dask.array.from_array

dask.array.from_array(x, chunks='auto', name=None, lock=False, asarray=None, fancy=True, getitem=None, meta=None, inline_array=False)[source]

Create dask array from something that looks like an array.

Input must have a .shape, .ndim, .dtype and support numpy-style slicing.

Parameters
xarray_like
chunksint, tuple

How to chunk the array. Must be one of the following forms:

  • A blocksize like 1000.

  • A blockshape like (1000, 1000).

  • Explicit sizes of all blocks along all dimensions like ((1000, 1000, 500), (400, 400)).

  • A size in bytes, like “100 MiB” which will choose a uniform block-like shape

  • The word “auto” which acts like the above, but uses a configuration value array.chunk-size for the chunk size

-1 or None as a blocksize indicate the size of the corresponding dimension.

namestr or bool, optional

The key name to use for the array. Defaults to a hash of x.

Hashing is useful if the same value of x is used to create multiple arrays, as Dask can then recognise that they’re the same and avoid duplicate computations. However, it can also be slow, and if the array is not contiguous it is copied for hashing. If the array uses stride tricks (such as numpy.broadcast_to() or skimage.util.view_as_windows()) to have a larger logical than physical size, this copy can cause excessive memory usage.

If you don’t need the deduplication provided by hashing, use name=False to generate a random name instead of hashing, which avoids the pitfalls described above. Using name=True is equivalent to the default.

By default, hashing uses python’s standard sha1. This behaviour can be changed by installing cityhash, xxhash or murmurhash. If installed, a large-factor speedup can be obtained in the tokenisation step.

Note

Because this name is used as the key in task graphs, you should ensure that it uniquely identifies the data contained within. If you’d like to provide a descriptive name that is still unique, combine the descriptive name with dask.base.tokenize() of the array_like. See Task Graphs for more.

lockbool or Lock, optional

If x doesn’t support concurrent reads then provide a lock here, or pass in True to have dask.array create one for you.

asarraybool, optional

If True then call np.asarray on chunks to convert them to numpy arrays. If False then chunks are passed through unchanged. If None (default) then we use True if the __array_function__ method is undefined.

fancybool, optional

If x doesn’t support fancy indexing (e.g. indexing with lists or arrays) then set to False. Default is True.

metaArray-like, optional

The metadata for the resulting dask array. This is the kind of array that will result from slicing the input array. Defaults to the input array.

inline_arraybool, default False

How to include the array in the task graph. By default (inline_array=False) the array is included in a task by itself, and each chunk refers to that task by its key.

>>> x = h5py.File("data.h5")["/x"]  
>>> a = da.from_array(x, chunks=500)  
>>> dict(a.dask)  
{
   'array-original-<name>': <HDF5 dataset ...>,
   ('array-<name>', 0): (getitem, "array-original-<name>", ...),
   ('array-<name>', 1): (getitem, "array-original-<name>", ...)
}

With inline_array=True, Dask will instead inline the array directly in the values of the task graph.

>>> a = da.from_array(x, chunks=500, inline_array=True)  
>>> dict(a.dask)  
{
   ('array-<name>', 0): (getitem, <HDF5 dataset ...>, ...),
   ('array-<name>', 1): (getitem, <HDF5 dataset ...>, ...)
}

Note that there’s no key in the task graph with just the array x anymore. Instead it’s placed directly in the values.

The right choice for inline_array depends on several factors, including the size of x, how expensive it is to create, which scheduler you’re using, and the pattern of downstream computations. As a heuristic, inline_array=True may be the right choice when the array x is cheap to serialize and deserialize (since it’s included in the graph many times) and if you’re experiencing ordering issues (see Ordering for more).

This has no effect when x is a NumPy array.

Examples

>>> x = h5py.File('...')['/data/path']  
>>> a = da.from_array(x, chunks=(1000, 1000))  

If your underlying datastore does not support concurrent reads then include the lock=True keyword argument or lock=mylock if you want multiple arrays to coordinate around the same lock.

>>> a = da.from_array(x, chunks=(1000, 1000), lock=True)  

If your underlying datastore has a .chunks attribute (as h5py and zarr datasets do) then a multiple of that chunk shape will be used if you do not provide a chunk shape.

>>> a = da.from_array(x, chunks='auto')  
>>> a = da.from_array(x, chunks='100 MiB')  
>>> a = da.from_array(x)  

If providing a name, ensure that it is unique

>>> import dask.base
>>> token = dask.base.tokenize(x)  
>>> a = da.from_array('myarray-' + token)  

NumPy ndarrays are eagerly sliced and then embedded in the graph.

>>> import dask.array
>>> a = dask.array.from_array(np.array([[1, 2], [3, 4]]), chunks=(1,1))
>>> a.dask[a.name, 0, 0][0]
array([1])

Chunks with exactly-specified, different sizes can be created.

>>> import numpy as np
>>> import dask.array as da
>>> x = np.random.random((100, 6))
>>> a = da.from_array(x, chunks=((67, 33), (6,)))