275 lines
10 KiB
ReStructuredText
275 lines
10 KiB
ReStructuredText
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:mod:`array` --- Efficient arrays of numeric values
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===================================================
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.. module:: array
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:synopsis: Space efficient arrays of uniformly typed numeric values.
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.. index:: single: arrays
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This module defines an object type which can compactly represent an array of
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basic values: characters, integers, floating point numbers. Arrays are sequence
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types and behave very much like lists, except that the type of objects stored in
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them is constrained. The type is specified at object creation time by using a
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:dfn:`type code`, which is a single character. The following type codes are
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defined:
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+-----------+----------------+-------------------+-----------------------+
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| Type code | C Type | Python Type | Minimum size in bytes |
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+===========+================+===================+=======================+
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| ``'c'`` | char | character | 1 |
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+-----------+----------------+-------------------+-----------------------+
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| ``'b'`` | signed char | int | 1 |
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+-----------+----------------+-------------------+-----------------------+
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| ``'B'`` | unsigned char | int | 1 |
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+-----------+----------------+-------------------+-----------------------+
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| ``'u'`` | Py_UNICODE | Unicode character | 2 (see note) |
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+-----------+----------------+-------------------+-----------------------+
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| ``'h'`` | signed short | int | 2 |
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+-----------+----------------+-------------------+-----------------------+
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| ``'H'`` | unsigned short | int | 2 |
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+-----------+----------------+-------------------+-----------------------+
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| ``'i'`` | signed int | int | 2 |
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+-----------+----------------+-------------------+-----------------------+
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| ``'I'`` | unsigned int | long | 2 |
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+-----------+----------------+-------------------+-----------------------+
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| ``'l'`` | signed long | int | 4 |
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+-----------+----------------+-------------------+-----------------------+
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| ``'L'`` | unsigned long | long | 4 |
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+-----------+----------------+-------------------+-----------------------+
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| ``'f'`` | float | float | 4 |
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+-----------+----------------+-------------------+-----------------------+
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| ``'d'`` | double | float | 8 |
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+-----------+----------------+-------------------+-----------------------+
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.. note::
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The ``'u'`` typecode corresponds to Python's unicode character. On narrow
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Unicode builds this is 2-bytes, on wide builds this is 4-bytes.
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The actual representation of values is determined by the machine architecture
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(strictly speaking, by the C implementation). The actual size can be accessed
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through the :attr:`itemsize` attribute. The values stored for ``'L'`` and
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``'I'`` items will be represented as Python long integers when retrieved,
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because Python's plain integer type cannot represent the full range of C's
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unsigned (long) integers.
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The module defines the following type:
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.. class:: array(typecode[, initializer])
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A new array whose items are restricted by *typecode*, and initialized
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from the optional *initializer* value, which must be a list, string, or iterable
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over elements of the appropriate type.
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.. versionchanged:: 2.4
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Formerly, only lists or strings were accepted.
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If given a list or string, the initializer is passed to the new array's
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:meth:`fromlist`, :meth:`fromstring`, or :meth:`fromunicode` method (see below)
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to add initial items to the array. Otherwise, the iterable initializer is
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passed to the :meth:`extend` method.
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.. data:: ArrayType
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Obsolete alias for :class:`array`.
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Array objects support the ordinary sequence operations of indexing, slicing,
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concatenation, and multiplication. When using slice assignment, the assigned
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value must be an array object with the same type code; in all other cases,
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:exc:`TypeError` is raised. Array objects also implement the buffer interface,
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and may be used wherever buffer objects are supported.
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The following data items and methods are also supported:
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.. attribute:: array.typecode
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The typecode character used to create the array.
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.. attribute:: array.itemsize
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The length in bytes of one array item in the internal representation.
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.. method:: array.append(x)
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Append a new item with value *x* to the end of the array.
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.. method:: array.buffer_info()
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Return a tuple ``(address, length)`` giving the current memory address and the
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length in elements of the buffer used to hold array's contents. The size of the
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memory buffer in bytes can be computed as ``array.buffer_info()[1] *
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array.itemsize``. This is occasionally useful when working with low-level (and
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inherently unsafe) I/O interfaces that require memory addresses, such as certain
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:c:func:`ioctl` operations. The returned numbers are valid as long as the array
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exists and no length-changing operations are applied to it.
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.. note::
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When using array objects from code written in C or C++ (the only way to
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effectively make use of this information), it makes more sense to use the buffer
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interface supported by array objects. This method is maintained for backward
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compatibility and should be avoided in new code. The buffer interface is
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documented in :ref:`bufferobjects`.
