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