Usage (Zarr V2)

Usage (Zarr V2)

Reading and writing a zarr hieararchy

Reading

The GroupSpec and ArraySpec classes represent Zarr v2 groups and arrays, respectively. To create an instance of a GroupSpec or ArraySpec from an existing Zarr group or array, pass the Zarr group / array to the .from_zarr method defined on the GroupSpec / ArraySpec classes. This will result in a pydantic-zarr model of the Zarr object.

By default GroupSpec.from_zarr(zarr_group) will traverse the entire hierarchy under zarr_group. This can be extremely slow if used on an extensive Zarr group on high latency storage. To limit the depth of traversal to a specific depth, use the depth keyword argument, e.g. GroupSpec.from_zarr(zarr_group, depth=1)

Note that from_zarr will not read the data inside an array.

Writing

To write a hierarchy to some zarr-compatible storage backend, GroupSpec and ArraySpec have to_zarr methods that take a Zarr store and a path and return a Zarr array or group created in the store at the given path.

Note that to_zarr will not write any array data. You have to do this separately.

from zarr import group
from zarr.creation import create
from zarr.storage import MemoryStore
from pydantic_zarr.v2 import GroupSpec

# create an in-memory Zarr group + array with attributes
grp = group(path='foo')
grp.attrs.put({'group_metadata': 10})
arr = create(path='foo/bar', store=grp.store, shape=(10,), compressor=None)
arr.attrs.put({'array_metadata': True})

spec = GroupSpec.from_zarr(grp)
print(spec.model_dump())
"""
{
    'zarr_version': 2,
    'attributes': {'group_metadata': 10},
    'members': {
        'bar': {
            'zarr_version': 2,
            'attributes': {'array_metadata': True},
            'shape': (10,),
            'chunks': (10,),
            'dtype': '<f8',
            'fill_value': 0.0,
            'order': 'C',
            'filters': None,
            'dimension_separator': '.',
            'compressor': None,
        }
    },
}
"""

# convert the spec to a dict so we can modify it
spec_dict2 = spec.model_dump()

# change the group metadata
spec_dict2['attributes'] = {'a': 100, 'b': 'metadata'}

# change the properties of an array member
spec_dict2['members']['bar']['shape'] = (100,)

# serialize the spec to the store
group2 = GroupSpec(**spec_dict2).to_zarr(grp.store, path='foo2')

print(group2)
#> <zarr.hierarchy.Group '/foo2'>

print(dict(group2.attrs))
#> {'a': 100, 'b': 'metadata'}

print(group2['bar'])
#> <zarr.core.Array '/foo2/bar' (100,) float64>

print(dict(group2['bar'].attrs))
#> {'array_metadata': True}

Creating from an array

The ArraySpec class has a from_array static method that takes an array-like object and returns an ArraySpec with shape and dtype fields matching those of the array-like object.

from pydantic_zarr.v2 import ArraySpec
import numpy as np

print(ArraySpec.from_array(np.arange(10)).model_dump())
"""
{
    'zarr_version': 2,
    'attributes': {},
    'shape': (10,),
    'chunks': (10,),
    'dtype': '<i8',
    'fill_value': 0,
    'order': 'C',
    'filters': None,
    'dimension_separator': '/',
    'compressor': None,
}
"""

Flattening and unflattening Zarr hierarchies

In the previous section we built a model of a Zarr hierarchy by defining GroupSpec and ArraySpec instances, then providing those objects as members to the constructor of another GroupSpec. In other words, with this approach we create "child nodes" and give those nodes to the "parent node", recursively.

Constructing deeply nested hierarchies this way can be tedious. For this reason, pydantic-zarr supports an alternative representation of the Zarr hierarchy in the form of a dictionary with str keys and ArraySpec / GroupSpec values, and methods to convert to / from these dictionaries.

