Doc/library/tarfile.rst

:mod:`tarfile` --- Read and write tar archive files
===================================================

.. module:: tarfile
   :synopsis: Read and write tar-format archive files.

.. moduleauthor:: Lars Gustäbel <lars@gustaebel.de>
.. sectionauthor:: Lars Gustäbel <lars@gustaebel.de>

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

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

The :mod:`tarfile` module makes it possible to read and write tar
archives, including those using gzip, bz2 and lzma compression.
Use the :mod:`zipfile` module to read or write :file:`.zip` files, or the
higher-level functions in :ref:`shutil <archiving-operations>`.

Some facts and figures:

* reads and writes :mod:`gzip`, :mod:`bz2` and :mod:`lzma` compressed archives
  if the respective modules are available.

* read/write support for the POSIX.1-1988 (ustar) format.

* read/write support for the GNU tar format including *longname* and *longlink*
  extensions, read-only support for all variants of the *sparse* extension
  including restoration of sparse files.

* read/write support for the POSIX.1-2001 (pax) format.

* handles directories, regular files, hardlinks, symbolic links, fifos,
  character devices and block devices and is able to acquire and restore file
  information like timestamp, access permissions and owner.

.. versionchanged:: 3.3
   Added support for :mod:`lzma` compression.


.. function:: open(name=None, mode='r', fileobj=None, bufsize=10240, \*\*kwargs)

   Return a :class:`TarFile` object for the pathname *name*. For detailed
   information on :class:`TarFile` objects and the keyword arguments that are
   allowed, see :ref:`tarfile-objects`.

   *mode* has to be a string of the form ``'filemode[:compression]'``, it defaults
   to ``'r'``. Here is a full list of mode combinations:

   +------------------+---------------------------------------------+
   | mode             | action                                      |
   +==================+=============================================+
   | ``'r' or 'r:*'`` | Open for reading with transparent           |
   |                  | compression (recommended).                  |
   +------------------+---------------------------------------------+
   | ``'r:'``         | Open for reading exclusively without        |
   |                  | compression.                                |
   +------------------+---------------------------------------------+
   | ``'r:gz'``       | Open for reading with gzip compression.     |
   +------------------+---------------------------------------------+
   | ``'r:bz2'``      | Open for reading with bzip2 compression.    |
   +------------------+---------------------------------------------+
   | ``'r:xz'``       | Open for reading with lzma compression.     |
   +------------------+---------------------------------------------+
   | ``'x'`` or       | Create a tarfile exclusively without        |
   | ``'x:'``         | compression.                                |
   |                  | Raise an :exc:`FileExistsError` exception   |
   |                  | if it already exists.                       |
   +------------------+---------------------------------------------+
   | ``'x:gz'``       | Create a tarfile with gzip compression.     |
   |                  | Raise an :exc:`FileExistsError` exception   |
   |                  | if it already exists.                       |
   +------------------+---------------------------------------------+
   | ``'x:bz2'``      | Create a tarfile with bzip2 compression.    |
   |                  | Raise an :exc:`FileExistsError` exception   |
   |                  | if it already exists.                       |
   +------------------+---------------------------------------------+
   | ``'x:xz'``       | Create a tarfile with lzma compression.     |
   |                  | Raise an :exc:`FileExistsError` exception   |
   |                  | if it already exists.                       |
   +------------------+---------------------------------------------+
   | ``'a' or 'a:'``  | Open for appending with no compression. The |
   |                  | file is created if it does not exist.       |
   +------------------+---------------------------------------------+
   | ``'w' or 'w:'``  | Open for uncompressed writing.              |
   +------------------+---------------------------------------------+
   | ``'w:gz'``       | Open for gzip compressed writing.           |
   +------------------+---------------------------------------------+
   | ``'w:bz2'``      | Open for bzip2 compressed writing.          |
   +------------------+---------------------------------------------+
   | ``'w:xz'``       | Open for lzma compressed writing.           |
   +------------------+---------------------------------------------+

   Note that ``'a:gz'``, ``'a:bz2'`` or ``'a:xz'`` is not possible. If *mode*
   is not suitable to open a certain (compressed) file for reading,
   :exc:`ReadError` is raised. Use *mode* ``'r'`` to avoid this.  If a
   compression method is not supported, :exc:`CompressionError` is raised.

