1 

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

     3 ===================================================

     4 

     5 .. module:: array

     6    :synopsis: Space efficient arrays of uniformly typed numeric values.

     7 

     8 

     9 .. index:: single: arrays

    10 

    11 This module defines an object type which can compactly represent an array of

    12 basic values: characters, integers, floating point numbers.  Arrays are sequence

    13 types and behave very much like lists, except that the type of objects stored in

    14 them is constrained.  The type is specified at object creation time by using a

    15 :dfn:`type code`, which is a single character.  The following type codes are

    16 defined:

    17 

    18 +-----------+----------------+-------------------+-----------------------+

    19 | Type code | C Type         | Python Type       | Minimum size in bytes |

    20 +===========+================+===================+=======================+

    21 | ``'c'``   | char           | character         | 1                     |

    22 +-----------+----------------+-------------------+-----------------------+

    23 | ``'b'``   | signed char    | int               | 1                     |

    24 +-----------+----------------+-------------------+-----------------------+

    25 | ``'B'``   | unsigned char  | int               | 1                     |

    26 +-----------+----------------+-------------------+-----------------------+

    27 | ``'u'``   | Py_UNICODE     | Unicode character | 2                     |

    28 +-----------+----------------+-------------------+-----------------------+

    29 | ``'h'``   | signed short   | int               | 2                     |

    30 +-----------+----------------+-------------------+-----------------------+

    31 | ``'H'``   | unsigned short | int               | 2                     |

    32 +-----------+----------------+-------------------+-----------------------+

    33 | ``'i'``   | signed int     | int               | 2                     |

    34 +-----------+----------------+-------------------+-----------------------+

    35 | ``'I'``   | unsigned int   | long              | 2                     |

    36 +-----------+----------------+-------------------+-----------------------+

    37 | ``'l'``   | signed long    | int               | 4                     |

    38 +-----------+----------------+-------------------+-----------------------+

    39 | ``'L'``   | unsigned long  | long              | 4                     |

    40 +-----------+----------------+-------------------+-----------------------+

    41 | ``'f'``   | float          | float             | 4                     |

    42 +-----------+----------------+-------------------+-----------------------+

    43 | ``'d'``   | double         | float             | 8                     |

    44 +-----------+----------------+-------------------+-----------------------+

    45 

    46 The actual representation of values is determined by the machine architecture

    47 (strictly speaking, by the C implementation).  The actual size can be accessed

    48 through the :attr:`itemsize` attribute.  The values stored  for ``'L'`` and

    49 ``'I'`` items will be represented as Python long integers when retrieved,

    50 because Python's plain integer type cannot represent the full range of C's

    51 unsigned (long) integers.

    52 

    53 The module defines the following type:

    54 

    55 

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

    57 

    58    Return a new array whose items are restricted by *typecode*, and initialized

    59    from the optional *initializer* value, which must be a list, string, or iterable

    60    over elements of the appropriate type.

    61 

    62    .. versionchanged:: 2.4

    63       Formerly, only lists or strings were accepted.

    64 

    65    If given a list or string, the initializer is passed to the new array's

    66    :meth:`fromlist`, :meth:`fromstring`, or :meth:`fromunicode` method (see below)

    67    to add initial items to the array.  Otherwise, the iterable initializer is

    68    passed to the :meth:`extend` method.

    69 

    70 

    71 .. data:: ArrayType

    72 

    73    Obsolete alias for :func:`array`.

    74 

    75 Array objects support the ordinary sequence operations of indexing, slicing,

    76 concatenation, and multiplication.  When using slice assignment, the assigned

    77 value must be an array object with the same type code; in all other cases,

    78 :exc:`TypeError` is raised. Array objects also implement the buffer interface,

    79 and may be used wherever buffer objects are supported.

    80 

    81 The following data items and methods are also supported:

    82 

    83 

    84 .. attribute:: array.typecode

    85 

    86    The typecode character used to create the array.

    87 

    88 

    89 .. attribute:: array.itemsize

    90 

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

    92 

    93 

    94 .. method:: array.append(x)

    95 

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

    97 

    98 

    99 .. method:: array.buffer_info()

   100 

   101    Return a tuple ``(address, length)`` giving the current memory address and the

   102    length in elements of the buffer used to hold array's contents.  The size of the

   103    memory buffer in bytes can be computed as ``array.buffer_info()[1] *

   104    array.itemsize``.  This is occasionally useful when working with low-level (and

   105    inherently unsafe) I/O interfaces that require memory addresses, such as certain

   106    :cfunc:`ioctl` operations.  The returned numbers are valid as long as the array

   107    exists and no length-changing operations are applied to it.

   108 

   109    .. note::

   110 

   111       When using array objects from code written in C or C++ (the only way to

   112       effectively make use of this information), it makes more sense to use the buffer

   113       interface supported by array objects.  This method is maintained for backward

   114       compatibility and should be avoided in new code.  The buffer interface is

   115       documented in :ref:`bufferobjects`.

   116 

   117 

   118 .. method:: array.byteswap()

   119 

   120    "Byteswap" all items of the array.  This is only supported for values which are

   121    1, 2, 4, or 8 bytes in size; for other types of values, :exc:`RuntimeError` is

   122    raised.  It is useful when reading data from a file written on a machine with a

   123    different byte order.

