/Doc/library/array.rst

  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
 11This module defines an object type which can compactly represent an array of
 12basic values: characters, integers, floating point numbers.  Arrays are sequence
 13types and behave very much like lists, except that the type of objects stored in
 14them 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
 16defined:
 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
 46The actual representation of values is determined by the machine architecture
 47(strictly speaking, by the C implementation).  The actual size can be accessed
 48through the :attr:`itemsize` attribute.  The values stored  for ``'L'`` and
 49``'I'`` items will be represented as Python long integers when retrieved,
 50because Python's plain integer type cannot represent the full range of C's
 51unsigned (long) integers.
 52
 53The 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
 75Array objects support the ordinary sequence operations of indexing, slicing,
 76concatenation, and multiplication.  When using slice assignment, the assigned
 77value 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,
 79and may be used wherever buffer objects are supported.
 80
 81The 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
243When 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
245empty, otherwise it is a string if the *typecode* is ``'c'``, otherwise it is a
246list of numbers.  The string is guaranteed to be able to be converted back to an
247array 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``.
249Examples::
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