   If *fileobj* is specified, it is used as an alternative to a :term:`file object`
   opened in binary mode for *name*. It is supposed to be at position 0.

   For modes ``'w:gz'``, ``'r:gz'``, ``'w:bz2'``, ``'r:bz2'``, ``'x:gz'``,
   ``'x:bz2'``, :func:`tarfile.open` accepts the keyword argument
   *compresslevel* (default ``9``) to specify the compression level of the file.

   For special purposes, there is a second format for *mode*:
   ``'filemode|[compression]'``.  :func:`tarfile.open` will return a :class:`TarFile`
   object that processes its data as a stream of blocks.  No random seeking will
   be done on the file. If given, *fileobj* may be any object that has a
   :meth:`read` or :meth:`write` method (depending on the *mode*). *bufsize*
   specifies the blocksize and defaults to ``20 * 512`` bytes. Use this variant
   in combination with e.g. ``sys.stdin``, a socket :term:`file object` or a tape
   device. However, such a :class:`TarFile` object is limited in that it does
   not allow random access, see :ref:`tar-examples`.  The currently
   possible modes:

   +-------------+--------------------------------------------+
   | Mode        | Action                                     |
   +=============+============================================+
   | ``'r|*'``   | Open a *stream* of tar blocks for reading  |
   |             | with transparent compression.              |
   +-------------+--------------------------------------------+
   | ``'r|'``    | Open a *stream* of uncompressed tar blocks |
   |             | for reading.                               |
   +-------------+--------------------------------------------+
   | ``'r|gz'``  | Open a gzip compressed *stream* for        |
   |             | reading.                                   |
   +-------------+--------------------------------------------+
   | ``'r|bz2'`` | Open a bzip2 compressed *stream* for       |
   |             | reading.                                   |
   +-------------+--------------------------------------------+
   | ``'r|xz'``  | Open an lzma compressed *stream* for       |
   |             | reading.                                   |
   +-------------+--------------------------------------------+
   | ``'w|'``    | Open an uncompressed *stream* for writing. |
   +-------------+--------------------------------------------+
   | ``'w|gz'``  | Open a gzip compressed *stream* for        |
   |             | writing.                                   |
   +-------------+--------------------------------------------+
   | ``'w|bz2'`` | Open a bzip2 compressed *stream* for       |
   |             | writing.                                   |
   +-------------+--------------------------------------------+
   | ``'w|xz'``  | Open an lzma compressed *stream* for       |
   |             | writing.                                   |
   +-------------+--------------------------------------------+

   .. versionchanged:: 3.5
      The ``'x'`` (exclusive creation) mode was added.

   .. versionchanged:: 3.6
      The *name* parameter accepts a :term:`path-like object`.


.. class:: TarFile

   Class for reading and writing tar archives. Do not use this class directly:
   use :func:`tarfile.open` instead. See :ref:`tarfile-objects`.


.. function:: is_tarfile(name)

   Return :const:`True` if *name* is a tar archive file, that the :mod:`tarfile`
   module can read.


The :mod:`tarfile` module defines the following exceptions:


.. exception:: TarError

   Base class for all :mod:`tarfile` exceptions.


.. exception:: ReadError

   Is raised when a tar archive is opened, that either cannot be handled by the
   :mod:`tarfile` module or is somehow invalid.


.. exception:: CompressionError

   Is raised when a compression method is not supported or when the data cannot be
   decoded properly.


.. exception:: StreamError

   Is raised for the limitations that are typical for stream-like :class:`TarFile`
   objects.


.. exception:: ExtractError

   Is raised for *non-fatal* errors when using :meth:`TarFile.extract`, but only if
   :attr:`TarFile.errorlevel`\ ``== 2``.


.. exception:: HeaderError

   Is raised by :meth:`TarInfo.frombuf` if the buffer it gets is invalid.


The following constants are available at the module level:

.. data:: ENCODING

   The default character encoding: ``'utf-8'`` on Windows, the value returned by
   :func:`sys.getfilesystemencoding` otherwise.


Each of the following constants defines a tar archive format that the
:mod:`tarfile` module is able to create. See section :ref:`tar-formats` for
details.