   124 

   125 

   126 .. method:: array.count(x)

   127 

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

   129 

   130 

   131 .. method:: array.extend(iterable)

   132 

   133    Append items from *iterable* to the end of the array.  If *iterable* is another

   134    array, it must have *exactly* the same type code; if not, :exc:`TypeError` will

   135    be raised.  If *iterable* is not an array, it must be iterable and its elements

   136    must be the right type to be appended to the array.

   137 

   138    .. versionchanged:: 2.4

   139       Formerly, the argument could only be another array.

   140 

   141 

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

   143 

   144    Read *n* items (as machine values) from the file object *f* and append them to

   145    the end of the array.  If less than *n* items are available, :exc:`EOFError` is

   146    raised, but the items that were available are still inserted into the array.

   147    *f* must be a real built-in file object; something else with a :meth:`read`

   148    method won't do.

   149 

   150 

   151 .. method:: array.fromlist(list)

   152 

   153    Append items from the list.  This is equivalent to ``for x in list:

   154    a.append(x)`` except that if there is a type error, the array is unchanged.

   155 

   156 

   157 .. method:: array.fromstring(s)

   158 

   159    Appends items from the string, interpreting the string as an array of machine

   160    values (as if it had been read from a file using the :meth:`fromfile` method).

   161 

   162 

   163 .. method:: array.fromunicode(s)

   164 

   165    Extends this array with data from the given unicode string.  The array must

   166    be a type ``'u'`` array; otherwise a :exc:`ValueError` is raised.  Use

   167    ``array.fromstring(unicodestring.encode(enc))`` to append Unicode data to an

   168    array of some other type.

   169 

   170 

   171 .. method:: array.index(x)

   172 

   173    Return the smallest *i* such that *i* is the index of the first occurrence of

   174    *x* in the array.

   175 

   176 

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

   178 

   179    Insert a new item with value *x* in the array before position *i*. Negative

   180    values are treated as being relative to the end of the array.

   181 

   182 

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

   184 

   185    Removes the item with the index *i* from the array and returns it. The optional

   186    argument defaults to ``-1``, so that by default the last item is removed and

   187    returned.

   188 

   189 

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

   191 

   192    .. deprecated:: 1.5.1

   193       Use the :meth:`fromfile` method.

   194 

   195    Read *n* items (as machine values) from the file object *f* and append them to

   196    the end of the array.  If less than *n* items are available, :exc:`EOFError` is

   197    raised, but the items that were available are still inserted into the array.

   198    *f* must be a real built-in file object; something else with a :meth:`read`

   199    method won't do.

   200 

   201 

   202 .. method:: array.remove(x)

   203 

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

   205 

   206 

   207 .. method:: array.reverse()

   208 

   209    Reverse the order of the items in the array.

   210 

   211 

   212 .. method:: array.tofile(f)

   213 

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

   215 

   216 

   217 .. method:: array.tolist()

   218 

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

   220 

   221 

   222 .. method:: array.tostring()

   223 

   224    Convert the array to an array of machine values and return the string

   225    representation (the same sequence of bytes that would be written to a file by

   226    the :meth:`tofile` method.)

   227 

   228 

   229 .. method:: array.tounicode()

   230 

   231    Convert the array to a unicode string.  The array must be a type ``'u'`` array;

   232    otherwise a :exc:`ValueError` is raised. Use ``array.tostring().decode(enc)`` to

   233    obtain a unicode string from an array of some other type.

   234 

   235 

   236 .. method:: array.write(f)

   237 

   238    .. deprecated:: 1.5.1

   239       Use the :meth:`tofile` method.

   240 

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

   242 

   243 When an array object is printed or converted to a string, it is represented as

   244 ``array(typecode, initializer)``.  The *initializer* is omitted if the array is

   245 empty, otherwise it is a string if the *typecode* is ``'c'``, otherwise it is a

   246 list of numbers.  The string is guaranteed to be able to be converted back to an

   247 array with the same type and value using :func:`eval`, so long as the

   248 :func:`array` function has been imported using ``from array import array``.

   249 Examples::

   250 

   251    array('l')

   252    array('c', 'hello world')

   253    array('u', u'hello \u2641')

   254    array('l', [1, 2, 3, 4, 5])

   255    array('d', [1.0, 2.0, 3.14])

   256 

   257 

   258 .. seealso::

   259 

   260    Module :mod:`struct`

   261       Packing and unpacking of heterogeneous binary data.

   262 

   263    Module :mod:`xdrlib`

   264       Packing and unpacking of External Data Representation (XDR) data as used in some

   265       remote procedure call systems.

   266 

   267    `The Numerical Python Manual <http://numpy.sourceforge.net/numdoc/HTML/numdoc.htm>`_

   268       The Numeric Python extension (NumPy) defines another array type; see

   269       http://numpy.sourceforge.net/ for further information about Numerical Python.

   270       (A PDF version of the NumPy manual is available at

   271       http://numpy.sourceforge.net/numdoc/numdoc.pdf).

   272