.. _arrays.classes:
#########################
Standard array subclasses
#########################
.. currentmodule:: numpy
The :class:`ndarray` in NumPy is a "new-style" Python
built-in-type. Therefore, it can be inherited from (in Python or in C)
if desired. Therefore, it can form a foundation for many useful
classes. Often whether to sub-class the array object or to simply use
the core array component as an internal part of a new class is a
difficult decision, and can be simply a matter of choice. NumPy has
several tools for simplifying how your new object interacts with other
array objects, and so the choice may not be significant in the
end. One way to simplify the question is by asking yourself if the
object you are interested in can be replaced as a single array or does
it really require two or more arrays at its core.
Note that :func:`asarray` always returns the base-class ndarray. If
you are confident that your use of the array object can handle any
subclass of an ndarray, then :func:`asanyarray` can be used to allow
subclasses to propagate more cleanly through your subroutine. In
principal a subclass could redefine any aspect of the array and
therefore, under strict guidelines, :func:`asanyarray` would rarely be
useful. However, most subclasses of the array object will not
redefine certain aspects of the array object such as the buffer
interface, or the attributes of the array. One important example,
however, of why your subroutine may not be able to handle an arbitrary
subclass of an array is that matrices redefine the "*" operator to be
matrix-multiplication, rather than element-by-element multiplication.
Special attributes and methods
==============================
.. seealso:: :ref:`Subclassing ndarray <basics.subclassing>`
NumPy provides several hooks that classes can customize:
.. py:method:: class.__array_ufunc__(ufunc, method, *inputs, **kwargs)
.. versionadded:: 1.13
.. note:: The API is `provisional
<https://docs.python.org/3/glossary.html#term-provisional-api>`_,
i.e., we do not yet guarantee backward compatibility.
Any class, ndarray subclass or not, can define this method or set it to
:obj:`None` in order to override the behavior of NumPy's ufuncs. This works
quite similarly to Python's ``__mul__`` and other binary operation routines.
- *ufunc* is the ufunc object that was called.
- *method* is a string indicating which Ufunc method was called
(one of ``"__call__"``, ``"reduce"``, ``"reduceat"``,
``"accumulate"``, ``"outer"``, ``"inner"``).
- *inputs* is a tuple of the input arguments to the ``ufunc``.
- *kwargs* is a dictionary containing the optional input arguments
of the ufunc. If given, any ``out`` arguments, both positional
and keyword, are passed as a :obj:`tuple` in *kwargs*. See the
discussion in :ref:`ufuncs` for details.
The method should return either the result of the operation, or
:obj:`NotImplemented` if the operation requested is not implemented.
If one of the input or output arguments has a :func:`__array_ufunc__`
method, it is executed *instead* of the ufunc. If more than one of the
arguments implements :func:`__array_ufunc__`, they are tried in the
order: subclasses before superclasses, inputs before outputs, otherwise
left to right. The first routine returning something other than
:obj:`NotImplemented` determines the result. If all of the
:func:`__array_ufunc__` operations return :obj:`NotImplemented`, a
:exc:`TypeError` is raised.
.. note:: We intend to re-implement numpy functions as (generalized)
Ufunc, in which case it will become possible for them to be
overridden by the ``__array_ufunc__`` method. A prime candidate is
:func:`~numpy.matmul`, which currently is not a Ufunc, but could be
relatively easily be rewritten as a (set of) generalized Ufuncs. The
same may happen with functions such as :func:`~numpy.median`,
:func:`~numpy.min`, and :func:`~numpy.argsort`.
Like with some other special methods in python, such as ``__hash__`` and
``__iter__``, it is possible to indicate that your class does *not*
support ufuncs by setting ``__array_ufunc__ = None``. Ufuncs always raise
:exc:`TypeError` when called on an object that sets
``__array_ufunc__ = None``.
The presence of :func:`__array_ufunc__` also influences how
:class:`ndarray` handles binary operations like ``arr + obj`` and ``arr
< obj`` when ``arr`` is an :class:`ndarray` and ``obj`` is an instance
of a custom class. There are two possibilities. If
``obj.__array_ufunc__`` is present and not :obj:`None`, then
``ndarray.__add__`` and friends will delegate to the ufunc machinery,
meaning that ``arr + obj`` becomes ``np.add(arr, obj)``, and then
:func:`~numpy.add` invokes ``obj.__array_ufunc__``. This is useful if you
want to define an object that acts like an array.
