Source

py-sdl2 / doc / modules / sdl2ext_array.rst

Full commit
.. currentmodule:: sdl2.ext

Converting sequences
====================
This module provides various functions and classes to access sequences and
buffer-style objects in different ways. It also provides conversion routines
to improve the interoperability of sequences with :mod:`ctypes` data types.

Providing read-write access for sequential data
-----------------------------------------------
Two classes allow you to access sequential data in different ways. The
:class:`CTypesView` provides byte-wise access to iterable objects and allows
you to convert the object representation to matching byte-widths for
:mod:`ctypes` or other modules.

Depending on the the underlying object and the chosen size of each particular
item of the object, the :class:`CTypesView` allows you to operate directly
on different representations of the object's contents. ::

    >>> text = bytearray("Hello, I am a simple ASCII string!")
    >>> ctview = CTypesView(text, itemsize=1)
    >>> ctview.view[0] = 0x61
    >>> print(text)
    aello, I am a simple ASCII string!"
    >>> ctview.to_uint16()[3] = 0x6554
    >>> print(text)
    aello,Te am a simple ASCII string!"

The snippet above provides a single-byte sized view on a :func:`bytearray`
object. Afterwards, the first item of the view is changed, which causes a
change on the :func:`bytearray`, on the first item as well, since both, the
:class:`CTypesView` and the :func:`bytearray` provide a byte-wise access to
the contents.

By using :meth:`CTypesView.to_uint16()`, we change the access representation to
a 2-byte unsigned integer :mod:`ctypes` pointer and change the fourth 2-byte
value, *I* to something else. ::

    >>> text = bytearray("Hello, I am a simple ASCII string!")
    >>> ctview = CTypesView(text, itemsize=2)
    >>> ctview.view[0] = 0x61
    >>> print(text)
    aello, I am a simple ASCII string!"
    >>> ctview.to_uint16()[3] = 0x6554
    >>> print(text)    aello,Te am a simple ASCII string!"

If the encapsuled object does not provide a (writable) :func:`buffer`
interface, but is iterable, the :class:`CTypesView` will create an
internal copy of the object data using Python's :mod:`array` module and
perform all operations on that copy. ::

    >>> mylist = [18, 52, 86, 120, 154, 188, 222, 240]
    >>> ctview = CTypesView(mylist, itemsize=1, docopy=True)
    >>> print(ctview.object)
    array('B', [18, 52, 86, 120, 154, 188, 222, 240])
    >>> ctview.view[3] = 0xFF
    >>> print(mylist)
    [18, 52, 86, 120, 154, 188, 222, 240]
    >>> print(ctview.object)
    array('B', [18, 52, 86, 255, 154, 188, 222, 240])

As for directly accessible objects, you can define your own itemsize to
be used. If the iterable does not provide a direct byte access to their
contents, this won't have any effect except for resizing the item
widths. ::

    >>> mylist = [18, 52, 86, 120, 154, 188, 222, 240]
    >>> ctview = CTypesView(mylist, itemsize=4, docopy=True)
    >>> print(ctview.object)
    array('I', [18L, 52L, 86L, 120L, 154L, 188L, 222L, 240L])

Accessing data over multiple dimensions
---------------------------------------
The second class, :class:`MemoryView` provides an interface to access
data over multiple dimensions. You can layout and access a simple
byte stream over e.g. two or more axes, providing a greater flexibility
for functional operations and complex data.

Let's assume, we are reading image data from a file stream into some buffer
object and want to access and manipulate the image data. Images feature two
axes, one being the width, the other being the height, defining a rectangular
graphics area.

When we read all data from the file, we have an one-dimensional view of the
image graphics. The :class:`MemoryView` allows us to define a
two-dimensional view over the image graphics, so that we can operate on
both, rows and columns of the image. ::

    >>> imagedata = bytearray("some 1-byte graphics data")
    >>> view = MemoryView(imagedata, 1, (5, 5))
    >>> print(view)
    [[s, o, m, e,  ], [1, -, b, y, t], [e,  , g, r, a], [p, h, i, c, s], [ , d, a, t, a]]
    >>> for row in view:
    ...     print(row)
    ...
    [s, o, m, e,  ]
    [1, -, b, y, t]
    [e,  , g, r, a]
    [p, h, i, c, s]
    [ , d, a, t, a]
    >>> for row in view:
    ...    row[1] = "X"
    ...    print row
    ...
    [s, X, m, e,  ]
    [1, X, b, y, t]
    [e, X, g, r, a]
    [p, X, i, c, s]
    [ , X, a, t, a]
    >>> print(imagedata)
    sXme 1XbyteXgrapXics Xata

On accessing a particular dimension of a :class:`MemoryView`, a new
:class:`MemoryView` is created, if it does not access a single
element. ::

    >>> firstrow = view[0]
    >>> type(firstrow)
    <class 'sdl2.ext.array.MemoryView'>
    >>> type(firstrow[0])
    <type 'bytearray'>

A :class:`MemoryView` features, similar to Python's builtin
:class:`memoryview`, dimensions and strides, accessible via the
:attr:`MemoryView.ndim` and :attr:`MemoryView.strides` attributes.