.. data:: USTAR_FORMAT

   POSIX.1-1988 (ustar) format.


.. data:: GNU_FORMAT

   GNU tar format.


.. data:: PAX_FORMAT

   POSIX.1-2001 (pax) format.


.. data:: DEFAULT_FORMAT

   The default format for creating archives. This is currently :const:`GNU_FORMAT`.


.. seealso::

   Module :mod:`zipfile`
      Documentation of the :mod:`zipfile` standard module.

   :ref:`archiving-operations`
      Documentation of the higher-level archiving facilities provided by the
      standard :mod:`shutil` module.

   `GNU tar manual, Basic Tar Format <https://www.gnu.org/software/tar/manual/html_node/Standard.html>`_
      Documentation for tar archive files, including GNU tar extensions.


.. _tarfile-objects:

TarFile Objects
---------------

The :class:`TarFile` object provides an interface to a tar archive. A tar
archive is a sequence of blocks. An archive member (a stored file) is made up of
a header block followed by data blocks. It is possible to store a file in a tar
archive several times. Each archive member is represented by a :class:`TarInfo`
object, see :ref:`tarinfo-objects` for details.

A :class:`TarFile` object can be used as a context manager in a :keyword:`with`
statement. It will automatically be closed when the block is completed. Please
note that in the event of an exception an archive opened for writing will not
be finalized; only the internally used file object will be closed. See the
:ref:`tar-examples` section for a use case.

.. versionadded:: 3.2
   Added support for the context management protocol.

.. class:: TarFile(name=None, mode='r', fileobj=None, format=DEFAULT_FORMAT, tarinfo=TarInfo, dereference=False, ignore_zeros=False, encoding=ENCODING, errors='surrogateescape', pax_headers=None, debug=0, errorlevel=0)

   All following arguments are optional and can be accessed as instance attributes
   as well.

   *name* is the pathname of the archive. *name* may be a :term:`path-like object`.
   It can be omitted if *fileobj* is given.
   In this case, the file object's :attr:`name` attribute is used if it exists.

   *mode* is either ``'r'`` to read from an existing archive, ``'a'`` to append
   data to an existing file, ``'w'`` to create a new file overwriting an existing
   one, or ``'x'`` to create a new file only if it does not already exist.

   If *fileobj* is given, it is used for reading or writing data. If it can be
   determined, *mode* is overridden by *fileobj*'s mode. *fileobj* will be used
   from position 0.

   .. note::

      *fileobj* is not closed, when :class:`TarFile` is closed.

   *format* controls the archive format. It must be one of the constants
   :const:`USTAR_FORMAT`, :const:`GNU_FORMAT` or :const:`PAX_FORMAT` that are
   defined at module level.

   The *tarinfo* argument can be used to replace the default :class:`TarInfo` class
   with a different one.

   If *dereference* is :const:`False`, add symbolic and hard links to the archive. If it
   is :const:`True`, add the content of the target files to the archive. This has no
   effect on systems that do not support symbolic links.

   If *ignore_zeros* is :const:`False`, treat an empty block as the end of the archive.
   If it is :const:`True`, skip empty (and invalid) blocks and try to get as many members
   as possible. This is only useful for reading concatenated or damaged archives.

   *debug* can be set from ``0`` (no debug messages) up to ``3`` (all debug
   messages). The messages are written to ``sys.stderr``.

   If *errorlevel* is ``0``, all errors are ignored when using :meth:`TarFile.extract`.
   Nevertheless, they appear as error messages in the debug output, when debugging
   is enabled.  If ``1``, all *fatal* errors are raised as :exc:`OSError`
   exceptions. If ``2``, all *non-fatal* errors are raised as :exc:`TarError`
   exceptions as well.

   The *encoding* and *errors* arguments define the character encoding to be
   used for reading or writing the archive and how conversion errors are going
   to be handled. The default settings will work for most users.
   See section :ref:`tar-unicode` for in-depth information.

   The *pax_headers* argument is an optional dictionary of strings which
   will be added as a pax global header if *format* is :const:`PAX_FORMAT`.

   .. versionchanged:: 3.2
      Use ``'surrogateescape'`` as the default for the *errors* argument.

