path: root/Doc/library/array.rst

:mod:`array` --- Efficient arrays of numeric values
===================================================

.. module:: array
   :synopsis: Efficient arrays of uniformly typed numeric values.


.. index:: single: arrays

This module defines an object type which can efficiently represent an array of
basic values: characters, integers, floating point numbers.  Arrays are sequence
types and behave very much like lists, except that the type of objects stored in
them is constrained.  The type is specified at object creation time by using a
:dfn:`type code`, which is a single character.  The following type codes are
defined:

+-----------+----------------+-------------------+-----------------------+
| Type code | C Type         | Python Type       | Minimum size in bytes |
+===========+================+===================+=======================+
| ``'c'``   | char           | character         | 1                     |
+-----------+----------------+-------------------+-----------------------+
| ``'b'``   | signed char    | int               | 1                     |
+-----------+----------------+-------------------+-----------------------+
| ``'B'``   | unsigned char  | int               | 1                     |
+-----------+----------------+-------------------+-----------------------+
| ``'u'``   | Py_UNICODE     | Unicode character | 2                     |
+-----------+----------------+-------------------+-----------------------+
| ``'h'``   | signed short   | int               | 2                     |
+-----------+----------------+-------------------+-----------------------+
| ``'H'``   | unsigned short | int               | 2                     |
+-----------+----------------+-------------------+-----------------------+
| ``'i'``   | signed int     | int               | 2                     |
+-----------+----------------+-------------------+-----------------------+
| ``'I'``   | unsigned int   | long              | 2                     |
+-----------+----------------+-------------------+-----------------------+
| ``'l'``   | signed long    | int               | 4                     |
+-----------+----------------+-------------------+-----------------------+
| ``'L'``   | unsigned long  | long              | 4                     |
+-----------+----------------+-------------------+-----------------------+
| ``'f'``   | float          | float             | 4                     |
+-----------+----------------+-------------------+-----------------------+
| ``'d'``   | double         | float             | 8                     |
+-----------+----------------+-------------------+-----------------------+

The actual representation of values is determined by the machine architecture
(strictly speaking, by the C implementation).  The actual size can be accessed
through the :attr:`itemsize` attribute.  The values stored  for ``'L'`` and
``'I'`` items will be represented as Python long integers when retrieved,
because Python's plain integer type cannot represent the full range of C's
unsigned (long) integers.

The module defines the following type:


.. function:: array(typecode[, initializer])

   Return a new array whose items are restricted by *typecode*, and initialized
   from the optional *initializer* value, which must be a list, string, or iterable
   over elements of the appropriate type.

   If given a list or string, the initializer is passed to the new array's
   :meth:`fromlist`, :meth:`fromstring`, or :meth:`fromunicode` method (see below)
   to add initial items to the array.  Otherwise, the iterable initializer is
   passed to the :meth:`extend` method.


.. data:: ArrayType

   Obsolete alias for :func:`array`.

Array objects support the ordinary sequence operations of indexing, slicing,
concatenation, and multiplication.  When using slice assignment, the assigned
value must be an array object with the same type code; in all other cases,
:exc:`TypeError` is raised. Array objects also implement the buffer interface,
and may be used wherever buffer objects are supported.

The following data items and methods are also supported:


.. attribute:: array.typecode

   The typecode character used to create the array.


.. attribute:: array.itemsize

   The length in bytes of one array item in the internal representation.


.. method:: array.append(x)

   Append a new item with value *x* to the end of the array.


.. method:: array.buffer_info()

   Return a tuple ``(address, length)`` giving the current memory address and the
   length in elements of the buffer used to hold array's contents.  The size of the
   memory buffer in bytes can be computed as ``array.buffer_info()[1] *
   array.itemsize``.  This is occasionally useful when working with low-level (and
   inherently unsafe) I/O interfaces that require memory addresses, such as certain
   :cfunc:`ioctl` operations.  The returned numbers are valid as long as the array
   exists and no length-changing operations are applied to it.

   .. note::

      When using array objects from code written in C or C++ (the only way to
      effectively make use of this information), it makes more sense to use the buffer
      interface supported by array objects.  This method is maintained for backward
      compatibility and should be avoided in new code.  The buffer interface is
      documented in :ref:`bufferobjects`.