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.. method:: array.byteswap()
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"Byteswap" all items of the array. This is only supported for values which are
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1, 2, 4, or 8 bytes in size; for other types of values, :exc:`RuntimeError` is
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raised. It is useful when reading data from a file written on a machine with a
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different byte order.
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.. method:: array.count(x)
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Return the number of occurrences of *x* in the array.
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.. method:: array.extend(iterable)
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Append items from *iterable* to the end of the array. If *iterable* is another
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array, it must have *exactly* the same type code; if not, :exc:`TypeError` will
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be raised. If *iterable* is not an array, it must be iterable and its elements
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must be the right type to be appended to the array.
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.. versionchanged:: 2.4
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Formerly, the argument could only be another array.
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.. method:: array.fromfile(f, n)
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Read *n* items (as machine values) from the file object *f* and append them to
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the end of the array. If less than *n* items are available, :exc:`EOFError` is
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raised, but the items that were available are still inserted into the array.
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*f* must be a real built-in file object; something else with a :meth:`read`
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method won't do.
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.. method:: array.fromlist(list)
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Append items from the list. This is equivalent to ``for x in list:
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a.append(x)`` except that if there is a type error, the array is unchanged.
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.. method:: array.fromstring(s)
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Appends items from the string, interpreting the string as an array of machine
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values (as if it had been read from a file using the :meth:`fromfile` method).
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.. method:: array.fromunicode(s)
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Extends this array with data from the given unicode string. The array must
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be a type ``'u'`` array; otherwise a :exc:`ValueError` is raised. Use
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``array.fromstring(unicodestring.encode(enc))`` to append Unicode data to an
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array of some other type.
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.. method:: array.index(x)
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Return the smallest *i* such that *i* is the index of the first occurrence of
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*x* in the array.
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.. method:: array.insert(i, x)
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Insert a new item with value *x* in the array before position *i*. Negative
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values are treated as being relative to the end of the array.
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.. method:: array.pop([i])
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Removes the item with the index *i* from the array and returns it. The optional
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argument defaults to ``-1``, so that by default the last item is removed and
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returned.
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.. method:: array.read(f, n)
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.. deprecated:: 1.5.1
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Use the :meth:`fromfile` method.
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Read *n* items (as machine values) from the file object *f* and append them to
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the end of the array. If less than *n* items are available, :exc:`EOFError` is
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raised, but the items that were available are still inserted into the array.
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*f* must be a real built-in file object; something else with a :meth:`read`
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method won't do.
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.. method:: array.remove(x)
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Remove the first occurrence of *x* from the array.
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.. method:: array.reverse()
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Reverse the order of the items in the array.
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.. method:: array.tofile(f)
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Write all items (as machine values) to the file object *f*.
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.. method:: array.tolist()
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Convert the array to an ordinary list with the same items.
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.. method:: array.tostring()
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Convert the array to an array of machine values and return the string
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representation (the same sequence of bytes that would be written to a file by
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the :meth:`tofile` method.)
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.. method:: array.tounicode()
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Convert the array to a unicode string. The array must be a type ``'u'`` array;
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otherwise a :exc:`ValueError` is raised. Use ``array.tostring().decode(enc)`` to
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obtain a unicode string from an array of some other type.
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.. method:: array.write(f)
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.. deprecated:: 1.5.1
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Use the :meth:`tofile` method.
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Write all items (as machine values) to the file object *f*.
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When an array object is printed or converted to a string, it is represented as
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``array(typecode, initializer)``. The *initializer* is omitted if the array is
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empty, otherwise it is a string if the *typecode* is ``'c'``, otherwise it is a
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list of numbers. The string is guaranteed to be able to be converted back to an
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array with the same type and value using :func:`eval`, so long as the
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:func:`array` function has been imported using ``from array import array``.
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Examples::
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array('l')
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array('c', 'hello world')
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array('u', u'hello \u2641')
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array('l', [1, 2, 3, 4, 5])
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array('d', [1.0, 2.0, 3.14])
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.. seealso::
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Module :mod:`struct`
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Packing and unpacking of heterogeneous binary data.
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Module :mod:`xdrlib`
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Packing and unpacking of External Data Representation (XDR) data as used in some
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remote procedure call systems.
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`The Numerical Python Documentation <http://docs.scipy.org/doc/>`_
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The Numeric Python extension (NumPy) defines another array type; see
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http://www.numpy.org/ for further information about Numerical Python.
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