   .. versionchanged:: 3.5
      The ``'x'`` (exclusive creation) mode was added.

   .. versionchanged:: 3.6
      The *name* parameter accepts a :term:`path-like object`.


.. classmethod:: TarFile.open(...)

   Alternative constructor. The :func:`tarfile.open` function is actually a
   shortcut to this classmethod.


.. method:: TarFile.getmember(name)

   Return a :class:`TarInfo` object for member *name*. If *name* can not be found
   in the archive, :exc:`KeyError` is raised.

   .. note::

      If a member occurs more than once in the archive, its last occurrence is assumed
      to be the most up-to-date version.


.. method:: TarFile.getmembers()

   Return the members of the archive as a list of :class:`TarInfo` objects. The
   list has the same order as the members in the archive.


.. method:: TarFile.getnames()

   Return the members as a list of their names. It has the same order as the list
   returned by :meth:`getmembers`.


.. method:: TarFile.list(verbose=True, *, members=None)

   Print a table of contents to ``sys.stdout``. If *verbose* is :const:`False`,
   only the names of the members are printed. If it is :const:`True`, output
   similar to that of :program:`ls -l` is produced. If optional *members* is
   given, it must be a subset of the list returned by :meth:`getmembers`.

   .. versionchanged:: 3.5
      Added the *members* parameter.


.. method:: TarFile.next()

   Return the next member of the archive as a :class:`TarInfo` object, when
   :class:`TarFile` is opened for reading. Return :const:`None` if there is no more
   available.


.. method:: TarFile.extractall(path=".", members=None, *, numeric_owner=False)

   Extract all members from the archive to the current working directory or
   directory *path*. If optional *members* is given, it must be a subset of the
   list returned by :meth:`getmembers`. Directory information like owner,
   modification time and permissions are set after all members have been extracted.
   This is done to work around two problems: A directory's modification time is
   reset each time a file is created in it. And, if a directory's permissions do
   not allow writing, extracting files to it will fail.

   If *numeric_owner* is :const:`True`, the uid and gid numbers from the tarfile
   are used to set the owner/group for the extracted files. Otherwise, the named
   values from the tarfile are used.

   .. warning::

      Never extract archives from untrusted sources without prior inspection.
      It is possible that files are created outside of *path*, e.g. members
      that have absolute filenames starting with ``"/"`` or filenames with two
      dots ``".."``.

   .. versionchanged:: 3.5
      Added the *numeric_owner* parameter.

   .. versionchanged:: 3.6
      The *path* parameter accepts a :term:`path-like object`.


.. method:: TarFile.extract(member, path="", set_attrs=True, *, numeric_owner=False)

   Extract a member from the archive to the current working directory, using its
   full name. Its file information is extracted as accurately as possible. *member*
   may be a filename or a :class:`TarInfo` object. You can specify a different
   directory using *path*. *path* may be a :term:`path-like object`.
   File attributes (owner, mtime, mode) are set unless *set_attrs* is false.

   If *numeric_owner* is :const:`True`, the uid and gid numbers from the tarfile
   are used to set the owner/group for the extracted files. Otherwise, the named
   values from the tarfile are used.

   .. note::

      The :meth:`extract` method does not take care of several extraction issues.
      In most cases you should consider using the :meth:`extractall` method.

   .. warning::

      See the warning for :meth:`extractall`.

   .. versionchanged:: 3.2
      Added the *set_attrs* parameter.

   .. versionchanged:: 3.5
      Added the *numeric_owner* parameter.

   .. versionchanged:: 3.6
      The *path* parameter accepts a :term:`path-like object`.


.. method:: TarFile.extractfile(member)

   Extract a member from the archive as a file object. *member* may be a filename
   or a :class:`TarInfo` object. If *member* is a regular file or a link, an
   :class:`io.BufferedReader` object is returned. Otherwise, :const:`None` is
   returned.

   .. versionchanged:: 3.3
      Return an :class:`io.BufferedReader` object.


