Accessing existing array with new shape and compressor using zarr.open

[d-v-b]

zarr,convenience.open silently ignores the compressor and shape kwargs if the array already exists:

import zarr, numcodecs, os, shutil

print(f'zarr: {zarr.__version__}')
print(f'numcodecs: {numcodecs.__version__}')
print('python: 3.7')

dirname = 'test.n5'
if os.path.exists(dirname):
    shutil.rmtree(dirname)

# create array 
arr1 = zarr.open(store=zarr.N5Store(dirname), path = 'test', shape=(3,3),compressor=None)
arr1[:] = 1
print(f'shape: {arr1.shape}, compressor: {arr1.compressor}')


# access existing array with new compressor and new shape
arr2 = zarr.open(store=zarr.N5Store(dirname), path = 'test', shape=(5,5),compressor=numcodecs.GZip(-1))
arr2[:] = 1
print(f'shape: {arr2.shape}, compressor: {arr2.compressor}')

This generates the following output:

zarr: 2.3.2
numcodecs: 0.6.3
python: 3.7
shape: (3, 3), compressor: N5ChunkWrapper(chunk_shape=(3, 3), compressor_config=None, dtype=dtype('float64'))
shape: (3, 3), compressor: N5ChunkWrapper(chunk_shape=(3, 3), compressor_config=None, dtype=dtype('float64'))

Problem description

Using zarr.convenience.open to access an existing array does not overwrite the compressor parameters or the shape of that array unless the w access mode is used (i.e. deleting the existing array). This function should either adopt the new compressor / shape or, if that is not possible, emit a warning / error that the user's input will be ignored.

Version and installation information

zarr: 2.3.2 (installed via conda)
numcodecs: 0.6.3
python: 3.7
OS: Ubuntu 18.04