b commentedon Feb 16, 2026
although nobody can have relied on this too much: we don't support accessing the values of such arrays once created: