# 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 `ctypes` data types.

## Providing read-write access for sequential data

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

Depending on the the underlying object and the chosen size of each particular item of the object, the `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 `bytearray()` object. Afterwards, the first item of the view is changed, which causes a change on the `bytearray()`, on the first item as well, since both, the `CTypesView` and the `bytearray()` provide a byte-wise access to the contents.

By using `CTypesView.to_uint16()`, we change the access representation to a 2-byte unsigned integer `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) `buffer()` interface, but is iterable, the `CTypesView` will create an internal copy of the object data using Python’s `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, `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 `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 `MemoryView`, a new `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 `MemoryView` features, similar to Python’s builtin `memoryview`, dimensions and strides, accessible via the `MemoryView.ndim` and `MemoryView.strides` attributes.

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

The `MemoryView.strides`, which have to be passed on creating a new `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 `sdl2.ext.``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 `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.

  `bytesize`

    Returns the length of the encapsuled object in bytes.

  `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.

  `object`

    The encapsuled object.

  `view`

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

  `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 `ctypes.POINTER()` points to an array of `ctypes.c_ubyte`.

  `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 `ctypes.POINTER()` points to an array of `ctypes.c_ushort`.

  `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 `ctypes.POINTER()` points to an array of `ctypes.c_uint`.

  `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 `ctypes.POINTER()` points to an array of `ctypes.c_ulonglong`.

class `sdl2.ext.``MemoryView`(source : object, itemsize : int, strides : tuple[, getfunc=None[, setfunc=None[, srcsize=None]]])

  The `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, `MemoryView` can act as solid fallback solution.

  `itemsize`

    The size of a single item in bytes.

  `ndim`

    The number of dimensions of the `MemoryView`.

  `size`

    The size in bytes of the underlying source object.

  `source`

    The underlying data source.

  `strides`

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

`sdl2.ext.``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 `TypeError`, if one or more elements in the passed sequence do not match the passed dtype.

`sdl2.ext.``to_list`(dataseq : iterable) → list

  Converts a ctypes array to a list.

`sdl2.ext.``to_tuple`(dataseq : iterable) → tuple

  Converts a ctypes array to a tuple.

`sdl2.ext.``create_array`(obj : object, itemsize : int) → array.array

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