Alternatively, if ``obj.__array_ufunc__`` is set to :obj:`None`, then as a
special case, special methods like ``ndarray.__add__`` will notice this
and *unconditionally* raise :exc:`TypeError`. This is useful if you want to
create objects that interact with arrays via binary operations, but
are not themselves arrays. For example, a units handling system might have
an object ``m`` representing the "meters" unit, and want to support the
syntax ``arr * m`` to represent that the array has units of "meters", but
not want to otherwise interact with arrays via ufuncs or otherwise. This
can be done by setting ``__array_ufunc__ = None`` and defining ``__mul__``
and ``__rmul__`` methods. (Note that this means that writing an
``__array_ufunc__`` that always returns :obj:`NotImplemented` is not
quite the same as setting ``__array_ufunc__ = None``: in the former
case, ``arr + obj`` will raise :exc:`TypeError`, while in the latter
case it is possible to define a ``__radd__`` method to prevent this.)
The above does not hold for in-place operators, for which :class:`ndarray`
never returns :obj:`NotImplemented`. Hence, ``arr += obj`` would always
lead to a :exc:`TypeError`. This is because for arrays in-place operations
cannot generically be replaced by a simple reverse operation. (For
instance, by default, ``arr += obj`` would be translated to ``arr =
arr + obj``, i.e., ``arr`` would be replaced, contrary to what is expected
for in-place array operations.)
.. note:: If you define ``__array_ufunc__``:
- If you are not a subclass of :class:`ndarray`, we recommend your
class define special methods like ``__add__`` and ``__lt__`` that
delegate to ufuncs just like ndarray does. An easy way to do this
is to subclass from :class:`~numpy.lib.mixins.NDArrayOperatorsMixin`.
- If you subclass :class:`ndarray`, we recommend that you put all your
override logic in ``__array_ufunc__`` and not also override special
methods. This ensures the class hierarchy is determined in only one
place rather than separately by the ufunc machinery and by the binary
operation rules (which gives preference to special methods of
subclasses; the alternative way to enforce a one-place only hierarchy,
of setting :func:`__array_ufunc__` to :obj:`None`, would seem very
unexpected and thus confusing, as then the subclass would not work at
all with ufuncs).
- :class:`ndarray` defines its own :func:`__array_ufunc__`, which,
evaluates the ufunc if no arguments have overrides, and returns
:obj:`NotImplemented` otherwise. This may be useful for subclasses
for which :func:`__array_ufunc__` converts any instances of its own
class to :class:`ndarray`: it can then pass these on to its
superclass using ``super().__array_ufunc__(*inputs, **kwargs)``,
and finally return the results after possible back-conversion. The
advantage of this practice is that it ensures that it is possible
to have a hierarchy of subclasses that extend the behaviour. See
:ref:`Subclassing ndarray <basics.subclassing>` for details.
.. note:: If a class defines the :func:`__array_ufunc__` method,
this disables the :func:`__array_wrap__`,
:func:`__array_prepare__`, :data:`__array_priority__` mechanism
described below for ufuncs (which may eventually be deprecated).
.. py:method:: class.__array_finalize__(obj)
This method is called whenever the system internally allocates a
new array from *obj*, where *obj* is a subclass (subtype) of the
:class:`ndarray`. It can be used to change attributes of *self*
after construction (so as to ensure a 2-d matrix for example), or
to update meta-information from the "parent." Subclasses inherit
a default implementation of this method that does nothing.
.. py:method:: class.__array_prepare__(array, context=None)
At the beginning of every :ref:`ufunc <ufuncs.output-type>`, this
method is called on the input object with the highest array
priority, or the output object if one was specified. The output
array is passed in and whatever is returned is passed to the ufunc.
Subclasses inherit a default implementation of this method which
simply returns the output array unmodified. Subclasses may opt to
use this method to transform the output array into an instance of
the subclass and update metadata before returning the array to the
ufunc for computation.
.. note:: For ufuncs, it is hoped to eventually deprecate this method in
favour of :func:`__array_ufunc__`.
.. py:method:: class.__array_wrap__(array, context=None)
At the end of every :ref:`ufunc <ufuncs.output-type>`, this method
is called on the input object with the highest array priority, or
the output object if one was specified. The ufunc-computed array
is passed in and whatever is returned is passed to the user.
Subclasses inherit a default implementation of this method, which
transforms the array into a new instance of the object's class.
Subclasses may opt to use this method to transform the output array
into an instance of the subclass and update metadata before
returning the array to the user.
.. note:: For ufuncs, it is hoped to eventually deprecate this method in
favour of :func:`__array_ufunc__`.
.. py:attribute:: class.__array_priority__
The value of this attribute is used to determine what type of
object to return in situations where there is more than one
possibility for the Python type of the returned object. Subclasses
inherit a default value of 0.0 for this attribute.
