/
stride_tricks.py
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/
stride_tricks.py
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"""
Utilities that manipulate strides to achieve desirable effects.
An explanation of strides can be found in the "ndarray.rst" file in the
NumPy reference guide.
"""
from __future__ import division, absolute_import, print_function
import numpy as np
__all__ = ['broadcast_to', 'broadcast_arrays']
class DummyArray(object):
"""Dummy object that just exists to hang __array_interface__ dictionaries
and possibly keep alive a reference to a base array.
"""
def __init__(self, interface, base=None):
self.__array_interface__ = interface
self.base = base
def _maybe_view_as_subclass(original_array, new_array):
if type(original_array) is not type(new_array):
# if input was an ndarray subclass and subclasses were OK,
# then view the result as that subclass.
new_array = new_array.view(type=type(original_array))
# Since we have done something akin to a view from original_array, we
# should let the subclass finalize (if it has it implemented, i.e., is
# not None).
if new_array.__array_finalize__:
new_array.__array_finalize__(original_array)
return new_array
def as_strided(x, shape=None, strides=None, subok=False):
""" Make an ndarray from the given array with the given shape and strides.
"""
# first convert input to array, possibly keeping subclass
x = np.array(x, copy=False, subok=subok)
interface = dict(x.__array_interface__)
if shape is not None:
interface['shape'] = tuple(shape)
if strides is not None:
interface['strides'] = tuple(strides)
array = np.asarray(DummyArray(interface, base=x))
if array.dtype.fields is None and x.dtype.fields is not None:
# This should only happen if x.dtype is [('', 'Vx')]
array.dtype = x.dtype
return _maybe_view_as_subclass(x, array)
def _broadcast_to(array, shape, subok, readonly):
shape = tuple(shape) if np.iterable(shape) else (shape,)
array = np.array(array, copy=False, subok=subok)
if not shape and array.shape:
raise ValueError('cannot broadcast a non-scalar to a scalar array')
if any(size < 0 for size in shape):
raise ValueError('all elements of broadcast shape must be non-'
'negative')
broadcast = np.nditer(
(array,), flags=['multi_index', 'refs_ok', 'zerosize_ok'],
op_flags=['readonly'], itershape=shape, order='C').itviews[0]
result = _maybe_view_as_subclass(array, broadcast)
if not readonly and array.flags.writeable:
result.flags.writeable = True
return result
def broadcast_to(array, shape, subok=False):
"""Broadcast an array to a new shape.
Parameters
----------
array : array_like
The array to broadcast.
shape : tuple
The shape of the desired array.
subok : bool, optional
If True, then sub-classes will be passed-through, otherwise
the returned array will be forced to be a base-class array (default).
Returns
-------
broadcast : array
A readonly view on the original array with the given shape. It is
typically not contiguous. Furthermore, more than one element of a
broadcasted array may refer to a single memory location.
Raises
------
ValueError
If the array is not compatible with the new shape according to NumPy's
broadcasting rules.
Notes
-----
.. versionadded:: 1.10.0
Examples
--------
>>> x = np.array([1, 2, 3])
>>> np.broadcast_to(x, (3, 3))
array([[1, 2, 3],
[1, 2, 3],
[1, 2, 3]])
"""
return _broadcast_to(array, shape, subok=subok, readonly=True)
def _broadcast_shape(*args):
"""Returns the shape of the ararys that would result from broadcasting the
supplied arrays against each other.
"""
if not args:
raise ValueError('must provide at least one argument')
if len(args) == 1:
# a single argument does not work with np.broadcast
return np.asarray(args[0]).shape
# use the old-iterator because np.nditer does not handle size 0 arrays
# consistently
b = np.broadcast(*args[:32])
# unfortunately, it cannot handle 32 or more arguments directly
for pos in range(32, len(args), 31):
# ironically, np.broadcast does not properly handle np.broadcast
# objects (it treats them as scalars)
# use broadcasting to avoid allocating the full array
b = broadcast_to(0, b.shape)
b = np.broadcast(b, *args[pos:(pos + 31)])
return b.shape
def broadcast_arrays(*args, **kwargs):
"""
Broadcast any number of arrays against each other.
Parameters
----------
`*args` : array_likes
The arrays to broadcast.
subok : bool, optional
If True, then sub-classes will be passed-through, otherwise
the returned arrays will be forced to be a base-class array (default).
Returns
-------
broadcasted : list of arrays
These arrays are views on the original arrays. They are typically
not contiguous. Furthermore, more than one element of a
broadcasted array may refer to a single memory location. If you
need to write to the arrays, make copies first.
Examples
--------
>>> x = np.array([[1,2,3]])
>>> y = np.array([[1],[2],[3]])
>>> np.broadcast_arrays(x, y)
[array([[1, 2, 3],
[1, 2, 3],
[1, 2, 3]]), array([[1, 1, 1],
[2, 2, 2],
[3, 3, 3]])]
Here is a useful idiom for getting contiguous copies instead of
non-contiguous views.
>>> [np.array(a) for a in np.broadcast_arrays(x, y)]
[array([[1, 2, 3],
[1, 2, 3],
[1, 2, 3]]), array([[1, 1, 1],
[2, 2, 2],
[3, 3, 3]])]
"""
# nditer is not used here to avoid the limit of 32 arrays.
# Otherwise, something like the following one-liner would suffice:
# return np.nditer(args, flags=['multi_index', 'zerosize_ok'],
# order='C').itviews
subok = kwargs.pop('subok', False)
if kwargs:
raise TypeError('broadcast_arrays() got an unexpected keyword '
'argument {}'.format(kwargs.pop()))
args = [np.array(_m, copy=False, subok=subok) for _m in args]
shape = _broadcast_shape(*args)
if all(array.shape == shape for array in args):
# Common case where nothing needs to be broadcasted.
return args
# TODO: consider making the results of broadcast_arrays readonly to match
# broadcast_to. This will require a deprecation cycle.
return [_broadcast_to(array, shape, subok=subok, readonly=False)
for array in args]