xref: /third_party/python/Doc/library/array.rst

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

.. module:: array
   :synopsis: Space efficient arrays of uniformly typed numeric values.

.. index:: single: arrays

--------------

This module defines an object type which can compactly 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 | Notes |
+===========+====================+===================+=======================+=======+
| ``'b'``   | signed char        | int               | 1                     |       |
+-----------+--------------------+-------------------+-----------------------+-------+
| ``'B'``   | unsigned char      | int               | 1                     |       |
+-----------+--------------------+-------------------+-----------------------+-------+
| ``'u'``   | wchar_t            | Unicode character | 2                     | \(1)  |
+-----------+--------------------+-------------------+-----------------------+-------+
| ``'h'``   | signed short       | int               | 2                     |       |
+-----------+--------------------+-------------------+-----------------------+-------+
| ``'H'``   | unsigned short     | int               | 2                     |       |
+-----------+--------------------+-------------------+-----------------------+-------+
| ``'i'``   | signed int         | int               | 2                     |       |
+-----------+--------------------+-------------------+-----------------------+-------+
| ``'I'``   | unsigned int       | int               | 2                     |       |
+-----------+--------------------+-------------------+-----------------------+-------+
| ``'l'``   | signed long        | int               | 4                     |       |
+-----------+--------------------+-------------------+-----------------------+-------+
| ``'L'``   | unsigned long      | int               | 4                     |       |
+-----------+--------------------+-------------------+-----------------------+-------+
| ``'q'``   | signed long long   | int               | 8                     |       |
+-----------+--------------------+-------------------+-----------------------+-------+
| ``'Q'``   | unsigned long long | int               | 8                     |       |
+-----------+--------------------+-------------------+-----------------------+-------+
| ``'f'``   | float              | float             | 4                     |       |
+-----------+--------------------+-------------------+-----------------------+-------+
| ``'d'``   | double             | float             | 8                     |       |
+-----------+--------------------+-------------------+-----------------------+-------+

Notes:

(1)
   It can be 16 bits or 32 bits depending on the platform.

   .. versionchanged:: 3.9
      ``array('u')`` now uses ``wchar_t`` as C type instead of deprecated
      ``Py_UNICODE``. This change doesn't affect its behavior because
      ``Py_UNICODE`` is alias of ``wchar_t`` since Python 3.3.

   .. deprecated-removed:: 3.3 4.0


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:`array.itemsize` attribute.

The module defines the following item:


.. data:: typecodes

   A string with all available type codes.


The module defines the following type:


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

   A new array whose items are restricted by *typecode*, and initialized
   from the optional *initializer* value, which must be a list, a
   :term:`bytes-like object`, 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:`frombytes`, or :meth:`fromunicode` method (see below)
   to add initial items to the array.  Otherwise, the iterable initializer is
   passed to the :meth:`extend` method.

   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 :term:`bytes-like objects <bytes-like object>` are supported.

   .. audit-event:: array.__new__ typecode,initializer array.array


   .. attribute:: typecode

      The typecode character used to create the array.


   .. attribute:: itemsize

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


   .. method:: append(x)

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


   .. method:: 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
      :c:func:`!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:: 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:: count(x)

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


   .. method:: 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:: frombytes(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).

      .. versionadded:: 3.2
         :meth:`!fromstring` is renamed to :meth:`frombytes` for clarity.


   .. method:: fromfile(f, n)

      Read *n* items (as machine values) from the :term:`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.


   .. method:: 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:: 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.frombytes(unicodestring.encode(enc))`` to append Unicode data to an
      array of some other type.


   .. method:: index(x[, start[, stop]])

      Return the smallest *i* such that *i* is the index of the first occurrence of
      *x* in the array.  The optional arguments *start* and *stop* can be
      specified to search for *x* within a subsection of the array.  Raise
      :exc:`ValueError` if *x* is not found.

      .. versionchanged:: 3.10
         Added optional *start* and *stop* parameters.


   .. method:: 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:: 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:: remove(x)

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


   .. method:: reverse()

      Reverse the order of the items in the array.


   .. method:: tobytes()

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

      .. versionadded:: 3.2
         :meth:`!tostring` is renamed to :meth:`tobytes` for clarity.


   .. method:: tofile(f)

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


   .. method:: tolist()

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


   .. method:: tounicode()

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


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 ``'u'``, 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
:class:`~array.array` class has been imported using ``from array import array``.
Examples::

   array('l')
   array('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.

   `NumPy <https://numpy.org/>`_
      The NumPy package defines another array type.