.. note:: For ufuncs, it is hoped to eventually deprecate this method in
favour of :func:`__array_ufunc__`.
.. py:method:: class.__array__([dtype])
If a class (ndarray subclass or not) having the :func:`__array__`
method is used as the output object of an :ref:`ufunc
<ufuncs.output-type>`, results will be written to the object
returned by :func:`__array__`. Similar conversion is done on
input arrays.
Matrix objects
==============
.. index::
single: matrix
:class:`matrix` objects inherit from the ndarray and therefore, they
have the same attributes and methods of ndarrays. There are six
important differences of matrix objects, however, that may lead to
unexpected results when you use matrices but expect them to act like
arrays:
1. Matrix objects can be created using a string notation to allow
Matlab-style syntax where spaces separate columns and semicolons
(';') separate rows.
2. Matrix objects are always two-dimensional. This has far-reaching
implications, in that m.ravel() is still two-dimensional (with a 1
in the first dimension) and item selection returns two-dimensional
objects so that sequence behavior is fundamentally different than
arrays.
3. Matrix objects over-ride multiplication to be
matrix-multiplication. **Make sure you understand this for
functions that you may want to receive matrices. Especially in
light of the fact that asanyarray(m) returns a matrix when m is
a matrix.**
4. Matrix objects over-ride power to be matrix raised to a power. The
same warning about using power inside a function that uses
asanyarray(...) to get an array object holds for this fact.
5. The default __array_priority\__ of matrix objects is 10.0, and
therefore mixed operations with ndarrays always produce matrices.
6. Matrices have special attributes which make calculations easier.
These are
.. autosummary::
:toctree: generated/
matrix.T
matrix.H
matrix.I
matrix.A
.. warning::
Matrix objects over-ride multiplication, '*', and power, '**', to
be matrix-multiplication and matrix power, respectively. If your
subroutine can accept sub-classes and you do not convert to base-
class arrays, then you must use the ufuncs multiply and power to
be sure that you are performing the correct operation for all
inputs.
The matrix class is a Python subclass of the ndarray and can be used
as a reference for how to construct your own subclass of the ndarray.
Matrices can be created from other matrices, strings, and anything
else that can be converted to an ``ndarray`` . The name "mat "is an
alias for "matrix "in NumPy.
.. autosummary::
:toctree: generated/
matrix
asmatrix
bmat
Example 1: Matrix creation from a string
>>> a=mat('1 2 3; 4 5 3')
>>> print (a*a.T).I
[[ 0.2924 -0.1345]
[-0.1345 0.0819]]
Example 2: Matrix creation from nested sequence
>>> mat([[1,5,10],[1.0,3,4j]])
matrix([[ 1.+0.j, 5.+0.j, 10.+0.j],
[ 1.+0.j, 3.+0.j, 0.+4.j]])
Example 3: Matrix creation from an array
>>> mat(random.rand(3,3)).T
matrix([[ 0.7699, 0.7922, 0.3294],
[ 0.2792, 0.0101, 0.9219],
[ 0.3398, 0.7571, 0.8197]])
Memory-mapped file arrays
=========================
.. index::
single: memory maps
.. currentmodule:: numpy
Memory-mapped files are useful for reading and/or modifying small
segments of a large file with regular layout, without reading the
entire file into memory. A simple subclass of the ndarray uses a
memory-mapped file for the data buffer of the array. For small files,
the over-head of reading the entire file into memory is typically not
significant, however for large files using memory mapping can save
considerable resources.
Memory-mapped-file arrays have one additional method (besides those
they inherit from the ndarray): :meth:`.flush() <memmap.flush>` which
must be called manually by the user to ensure that any changes to the
array actually get written to disk.
.. autosummary::
:toctree: generated/
memmap
memmap.flush
Example:
>>> a = memmap('newfile.dat', dtype=float, mode='w+', shape=1000)
>>> a[10] = 10.0
>>> a[30] = 30.0
>>> del a
>>> b = fromfile('newfile.dat', dtype=float)
>>> print b[10], b[30]
10.0 30.0
>>> a = memmap('newfile.dat', dtype=float)
>>> print a[10], a[30]
10.0 30.0
Character arrays (:mod:`numpy.char`)
====================================
.. seealso:: :ref:`routines.array-creation.char`
.. index::
single: character arrays
.. note::
The `chararray` class exists for backwards compatibility with
Numarray, it is not recommended for new development. Starting from numpy
1.4, if one needs arrays of strings, it is recommended to use arrays of
`dtype` `object_`, `string_` or `unicode_`, and use the free functions
in the `numpy.char` module for fast vectorized string operations.