.. method:: TarFile.add(name, arcname=None, recursive=True, *, filter=None)

   Add the file *name* to the archive. *name* may be any type of file
   (directory, fifo, symbolic link, etc.). If given, *arcname* specifies an
   alternative name for the file in the archive. Directories are added
   recursively by default. This can be avoided by setting *recursive* to
   :const:`False`. Recursion adds entries in sorted order.
   If *filter* is given, it
   should be a function that takes a :class:`TarInfo` object argument and
   returns the changed :class:`TarInfo` object. If it instead returns
   :const:`None` the :class:`TarInfo` object will be excluded from the
   archive. See :ref:`tar-examples` for an example.

   .. versionchanged:: 3.2
      Added the *filter* parameter.

   .. versionchanged:: 3.7
      Recursion adds entries in sorted order.


.. method:: TarFile.addfile(tarinfo, fileobj=None)

   Add the :class:`TarInfo` object *tarinfo* to the archive. If *fileobj* is given,
   it should be a :term:`binary file`, and
   ``tarinfo.size`` bytes are read from it and added to the archive.  You can
   create :class:`TarInfo` objects directly, or by using :meth:`gettarinfo`.


.. method:: TarFile.gettarinfo(name=None, arcname=None, fileobj=None)

   Create a :class:`TarInfo` object from the result of :func:`os.stat` or
   equivalent on an existing file.  The file is either named by *name*, or
   specified as a :term:`file object` *fileobj* with a file descriptor.
   *name* may be a :term:`path-like object`.  If
   given, *arcname* specifies an alternative name for the file in the
   archive, otherwise, the name is taken from *fileobj*’s
   :attr:`~io.FileIO.name` attribute, or the *name* argument.  The name
   should be a text string.

   You can modify
   some of the :class:`TarInfo`’s attributes before you add it using :meth:`addfile`.
   If the file object is not an ordinary file object positioned at the
   beginning of the file, attributes such as :attr:`~TarInfo.size` may need
   modifying.  This is the case for objects such as :class:`~gzip.GzipFile`.
   The :attr:`~TarInfo.name` may also be modified, in which case *arcname*
   could be a dummy string.

   .. versionchanged:: 3.6
      The *name* parameter accepts a :term:`path-like object`.


.. method:: TarFile.close()

   Close the :class:`TarFile`. In write mode, two finishing zero blocks are
   appended to the archive.


.. attribute:: TarFile.pax_headers

   A dictionary containing key-value pairs of pax global headers.



.. _tarinfo-objects:

TarInfo Objects
---------------

A :class:`TarInfo` object represents one member in a :class:`TarFile`. Aside
from storing all required attributes of a file (like file type, size, time,
permissions, owner etc.), it provides some useful methods to determine its type.
It does *not* contain the file's data itself.

:class:`TarInfo` objects are returned by :class:`TarFile`'s methods
:meth:`getmember`, :meth:`getmembers` and :meth:`gettarinfo`.


.. class:: TarInfo(name="")

   Create a :class:`TarInfo` object.


.. classmethod:: TarInfo.frombuf(buf, encoding, errors)

   Create and return a :class:`TarInfo` object from string buffer *buf*.

   Raises :exc:`HeaderError` if the buffer is invalid.


.. classmethod:: TarInfo.fromtarfile(tarfile)

   Read the next member from the :class:`TarFile` object *tarfile* and return it as
   a :class:`TarInfo` object.


.. method:: TarInfo.tobuf(format=DEFAULT_FORMAT, encoding=ENCODING, errors='surrogateescape')

   Create a string buffer from a :class:`TarInfo` object. For information on the
   arguments see the constructor of the :class:`TarFile` class.

   .. versionchanged:: 3.2
      Use ``'surrogateescape'`` as the default for the *errors* argument.


A ``TarInfo`` object has the following public data attributes:


.. attribute:: TarInfo.name

   Name of the archive member.


.. attribute:: TarInfo.size

   Size in bytes.


.. attribute:: TarInfo.mtime

   Time of last modification.


.. attribute:: TarInfo.mode

   Permission bits.


.. attribute:: TarInfo.type

   File type.  *type* is usually one of these constants: :const:`REGTYPE`,
   :const:`AREGTYPE`, :const:`LNKTYPE`, :const:`SYMTYPE`, :const:`DIRTYPE`,
   :const:`FIFOTYPE`, :const:`CONTTYPE`, :const:`CHRTYPE`, :const:`BLKTYPE`,
   :const:`GNUTYPE_SPARSE`.  To determine the type of a :class:`TarInfo` object
   more conveniently, use the ``is*()`` methods below.


