reference:arrays.classes

The ndarray 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 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 asanyarray can be used to allow subclasses to propagate more cleanly through your subroutine. In principle, a subclass could redefine any aspect of the array and therefore, under strict guidelines, 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

NumPy provides several hooks that classes can customize:

Matrix objects

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:

Matrix objects can be created using a string notation to allow Matlab-style syntax where spaces separate columns and semicolons (';') separate rows.
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.
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.
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.
The default __array_priority\__ of matrix objects is 10.0, and therefore mixed operations with ndarrays always produce matrices.

Matrices have special attributes which make calculations easier. These are

.. autosummary:: 
    :toctree:generated/
    matrix.T
       matrix.H
       matrix.I
       matrix.A

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

Example 2: Matrix creation from a nested sequence

Example 3: Matrix creation from an array

Memory-mapped file arrays

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): .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:

Character arrays (`numpy.char`)

These are enhanced arrays of either str_ type or bytes_ type. These arrays inherit from the ndarray, but specially-define the operations +, *, and % on a (broadcasting) element-by-element basis. These operations are not available on the standard ndarray of character type. In addition, the chararray has all of the standard str (and bytes) methods, executing them on an element-by-element basis. Perhaps the easiest way to create a chararray is to use 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 chararray constructor, or via the numpy.char.array function:

.. autosummary:: 
    :toctree:generated/
    char.chararray
    char.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.

Record arrays

NumPy provides the recarray class which allows accessing the fields of a structured array as attributes, and a corresponding scalar data type object record.

.. autosummary:: 
    :toctree:generated/
    recarray
    record

Masked arrays (`numpy.ma`)

Standard container class

For backward compatibility and as a standard "container "class, the UserArray from Numeric has been brought over to NumPy and named 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

Array iterators

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 = next(myiter) repeatedly until StopIteration is raised by the iterator. There are several ways to iterate over an array that may be useful: default iteration, flat iteration, and $N$ -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 $N - 1$ from the array. This can be a useful construct for defining recursive algorithms. To loop over the entire array requires $N$ for-loops.

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.

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.

Iterator for broadcasting

.. autosummary:: 
    :toctree:generated/
    broadcast

The general concept of broadcasting is also available from Python using the broadcast iterator. This object takes $N$ objects as inputs and returns an iterator that returns tuples providing each of the input sequence elements in the broadcasted result.