190 lines
7.0 KiB
ReStructuredText
190 lines
7.0 KiB
ReStructuredText
:mod:`posixfile` --- File-like objects with locking support
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===========================================================
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.. module:: posixfile
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:platform: Unix
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:synopsis: A file-like object with support for locking.
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:deprecated:
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.. moduleauthor:: Jaap Vermeulen
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.. sectionauthor:: Jaap Vermeulen
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.. index:: pair: POSIX; file object
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.. deprecated:: 1.5
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The locking operation that this module provides is done better and more portably
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by the :func:`fcntl.lockf` call.
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.. index:: single: fcntl() (in module fcntl)
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This module implements some additional functionality over the built-in file
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objects. In particular, it implements file locking, control over the file
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flags, and an easy interface to duplicate the file object. The module defines a
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new file object, the posixfile object. It has all the standard file object
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methods and adds the methods described below. This module only works for
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certain flavors of Unix, since it uses :func:`fcntl.fcntl` for file locking.
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To instantiate a posixfile object, use the :func:`posixfile.open` function. The
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resulting object looks and feels roughly the same as a standard file object.
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The :mod:`posixfile` module defines the following constants:
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.. data:: SEEK_SET
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Offset is calculated from the start of the file.
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.. data:: SEEK_CUR
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Offset is calculated from the current position in the file.
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.. data:: SEEK_END
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Offset is calculated from the end of the file.
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The :mod:`posixfile` module defines the following functions:
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.. function:: open(filename[, mode[, bufsize]])
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Create a new posixfile object with the given filename and mode. The *filename*,
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*mode* and *bufsize* arguments are interpreted the same way as by the built-in
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:func:`open` function.
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.. function:: fileopen(fileobject)
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Create a new posixfile object with the given standard file object. The resulting
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object has the same filename and mode as the original file object.
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The posixfile object defines the following additional methods:
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.. method:: posixfile.lock(fmt, [len[, start[, whence]]])
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Lock the specified section of the file that the file object is referring to.
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The format is explained below in a table. The *len* argument specifies the
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length of the section that should be locked. The default is ``0``. *start*
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specifies the starting offset of the section, where the default is ``0``. The
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*whence* argument specifies where the offset is relative to. It accepts one of
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the constants :const:`SEEK_SET`, :const:`SEEK_CUR` or :const:`SEEK_END`. The
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default is :const:`SEEK_SET`. For more information about the arguments refer to
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the :manpage:`fcntl(2)` manual page on your system.
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.. method:: posixfile.flags([flags])
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Set the specified flags for the file that the file object is referring to. The
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new flags are ORed with the old flags, unless specified otherwise. The format
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is explained below in a table. Without the *flags* argument a string indicating
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the current flags is returned (this is the same as the ``?`` modifier). For
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more information about the flags refer to the :manpage:`fcntl(2)` manual page on
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your system.
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.. method:: posixfile.dup()
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Duplicate the file object and the underlying file pointer and file descriptor.
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The resulting object behaves as if it were newly opened.
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.. method:: posixfile.dup2(fd)
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Duplicate the file object and the underlying file pointer and file descriptor.
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The new object will have the given file descriptor. Otherwise the resulting
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object behaves as if it were newly opened.
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.. method:: posixfile.file()
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Return the standard file object that the posixfile object is based on. This is
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sometimes necessary for functions that insist on a standard file object.
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All methods raise :exc:`IOError` when the request fails.
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Format characters for the :meth:`lock` method have the following meaning:
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+--------+-----------------------------------------------+
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| Format | Meaning |
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+========+===============================================+
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| ``u`` | unlock the specified region |
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+--------+-----------------------------------------------+
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| ``r`` | request a read lock for the specified section |
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+--------+-----------------------------------------------+
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| ``w`` | request a write lock for the specified |
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| | section |
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+--------+-----------------------------------------------+
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In addition the following modifiers can be added to the format:
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+----------+--------------------------------+-------+
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| Modifier | Meaning | Notes |
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+==========+================================+=======+
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| ``|`` | wait until the lock has been | |
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| | granted | |
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+----------+--------------------------------+-------+
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| ``?`` | return the first lock | \(1) |
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| | conflicting with the requested | |
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| | lock, or ``None`` if there is | |
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| | no conflict. | |
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+----------+--------------------------------+-------+
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Note:
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(1)
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The lock returned is in the format ``(mode, len, start, whence, pid)`` where
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*mode* is a character representing the type of lock ('r' or 'w'). This modifier
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prevents a request from being granted; it is for query purposes only.
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Format characters for the :meth:`flags` method have the following meanings:
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+--------+-----------------------------------------------+
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| Format | Meaning |
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+========+===============================================+
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| ``a`` | append only flag |
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+--------+-----------------------------------------------+
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| ``c`` | close on exec flag |
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+--------+-----------------------------------------------+
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| ``n`` | no delay flag (also called non-blocking flag) |
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+--------+-----------------------------------------------+
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| ``s`` | synchronization flag |
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+--------+-----------------------------------------------+
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In addition the following modifiers can be added to the format:
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+----------+---------------------------------+-------+
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| Modifier | Meaning | Notes |
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+==========+=================================+=======+
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| ``!`` | turn the specified flags 'off', | \(1) |
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| | instead of the default 'on' | |
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+----------+---------------------------------+-------+
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| ``=`` | replace the flags, instead of | \(1) |
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| | the default 'OR' operation | |
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+----------+---------------------------------+-------+
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| ``?`` | return a string in which the | \(2) |
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| | characters represent the flags | |
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| | that are set. | |
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+----------+---------------------------------+-------+
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Notes:
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(1)
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The ``!`` and ``=`` modifiers are mutually exclusive.
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(2)
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This string represents the flags after they may have been altered by the same
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call.
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Examples::
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import posixfile
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file = posixfile.open('testfile', 'w')
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file.lock('w|')
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...
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file.lock('u')
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file.close()
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