.. method:: array.byteswap()

   "Byteswap" all items of the array.  This is only supported for values which are
   1, 2, 4, or 8 bytes in size; for other types of values, :exc:`RuntimeError` is
   raised.  It is useful when reading data from a file written on a machine with a
   different byte order.


.. method:: array.count(x)

   Return the number of occurrences of *x* in the array.


.. method:: array.extend(iterable)

   Append items from *iterable* to the end of the array.  If *iterable* is another
   array, it must have *exactly* the same type code; if not, :exc:`TypeError` will
   be raised.  If *iterable* is not an array, it must be iterable and its elements
   must be the right type to be appended to the array.


.. method:: array.fromfile(f, n)

   Read *n* items (as machine values) from the file object *f* and append them to
   the end of the array.  If less than *n* items are available, :exc:`EOFError` is
   raised, but the items that were available are still inserted into the array.
   *f* must be a real built-in file object; something else with a :meth:`read`
   method won't do.


.. method:: array.fromlist(list)

   Append items from the list.  This is equivalent to ``for x in list:
   a.append(x)`` except that if there is a type error, the array is unchanged.


.. method:: array.fromstring(s)

   Appends items from the string, interpreting the string as an array of machine
   values (as if it had been read from a file using the :meth:`fromfile` method).


.. method:: array.fromunicode(s)

   Extends this array with data from the given unicode string.  The array must
   be a type ``'u'`` array; otherwise a :exc:`ValueError` is raised.  Use
   ``array.fromstring(unicodestring.encode(enc))`` to append Unicode data to an
   array of some other type.


.. method:: array.index(x)

   Return the smallest *i* such that *i* is the index of the first occurrence of
   *x* in the array.


.. method:: array.insert(i, x)

   Insert a new item with value *x* in the array before position *i*. Negative
   values are treated as being relative to the end of the array.


.. method:: array.pop([i])

   Removes the item with the index *i* from the array and returns it. The optional
   argument defaults to ``-1``, so that by default the last item is removed and
   returned.


.. method:: array.read(f, n)

   .. deprecated:: 1.5.1
      Use the :meth:`fromfile` method.

   Read *n* items (as machine values) from the file object *f* and append them to
   the end of the array.  If less than *n* items are available, :exc:`EOFError` is
   raised, but the items that were available are still inserted into the array.
   *f* must be a real built-in file object; something else with a :meth:`read`
   method won't do.


.. method:: array.remove(x)

   Remove the first occurrence of *x* from the array.


.. method:: array.reverse()

   Reverse the order of the items in the array.


.. method:: array.tofile(f)

   Write all items (as machine values) to the file object *f*.


.. method:: array.tolist()

   Convert the array to an ordinary list with the same items.


.. method:: array.tostring()

   Convert the array to an array of machine values and return the string
   representation (the same sequence of bytes that would be written to a file by
   the :meth:`tofile` method.)


.. method:: array.tounicode()

   Convert the array to a unicode string.  The array must be a type ``'u'`` array;
   otherwise a :exc:`ValueError` is raised. Use ``array.tostring().decode(enc)`` to
   obtain a unicode string from an array of some other type.


.. method:: array.write(f)

   .. deprecated:: 1.5.1
      Use the :meth:`tofile` method.

   Write all items (as machine values) to the file object *f*.

When an array object is printed or converted to a string, it is represented as
``array(typecode, initializer)``.  The *initializer* is omitted if the array is
empty, otherwise it is a string if the *typecode* is ``'c'``, otherwise it is a
list of numbers.  The string is guaranteed to be able to be converted back to an
array with the same type and value using :func:`eval`, so long as the
:func:`array` function has been imported using ``from array import array``.
Examples::

   array('l')
   array('c', 'hello world')
   array('u', u'hello \u2641')
   array('l', [1, 2, 3, 4, 5])
   array('d', [1.0, 2.0, 3.14])


.. seealso::

   Module :mod:`struct`
      Packing and unpacking of heterogeneous binary data.

   Module :mod:`xdrlib`
      Packing and unpacking of External Data Representation (XDR) data as used in some
      remote procedure call systems.

   `The Numerical Python Manual <http://numpy.sourceforge.net/numdoc/HTML/numdoc.htm>`_
      The Numeric Python extension (NumPy) defines another array type; see
      http://numpy.sourceforge.net/ for further information about Numerical Python.
      (A PDF version of the NumPy manual is available at
      http://numpy.sourceforge.net/numdoc/numdoc.pdf).