    >>> view.ndim
    2
    >>> view.strides
    (5, 5)

The :attr:`MemoryView.strides`, which have to be passed on creating a
new :class:`MemoryView`, define the layout of the data over different
dimensions. In the example above, we created a 5x5 two-dimensional view
to the image graphics. ::

    >>> twobytes = MemoryView(imagedata, 2, (5, 1))
    >>> print(twobytes)
    [[sX, me,  1, Xb, yt], [eX, gr, ap, Xi, cs]]


Array API
---------

.. class:: CTypesView(obj : iterable[, itemsize=1[, docopy=False[, objsize=None]]])

   A proxy class for byte-wise accessible data types to be used in
   ctypes bindings. The CTypesView provides a read-write access to
   arbitrary objects that are iterable.

   In case the object does not provide a :func:`buffer()` interface for
   direct access, the CTypesView can copy the object's contents into an
   internal buffer, from which data can be retrieved, once the necessary
   operations have been performed.

   Depending on the item type stored in the iterable object, you might
   need to provide a certain *itemsize*, which denotes the size per
   item in bytes. The *objsize* argument might be necessary of iterables,
   for which len() does not return the correct amount of objects or is not
   implemented.

   .. attribute:: bytesize

      Returns the length of the encapsuled object in bytes.

   .. attribute:: is_shared

      Indicates, if changes on the CTypesView data effect the encapsuled
      object directly. if not, this means that the object was copied
      internally and needs to be updated by the user code outside of the
      CTypesView.

   .. attribute:: object

      The encapsuled object.

   .. attribute:: view

      Provides a read-write aware view of the encapsuled object data
      that is suitable for usage from :mod:`ctypes`.

   .. method:: to_bytes() -> ctypes.POINTER

      Returns a byte representation of the encapsuled object. The return
      value allows a direct read-write access to the object data, if it
      is not copied. The :func:`ctypes.POINTER` points to an array of
      :class:`ctypes.c_ubyte`.

   .. method:: to_uint16() -> ctypes.POINTER

      Returns a 16-bit representation of the encapsuled object. The return
      value allows a direct read-write access to the object data, if it
      is not copied. The :func:`ctypes.POINTER` points to an array of
      :class:`ctypes.c_ushort`.

   .. method:: to_uint32() -> ctypes.POINTER

      Returns a 32-bit representation of the encapsuled object. The return
      value allows a direct read-write access to the object data, if it
      is not copied. The :func:`ctypes.POINTER` points to an array of
      :class:`ctypes.c_uint`.

   .. method:: to_uint64() -> ctypes.POINTER

      Returns a 64-bit representation of the encapsuled object. The return
      value allows a direct read-write access to the object data, if it
      is not copied. The :func:`ctypes.POINTER` points to an array of
      :class:`ctypes.c_ulonglong`.

.. class:: MemoryView(source : object, itemsize : int, strides : tuple[, getfunc=None[, setfunc=None[, srcsize=None]]])

   The :class:`MemoryView` provides a read-write access to arbitrary
   data objects, which can be indexed.

   *itemsize* denotes the size of a single item. *strides* defines
   the dimensions and the length (n items * *itemsize*) for each
   dimension. *getfunc* and *setfunc* are optional parameters to
   provide specialised read and write access to the underlying
   *source*. *srcsize* can be used to provide the correct source
   size, if ``len(source)`` does not return the absolute size of the
   source object in all dimensions.

   .. note::

      The MemoryView is a pure Python-based implementation and makes
      heavy use of recursion for multi-dimensional access. If you aim
      for speed on accessing a n-dimensional object, you want to
      consider using a specialised library such as numpy. If you need
      n-dimensional access support, where such a library is not
      supported, or if you need to provide access to objects, which do
      not fulfill the requirements of that particular libray,
      :class:`MemoryView` can act as solid fallback solution.

   .. attribute:: itemsize

      The size of a single item in bytes.

   .. attribute:: ndim

      The number of dimensions of the :class:`MemoryView`.

   .. attribute:: size

      The size in bytes of the underlying source object.

   .. attribute:: source

      The underlying data source.

   .. attribute:: strides

      A tuple defining the length in bytes for accessing all
      elements in each dimension of the :class:`MemoryView`.

.. function:: to_ctypes(dataseq : iterable, dtype[, mcount=0]) -> array, int

    Converts an arbitrary sequence to a ctypes array of the specified
    *dtype* and returns the ctypes array and amount of items as
    two-value tuple.

    Raises a :exc:`TypeError`, if one or more elements in the passed
    sequence do not match the passed *dtype*.

.. function:: to_list(dataseq : iterable) -> list

   Converts a ctypes array to a list.

.. function:: to_tuple(dataseq : iterable) -> tuple

   Converts a ctypes array to a tuple.

.. function:: create_array(obj : object, itemsize : int) -> array.array

   Creates an :class:`array.array` based copy of the passed object.
   *itemsize* denotes the size in bytes for a single element within
   *obj*.