Write dask array to a stack of .npy files

dask/array/__init__.py

@@ -4,7 +4,7 @@
 from .core import (Array, stack, concatenate, take, tensordot, transpose,
         from_array, choose, where, coarsen, insert, broadcast_to, fromfunction,
         unique, store, squeeze, topk, bincount, histogram, map_blocks, atop,
-        to_hdf5, dot, cov, array)
+        to_hdf5, dot, cov, array, to_npy_stack, from_npy_stack)
 from .core import (logaddexp, logaddexp2, conj, exp, log, log2, log10, log1p,
         expm1, sqrt, square, sin, cos, tan, arcsin, arccos, arctan, arctan2,
         hypot, sinh, cosh, tanh, arcsinh, arccosh, arctanh, deg2rad, rad2deg,

dask/array/core.py

@@ -14,6 +14,9 @@
                            memoize, map, groupby, valmap, accumulate, merge,
                            curry, reduce, interleave, sliding_window, partial)
 import numpy as np
+import os
+import pickle
+import uuid
 
 from threading import Lock
 from . import chunk
@@ -1089,6 +1092,7 @@ def squeeze(self):
         return squeeze(self)
 
     def rechunk(self, chunks):
+        """ See da.rechunk for docstring """
         from .rechunk import rechunk
         return rechunk(self, chunks)
 
@@ -2580,3 +2584,70 @@ def cov(m, y=None, rowvar=1, bias=0, ddof=None):
         return (dot(X.T, X.conj()) / fact).squeeze()
     else:
         return (dot(X, X.T.conj()) / fact).squeeze()
+
+
+def to_npy_stack(dirname, x, axis=0):
+    """ Write dask array to a stack of .npy files
+
+    This partitions the dask.array along one axis and stores each block along
+    that axis as a single .npy file in the specified directory
+
+    Example
+    -------
+
+    >>> x = da.ones((5, 10, 10), chunks=(2, 4, 4))  # doctest: +SKIP
+    >>> da.to_npy_stack('data/', x, axis=0)  # doctest: +SKIP
+
+    $ tree data/
+    data/
+    |-- 0.npy
+    |-- 1.npy
+    |-- 2.npy
+    |-- info
+
+    The ``.npy`` files store numpy arrays for ``x[0:2], x[2:4], and x[4:5]``
+    respectively, as is specified by the chunk size along the zeroth axis.  The
+    info file stores the dtype, chunks, and axis information of the array.
+
+    You can load these stacks with the ``da.from_npy_stack`` function.
+
+    >>> y = da.from_npy_stack('data/')  # doctest: +SKIP
+
+    See also:
+        from_npy_stack
+    """
+
+    chunks = tuple((c if i == axis else (sum(c),))
+                   for i, c in enumerate(x.chunks))
+    xx = x.rechunk(chunks)
+
+    if not os.path.exists(dirname):
+        os.path.mkdir(dirname)
+
+    meta = {'chunks': chunks, 'dtype': x.dtype, 'axis': axis}
+
+    with open(os.path.join(dirname, 'info'), 'wb') as f:
+        pickle.dump(meta, f)
+
+    name = 'to-npy-stack-' + str(uuid.uuid1())
+    dsk = dict(((name, i), (np.save, os.path.join(dirname, '%d.npy' % i), key))
+              for i, key in enumerate(core.flatten(xx._keys())))
+
+    Array._get(merge(dsk, xx.dask), list(dsk))
+
+def from_npy_stack(dirname):
+    """ See da.to_npy_stack for docstring """
+    with open(os.path.join(dirname, 'info'), 'rb') as f:
+        info = pickle.load(f)
+
+    dtype = info['dtype']
+    chunks = info['chunks']
+    axis = info['axis']
+
+    name = 'from-npy-stack-%s' % dirname
+    keys = list(product([name], *[range(len(c)) for c in chunks]))
+    values = [(np.load, os.path.join(dirname, '%d.npy' % i))
+              for i in range(len(chunks[axis]))]
+    dsk = dict(zip(keys, values))
+
+    return Array(dsk, name, chunks, dtype)

dask/array/tests/test_array_core.py

@@ -4,6 +4,9 @@
 pytest.importorskip('numpy')
 
 from operator import add
+from tempfile import mkdtemp
+import shutil
+import os
 
 from toolz import merge
 from toolz.curried import identity
@@ -1401,3 +1404,34 @@ def test_cov():
     assert eq(da.cov(e, d), np.cov(y, x))
 
     assert raises(ValueError, lambda: da.cov(d, ddof=1.5))
+
+
+def test_memmap():
+    with tmpfile('npy') as fn_1:
+        with tmpfile('npy') as fn_2:
+            x = da.arange(100, chunks=15)
+            target = np.memmap(fn_1, shape=x.shape, mode='w+', dtype=x.dtype)
+
+            x.store(target)
+
+            assert eq(target, x)
+
+            np.save(fn_2, target)
+
+            assert eq(np.load(fn_2, mmap_mode='r'), x)
+
+
+def test_to_npy_stack():
+    x = np.arange(5*10*10).reshape((5, 10, 10))
+    d = da.from_array(x, chunks=(2, 4, 4))
+
+    dirname = mkdtemp()
+    try:
+        da.to_npy_stack(dirname, d, axis=0)
+        assert os.path.exists(os.path.join(dirname, '0.npy'))
+        assert (np.load(os.path.join(dirname, '1.npy')) == x[2:4]).all()
+
+        e = da.from_npy_stack(dirname)
+        assert eq(d, e)
+    finally:
+        shutil.rmtree(dirname)