Making a GroupSpec object from a flat hierarchy

This example demonstrates how to create a GroupSpec from a dict representation of a Zarr hierarchy.

from pydantic_zarr.v2 import GroupSpec, ArraySpec
# other than the key representing the root path "", 
# the keys must be valid paths in the Zarr storage hierarchy
# note that the `members` attribute is `None` for the `GroupSpec` instances in this `dict`.
tree = {
    "": GroupSpec(members=None, attributes={"root": True}), 
    "/a": GroupSpec(members=None, attributes={"root": False}),
    "/a/b": ArraySpec(shape=(10,10), dtype="uint8", chunks=(1,1))
    }

print(GroupSpec.from_flat(tree).model_dump())
"""
{
    'zarr_version': 2,
    'attributes': {'root': True},
    'members': {
        'a': {
            'zarr_version': 2,
            'attributes': {'root': False},
            'members': {
                'b': {
                    'zarr_version': 2,
                    'attributes': {},
                    'shape': (10, 10),
                    'chunks': (1, 1),
                    'dtype': '|u1',
                    'fill_value': 0,
                    'order': 'C',
                    'filters': None,
                    'dimension_separator': '/',
                    'compressor': None,
                }
            },
        }
    },
}
"""

flattening GroupSpec objects

This is similar to the example above, except that we are working in reverse -- we are making the flat dict from the GroupSpec object.

from pydantic_zarr.v2 import GroupSpec, ArraySpec
# other than the key representing the root path "", 
# the keys must be valid paths in the Zarr storage hierarchy
# note that the `members` attribute is `None` for the `GroupSpec` instances in this `dict`.

a_b = ArraySpec(shape=(10,10), dtype="uint8", chunks=(1,1))
a = GroupSpec(members={'b': a_b}, attributes={"root": False})
root = GroupSpec(members={'a': a}, attributes={"root": True})

print(root.to_flat())
"""
{
    '': GroupSpec(zarr_version=2, attributes={'root': True}, members=None),
    '/a': GroupSpec(zarr_version=2, attributes={'root': False}, members=None),
    '/a/b': ArraySpec(
        zarr_version=2,
        attributes={},
        shape=(10, 10),
        chunks=(1, 1),
        dtype='|u1',
        fill_value=0,
        order='C',
        filters=None,
        dimension_separator='/',
        compressor=None,
    ),
}
"""

Implicit groups

zarr-python supports creating Zarr arrays or groups deep in the hierarchy without explicitly creating the intermediate groups first. from_flat models this behavior. For example, {'/a/b/c': ArraySpec(...)} implicitly defines the existence of a groups named a and b (which is contained in a). from_flat will create the expected GroupSpec object from such dict instances.

from pydantic_zarr.v2 import GroupSpec, ArraySpec
tree = {'/a/b/c': ArraySpec(shape=(1,), dtype='uint8', chunks=(1,))}
print(GroupSpec.from_flat(tree).model_dump())
"""
{
    'zarr_version': 2,
    'attributes': {},
    'members': {
        'a': {
            'zarr_version': 2,
            'attributes': {},
            'members': {
                'b': {
                    'zarr_version': 2,
                    'attributes': {},
                    'members': {
                        'c': {
                            'zarr_version': 2,
                            'attributes': {},
                            'shape': (1,),
                            'chunks': (1,),
                            'dtype': '|u1',
                            'fill_value': 0,
                            'order': 'C',
                            'filters': None,
                            'dimension_separator': '/',
                            'compressor': None,
                        }
                    },
                }
            },
        }
    },
}
"""

Comparing GroupSpec and ArraySpec models

GroupSpec and ArraySpec both have like methods that take another GroupSpec or ArraySpec as an argument and return True (the models are like each other) or False (the models are not like each other).

The like method works by converting both input models to dict via pydantic.BaseModel.model_dump, and comparing the dict representation of the models. This means that instances of two different subclasses of GroupSpec, which would not be considered equal according to the == operator, will be considered like if and only if they serialize to identical dict instances.

