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Doc/library/fileinput.rst
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Victor Stinner
bpo-39674: Revert "bpo-37330: open() no longer accept 'U' in file mode (GH-16959)" (GH-18767)
04 мар 2020, 20:50
04 мар 2020, 20:50942f7a2
:mod:`fileinput` --- Iterate over lines from multiple input streams
===================================================================

.. module:: fileinput
   :synopsis: Loop over standard input or a list of files.

.. moduleauthor:: Guido van Rossum <guido@python.org>
.. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>

**Source code:** :source:`Lib/fileinput.py`

--------------

This module implements a helper class and functions to quickly write a
loop over standard input or a list of files. If you just want to read or
write one file see :func:`open`.

The typical use is::

   import fileinput
   for line in fileinput.input():
       process(line)

This iterates over the lines of all files listed in ``sys.argv[1:]``, defaulting
to ``sys.stdin`` if the list is empty.  If a filename is ``'-'``, it is also
replaced by ``sys.stdin`` and the optional arguments *mode* and *openhook*
are ignored.  To specify an alternative list of filenames, pass it as the
first argument to :func:`.input`.  A single file name is also allowed.

All files are opened in text mode by default, but you can override this by
specifying the *mode* parameter in the call to :func:`.input` or
:class:`FileInput`.  If an I/O error occurs during opening or reading a file,
:exc:`OSError` is raised.

.. versionchanged:: 3.3
   :exc:`IOError` used to be raised; it is now an alias of :exc:`OSError`.

If ``sys.stdin`` is used more than once, the second and further use will return
no lines, except perhaps for interactive use, or if it has been explicitly reset
(e.g. using ``sys.stdin.seek(0)``).

Empty files are opened and immediately closed; the only time their presence in
the list of filenames is noticeable at all is when the last file opened is
empty.

Lines are returned with any newlines intact, which means that the last line in
a file may not have one.

You can control how files are opened by providing an opening hook via the
*openhook* parameter to :func:`fileinput.input` or :class:`FileInput()`. The
hook must be a function that takes two arguments, *filename* and *mode*, and
returns an accordingly opened file-like object. Two useful hooks are already
provided by this module.

The following function is the primary interface of this module:


.. function:: input(files=None, inplace=False, backup='', *, mode='r', openhook=None)

   Create an instance of the :class:`FileInput` class.  The instance will be used
   as global state for the functions of this module, and is also returned to use
   during iteration.  The parameters to this function will be passed along to the
   constructor of the :class:`FileInput` class.

   The :class:`FileInput` instance can be used as a context manager in the
   :keyword:`with` statement.  In this example, *input* is closed after the
   :keyword:`!with` statement is exited, even if an exception occurs::

      with fileinput.input(files=('spam.txt', 'eggs.txt')) as f:
          for line in f:
              process(line)

   .. versionchanged:: 3.2
      Can be used as a context manager.

   .. versionchanged:: 3.8
      The keyword parameters *mode* and *openhook* are now keyword-only.


The following functions use the global state created by :func:`fileinput.input`;
if there is no active state, :exc:`RuntimeError` is raised.


.. function:: filename()

   Return the name of the file currently being read.  Before the first line has
   been read, returns ``None``.


.. function:: fileno()

   Return the integer "file descriptor" for the current file. When no file is
   opened (before the first line and between files), returns ``-1``.


.. function:: lineno()

   Return the cumulative line number of the line that has just been read.  Before
   the first line has been read, returns ``0``.  After the last line of the last
   file has been read, returns the line number of that line.


.. function:: filelineno()

   Return the line number in the current file.  Before the first line has been
   read, returns ``0``.  After the last line of the last file has been read,
   returns the line number of that line within the file.


.. function:: isfirstline()

   Return ``True`` if the line just read is the first line of its file, otherwise
   return ``False``.


.. function:: isstdin()

   Return ``True`` if the last line was read from ``sys.stdin``, otherwise return
   ``False``.


.. function:: nextfile()

   Close the current file so that the next iteration will read the first line from
   the next file (if any); lines not read from the file will not count towards the
   cumulative line count.  The filename is not changed until after the first line
   of the next file has been read.  Before the first line has been read, this
   function has no effect; it cannot be used to skip the first file.  After the
   last line of the last file has been read, this function has no effect.


.. function:: close()

   Close the sequence.

The class which implements the sequence behavior provided by the module is
available for subclassing as well:


.. class:: FileInput(files=None, inplace=False, backup='', *, mode='r', openhook=None)

   Class :class:`FileInput` is the implementation; its methods :meth:`filename`,
   :meth:`fileno`, :meth:`lineno`, :meth:`filelineno`, :meth:`isfirstline`,
   :meth:`isstdin`, :meth:`nextfile` and :meth:`close` correspond to the
   functions of the same name in the module. In addition it has a
   :meth:`~io.TextIOBase.readline` method which returns the next input line,
   and a :meth:`__getitem__` method which implements the sequence behavior.
   The sequence must be accessed in strictly sequential order; random access
   and :meth:`~io.TextIOBase.readline` cannot be mixed.

   With *mode* you can specify which file mode will be passed to :func:`open`. It
   must be one of ``'r'``, ``'rU'``, ``'U'`` and ``'rb'``.

   The *openhook*, when given, must be a function that takes two arguments,
   *filename* and *mode*, and returns an accordingly opened file-like object. You
   cannot use *inplace* and *openhook* together.

   A :class:`FileInput` instance can be used as a context manager in the
   :keyword:`with` statement.  In this example, *input* is closed after the
   :keyword:`!with` statement is exited, even if an exception occurs::

      with FileInput(files=('spam.txt', 'eggs.txt')) as input:
          process(input)


   .. versionchanged:: 3.2
      Can be used as a context manager.

   .. deprecated:: 3.4
      The ``'rU'`` and ``'U'`` modes.

   .. deprecated:: 3.8
      Support for :meth:`__getitem__` method is deprecated.

   .. versionchanged:: 3.8
      The keyword parameter *mode* and *openhook* are now keyword-only.



**Optional in-place filtering:** if the keyword argument ``inplace=True`` is
passed to :func:`fileinput.input` or to the :class:`FileInput` constructor, the
file is moved to a backup file and standard output is directed to the input file
(if a file of the same name as the backup file already exists, it will be
replaced silently).  This makes it possible to write a filter that rewrites its
input file in place.  If the *backup* parameter is given (typically as
``backup='.<some extension>'``), it specifies the extension for the backup file,
and the backup file remains around; by default, the extension is ``'.bak'`` and
it is deleted when the output file is closed.  In-place filtering is disabled
when standard input is read.


The two following opening hooks are provided by this module:

.. function:: hook_compressed(filename, mode)

   Transparently opens files compressed with gzip and bzip2 (recognized by the
   extensions ``'.gz'`` and ``'.bz2'``) using the :mod:`gzip` and :mod:`bz2`
   modules.  If the filename extension is not ``'.gz'`` or ``'.bz2'``, the file is
   opened normally (ie, using :func:`open` without any decompression).

   Usage example:  ``fi = fileinput.FileInput(openhook=fileinput.hook_compressed)``


.. function:: hook_encoded(encoding, errors=None)

   Returns a hook which opens each file with :func:`open`, using the given
   *encoding* and *errors* to read the file.

   Usage example: ``fi =
   fileinput.FileInput(openhook=fileinput.hook_encoded("utf-8",
   "surrogateescape"))``

   .. versionchanged:: 3.6
      Added the optional *errors* parameter.