These are enhanced arrays of either :class:`string_` type or
:class:`unicode_` type. These arrays inherit from the
:class:`ndarray`, but specially-define the operations ``+``, ``*``,
and ``%`` on a (broadcasting) element-by-element basis. These
operations are not available on the standard :class:`ndarray` of
character type. In addition, the :class:`chararray` has all of the
standard :class:`string <str>` (and :class:`unicode`) methods,
executing them on an element-by-element basis. Perhaps the easiest
way to create a chararray is to use :meth:`self.view(chararray)
<ndarray.view>` where *self* is an ndarray of str or unicode
data-type. However, a chararray can also be created using the
:meth:`numpy.chararray` constructor, or via the
:func:`numpy.char.array <core.defchararray.array>` function:
.. autosummary::
:toctree: generated/
chararray
core.defchararray.array
Another difference with the standard ndarray of str data-type is
that the chararray inherits the feature introduced by Numarray that
white-space at the end of any element in the array will be ignored
on item retrieval and comparison operations.
.. _arrays.classes.rec:
Record arrays (:mod:`numpy.rec`)
================================
.. seealso:: :ref:`routines.array-creation.rec`, :ref:`routines.dtype`,
:ref:`arrays.dtypes`.
NumPy provides the :class:`recarray` class which allows accessing the
fields of a structured array as attributes, and a corresponding
scalar data type object :class:`record`.
.. currentmodule:: numpy
.. autosummary::
:toctree: generated/
recarray
record
Masked arrays (:mod:`numpy.ma`)
===============================
.. seealso:: :ref:`maskedarray`
Standard container class
========================
.. currentmodule:: numpy
For backward compatibility and as a standard "container "class, the
UserArray from Numeric has been brought over to NumPy and named
:class:`numpy.lib.user_array.container` The container class is a
Python class whose self.array attribute is an ndarray. Multiple
inheritance is probably easier with numpy.lib.user_array.container
than with the ndarray itself and so it is included by default. It is
not documented here beyond mentioning its existence because you are
encouraged to use the ndarray class directly if you can.
.. autosummary::
:toctree: generated/
numpy.lib.user_array.container
.. index::
single: user_array
single: container class
Array Iterators
===============
.. currentmodule:: numpy
.. index::
single: array iterator
Iterators are a powerful concept for array processing. Essentially,
iterators implement a generalized for-loop. If *myiter* is an iterator
object, then the Python code::
for val in myiter:
...
some code involving val
...
calls ``val = myiter.next()`` repeatedly until :exc:`StopIteration` is
raised by the iterator. There are several ways to iterate over an
array that may be useful: default iteration, flat iteration, and
:math:`N`-dimensional enumeration.
Default iteration
-----------------
The default iterator of an ndarray object is the default Python
iterator of a sequence type. Thus, when the array object itself is
used as an iterator. The default behavior is equivalent to::
for i in range(arr.shape[0]):
val = arr[i]
This default iterator selects a sub-array of dimension :math:`N-1`
from the array. This can be a useful construct for defining recursive
algorithms. To loop over the entire array requires :math:`N` for-loops.
>>> a = arange(24).reshape(3,2,4)+10
>>> for val in a:
... print 'item:', val
item: [[10 11 12 13]
[14 15 16 17]]
item: [[18 19 20 21]
[22 23 24 25]]
item: [[26 27 28 29]
[30 31 32 33]]
Flat iteration
--------------
.. autosummary::
:toctree: generated/
ndarray.flat
As mentioned previously, the flat attribute of ndarray objects returns
an iterator that will cycle over the entire array in C-style
contiguous order.
>>> for i, val in enumerate(a.flat):
... if i%5 == 0: print i, val
0 10
5 15
10 20
15 25
20 30
Here, I've used the built-in enumerate iterator to return the iterator
index as well as the value.
N-dimensional enumeration
-------------------------
.. autosummary::
:toctree: generated/
ndenumerate
Sometimes it may be useful to get the N-dimensional index while
iterating. The ndenumerate iterator can achieve this.
>>> for i, val in ndenumerate(a):
... if sum(i)%5 == 0: print i, val
(0, 0, 0) 10
(1, 1, 3) 25
(2, 0, 3) 29
(2, 1, 2) 32
Iterator for broadcasting
-------------------------
.. autosummary::
:toctree: generated/
broadcast
The general concept of broadcasting is also available from Python
using the :class:`broadcast` iterator. This object takes :math:`N`
objects as inputs and returns an iterator that returns tuples
providing each of the input sequence elements in the broadcasted
result.
>>> for val in broadcast([[1,0],[2,3]],[0,1]):
... print val
(1, 0)
(0, 1)
(2, 0)
(3, 1)