.. _array-array:

==========================
Working with Python arrays
==========================

Python has a builtin array module supporting dynamic 1-dimensional arrays of
primitive types. It is possible to access the underlying C array of a Python
array from within Cython. At the same time they are ordinary Python objects
which can be stored in lists and serialized between processes when using
:obj:`multiprocessing`.

Compared to the manual approach with :c:func:`malloc` and :c:func:`free`, this
gives the safe and automatic memory management of Python, and compared to a
Numpy array there is no need to install a dependency, as the :obj:`array`
module is built into both Python and Cython.

Safe usage with memory views
----------------------------

::

    from cpython cimport array as c_array
    from array import array
    cdef c_array.array a = array('i', [1, 2, 3])
    cdef int[:] ca = a

    print ca[0]

A Python array is constructed with a type signature and sequence of
initial values. For the possible type signatures, refer to the Python
documentation for the `array module <http://docs.python.org/library/array.html>`_.

Notice that when a Python array is assigned to a variable typed as
memory view, there will be a slight overhead to construct the memory
view. However, from that point on the variable can be passed to other
functions without overhead, so long as it is typed::

    from cpython cimport array as c_array
    from array import array
    cdef c_array.array a = array('i', [1, 2, 3])
    cdef int[:] ca = a

    cdef int overhead(object a):
        cdef int[:] ca = a
        return ca[0]

    cdef int no_overhead(int[:] ca):
        return ca[0]

    print overhead(a)  # new memory view will be constructed, overhead
    print no_overhead(ca)  # ca is already a memory view, so no overhead

Zero-overhead, unsafe access to raw C pointer
---------------------------------------------
To avoid any overhead and to be able to pass a C pointer to other
functions, it is possible to access the underlying contiguous array as a
pointer. There is no type or bounds checking, so be careful to use the
right type and signedness.

::

    from cpython cimport array as c_array
    from array import array

    cdef c_array.array a = array('i', [1, 2, 3])

    # access underlying pointer:
    print a.data.as_ints[0]

    from libc.string cimport memset
    memset(a.data.as_voidptr, 0, len(a) * sizeof(int))

Cloning, extending arrays
-------------------------
To avoid having to use the array constructor from the Python module,
it is possible to create a new array with the same type as a template,
and preallocate a given number of elements. The array is initialized to
zero when requested.

::

    from cpython cimport array as c_array
    from array import array

    cdef c_array.array int_array_template = array('i', [])
    cdef c_array.array newarray

    # create an array with 3 elements with same type as template
    newarray = c_array.clone(int_array_template, 3, zero=False)

An array can also be extended and resized; this avoids repeated memory
reallocation which would occur if elements would be appended or removed
one by one.

::

    from cpython cimport array as c_array
    from array import array

    cdef c_array.array a = array('i', [1, 2, 3])
    cdef c_array.array b = array('i', [4, 5, 6])

    # extend a with b, resize as needed
    c_array.extend(a, b)
    # resize a, leaving just original three elements
    c_array.resize(a, len(a) - len(b))