.. attribute:: TarInfo.linkname

   Name of the target file name, which is only present in :class:`TarInfo` objects
   of type :const:`LNKTYPE` and :const:`SYMTYPE`.


.. attribute:: TarInfo.uid

   User ID of the user who originally stored this member.


.. attribute:: TarInfo.gid

   Group ID of the user who originally stored this member.


.. attribute:: TarInfo.uname

   User name.


.. attribute:: TarInfo.gname

   Group name.


.. attribute:: TarInfo.pax_headers

   A dictionary containing key-value pairs of an associated pax extended header.


A :class:`TarInfo` object also provides some convenient query methods:


.. method:: TarInfo.isfile()

   Return :const:`True` if the :class:`Tarinfo` object is a regular file.


.. method:: TarInfo.isreg()

   Same as :meth:`isfile`.


.. method:: TarInfo.isdir()

   Return :const:`True` if it is a directory.


.. method:: TarInfo.issym()

   Return :const:`True` if it is a symbolic link.


.. method:: TarInfo.islnk()

   Return :const:`True` if it is a hard link.


.. method:: TarInfo.ischr()

   Return :const:`True` if it is a character device.


.. method:: TarInfo.isblk()

   Return :const:`True` if it is a block device.


.. method:: TarInfo.isfifo()

   Return :const:`True` if it is a FIFO.


.. method:: TarInfo.isdev()

   Return :const:`True` if it is one of character device, block device or FIFO.


.. _tarfile-commandline:
.. program:: tarfile

Command-Line Interface
----------------------

.. versionadded:: 3.4

The :mod:`tarfile` module provides a simple command-line interface to interact
with tar archives.

If you want to create a new tar archive, specify its name after the :option:`-c`
option and then list the filename(s) that should be included:

.. code-block:: shell-session

    $ python -m tarfile -c monty.tar  spam.txt eggs.txt

Passing a directory is also acceptable:

.. code-block:: shell-session

    $ python -m tarfile -c monty.tar life-of-brian_1979/

If you want to extract a tar archive into the current directory, use
the :option:`-e` option:

.. code-block:: shell-session

    $ python -m tarfile -e monty.tar

You can also extract a tar archive into a different directory by passing the
directory's name:

.. code-block:: shell-session

    $ python -m tarfile -e monty.tar  other-dir/

For a list of the files in a tar archive, use the :option:`-l` option:

.. code-block:: shell-session

    $ python -m tarfile -l monty.tar


Command-line options
~~~~~~~~~~~~~~~~~~~~

.. cmdoption:: -l <tarfile>
               --list <tarfile>

   List files in a tarfile.

.. cmdoption:: -c <tarfile> <source1> ... <sourceN>
               --create <tarfile> <source1> ... <sourceN>

   Create tarfile from source files.

.. cmdoption:: -e <tarfile> [<output_dir>]
               --extract <tarfile> [<output_dir>]

   Extract tarfile into the current directory if *output_dir* is not specified.

.. cmdoption:: -t <tarfile>
               --test <tarfile>

   Test whether the tarfile is valid or not.

.. cmdoption:: -v, --verbose

   Verbose output.

.. _tar-examples:

Examples
--------

How to extract an entire tar archive to the current working directory::

   import tarfile
   tar = tarfile.open("sample.tar.gz")
   tar.extractall()
   tar.close()

How to extract a subset of a tar archive with :meth:`TarFile.extractall` using
a generator function instead of a list::

   import os
   import tarfile

   def py_files(members):
       for tarinfo in members:
           if os.path.splitext(tarinfo.name)[1] == ".py":
               yield tarinfo

   tar = tarfile.open("sample.tar.gz")
   tar.extractall(members=py_files(tar))
   tar.close()

How to create an uncompressed tar archive from a list of filenames::

   import tarfile
   tar = tarfile.open("sample.tar", "w")
   for name in ["foo", "bar", "quux"]:
       tar.add(name)
   tar.close()

The same example using the :keyword:`with` statement::

    import tarfile
    with tarfile.open("sample.tar", "w") as tar:
        for name in ["foo", "bar", "quux"]:
            tar.add(name)

How to read a gzip compressed tar archive and display some member information::

   import tarfile
   tar = tarfile.open("sample.tar.gz", "r:gz")
   for tarinfo in tar:
       print(tarinfo.name, "is", tarinfo.size, "bytes in size and is", end="")
       if tarinfo.isreg():
           print("a regular file.")
       elif tarinfo.isdir():
           print("a directory.")
       else:
           print("something else.")
   tar.close()

How to create an archive and reset the user information using the *filter*
parameter in :meth:`TarFile.add`::

    import tarfile
    def reset(tarinfo):
        tarinfo.uid = tarinfo.gid = 0
        tarinfo.uname = tarinfo.gname = "root"
        return tarinfo
    tar = tarfile.open("sample.tar.gz", "w:gz")
    tar.add("foo", filter=reset)
    tar.close()


.. _tar-formats:

Supported tar formats
---------------------

There are three tar formats that can be created with the :mod:`tarfile` module:

* The POSIX.1-1988 ustar format (:const:`USTAR_FORMAT`). It supports filenames
  up to a length of at best 256 characters and linknames up to 100 characters. The
  maximum file size is 8 GiB. This is an old and limited but widely
  supported format.

* The GNU tar format (:const:`GNU_FORMAT`). It supports long filenames and
  linknames, files bigger than 8 GiB and sparse files. It is the de facto
  standard on GNU/Linux systems. :mod:`tarfile` fully supports the GNU tar
  extensions for long names, sparse file support is read-only.

* The POSIX.1-2001 pax format (:const:`PAX_FORMAT`). It is the most flexible
  format with virtually no limits. It supports long filenames and linknames, large
  files and stores pathnames in a portable way. However, not all tar
  implementations today are able to handle pax archives properly.

  The *pax* format is an extension to the existing *ustar* format. It uses extra
  headers for information that cannot be stored otherwise. There are two flavours
  of pax headers: Extended headers only affect the subsequent file header, global
  headers are valid for the complete archive and affect all following files. All
  the data in a pax header is encoded in *UTF-8* for portability reasons.

There are some more variants of the tar format which can be read, but not
created:

* The ancient V7 format. This is the first tar format from Unix Seventh Edition,
  storing only regular files and directories. Names must not be longer than 100
  characters, there is no user/group name information. Some archives have
  miscalculated header checksums in case of fields with non-ASCII characters.

* The SunOS tar extended format. This format is a variant of the POSIX.1-2001
  pax format, but is not compatible.

.. _tar-unicode:

Unicode issues
--------------

The tar format was originally conceived to make backups on tape drives with the
main focus on preserving file system information. Nowadays tar archives are
commonly used for file distribution and exchanging archives over networks. One
problem of the original format (which is the basis of all other formats) is
that there is no concept of supporting different character encodings. For
example, an ordinary tar archive created on a *UTF-8* system cannot be read
correctly on a *Latin-1* system if it contains non-*ASCII* characters. Textual
metadata (like filenames, linknames, user/group names) will appear damaged.
Unfortunately, there is no way to autodetect the encoding of an archive. The
pax format was designed to solve this problem. It stores non-ASCII metadata
using the universal character encoding *UTF-8*.

The details of character conversion in :mod:`tarfile` are controlled by the
*encoding* and *errors* keyword arguments of the :class:`TarFile` class.

*encoding* defines the character encoding to use for the metadata in the
archive. The default value is :func:`sys.getfilesystemencoding` or ``'ascii'``
as a fallback. Depending on whether the archive is read or written, the
metadata must be either decoded or encoded. If *encoding* is not set
appropriately, this conversion may fail.

The *errors* argument defines how characters are treated that cannot be
converted. Possible values are listed in section :ref:`error-handlers`.
The default scheme is ``'surrogateescape'`` which Python also uses for its
file system calls, see :ref:`os-filenames`.

In case of :const:`PAX_FORMAT` archives, *encoding* is generally not needed
because all the metadata is stored using *UTF-8*. *encoding* is only used in
the rare cases when binary pax headers are decoded or when strings with
surrogate characters are stored.