The like method takes keyword arguments include and exclude, which determine the attributes included or excluded from the model comparison. So it's possible to use like to check if two ArraySpec instances have the same shape, dtype and chunks by calling array_a.like(array_b, include={'shape', 'dtype', 'chunks'}). This is useful if you don't care about the compressor or filters and just want to ensure that you can safely write an in-memory array to a Zarr array, which depends just on the two arrays having matching shape, dtype, and chunks attributes.

from pydantic_zarr.v2 import ArraySpec, GroupSpec
import zarr
arr_a = ArraySpec(shape=(1,), dtype='uint8', chunks=(1,))
# make an array with a different shape
arr_b = ArraySpec(shape=(2,), dtype='uint8', chunks=(1,))

# Returns False, because of mismatched shape
print(arr_a.like(arr_b)) 
#> False

# Returns True, because we exclude shape.
print(arr_a.like(arr_b, exclude={'shape'}))
#> True

# `ArraySpec.like` will convert a zarr.Array to ArraySpec
store = zarr.MemoryStore()
# This is a zarr.Array
arr_a_stored = arr_a.to_zarr(store, path='arr_a')

# arr_a is like the zarr.Array version of itself
print(arr_a.like(arr_a_stored)) 
#> True

# Returns False, because of mismatched shape
print(arr_b.like(arr_a_stored)) 
#> False

# Returns True, because we exclude shape.
print(arr_b.like(arr_a_stored, exclude={'shape'}))
#> True

# The same thing, but for groups
g_a = GroupSpec(attributes={'foo': 10}, members={'a': arr_a, 'b': arr_b})
g_b = GroupSpec(attributes={'foo': 11}, members={'a': arr_a, 'b': arr_b})

# g_a is like itself
print(g_a.like(g_a))
#> True

# Returns False, because of mismatched attributes
print(g_a.like(g_b)) 
#> False

# Returns True, because we ignore attributes
print(g_a.like(g_b, exclude={'attributes'}))
#> True

# g_a is like its zarr.Group counterpart
print(g_a.like(g_a.to_zarr(store, path='g_a')))
#> True

Using generic types

This example shows how to specialize GroupSpec and ArraySpec with type parameters. By specializing GroupSpec or ArraySpec in this way, python type checkers and Pydantic can type-check elements of a Zarr hierarchy.

import sys
from pydantic_zarr.v2 import GroupSpec, ArraySpec, TItem, TAttr
from pydantic import ValidationError
from typing import Any

if sys.version_info < (3, 12):
    from typing_extensions import TypedDict
else:
    from typing import TypedDict

# a Pydantic BaseModel would also work here
class GroupAttrs(TypedDict):
    a: int
    b: int

# a Zarr group with attributes consistent with GroupAttrs
SpecificAttrsGroup = GroupSpec[GroupAttrs, TItem]

try:
    SpecificAttrsGroup(attributes={'a' : 10, 'b': 'foo'})
except ValidationError as exc:
    print(exc)
    """
    1 validation error for GroupSpec[GroupAttrs, ~TItem]
    attributes.b
      Input should be a valid integer, unable to parse string as an integer [type=int_parsing, input_value='foo', input_type=str]
        For further information visit https://errors.pydantic.dev/2.6/v/int_parsing
    """

# this passes validation
print(SpecificAttrsGroup(attributes={'a': 100, 'b': 100}))
#> zarr_version=2 attributes={'a': 100, 'b': 100} members={}

# a Zarr group that only contains arrays -- no subgroups!
# we re-use the TAttr type variable defined in pydantic_zarr.core
ArraysOnlyGroup = GroupSpec[TAttr, ArraySpec]

try:
    ArraysOnlyGroup(attributes={}, members={'foo': GroupSpec(attributes={})})
except ValidationError as exc:
    print(exc)
    """
    1 validation error for GroupSpec[~TAttr, ArraySpec]
    members.foo
      Input should be a valid dictionary or instance of ArraySpec [type=model_type, input_value=GroupSpec(zarr_version=2,...tributes={}, members={}), input_type=GroupSpec]
        For further information visit https://errors.pydantic.dev/2.6/v/model_type
    """

# this passes validation
items = {'foo': ArraySpec(attributes={},
                          shape=(1,),
                          dtype='uint8',
                          chunks=(1,),
                          compressor=None)}
print(ArraysOnlyGroup(attributes={}, members=items).model_dump())
"""
{
    'zarr_version': 2,
    'attributes': {},
    'members': {
        'foo': {
            'zarr_version': 2,
            'attributes': {},
            'shape': (1,),
            'chunks': (1,),
            'dtype': '|u1',
            'fill_value': 0,
            'order': 'C',
            'filters': None,
            'dimension_separator': '/',
            'compressor': None,
        }
    },
}
"""