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Doc/library/tempfile.rst
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[3.13] gh-142302: Fix mkstemp() documentation: clarify file descriptor inheritance behavior (GH-142338) (#143487)
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:mod:`!tempfile` --- Generate temporary files and directories
=============================================================

.. module:: tempfile
   :synopsis: Generate temporary files and directories.

.. sectionauthor:: Zack Weinberg <zack@codesourcery.com>

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

.. index::
   pair: temporary; file name
   pair: temporary; file

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

This module creates temporary files and directories.  It works on all
supported platforms. :class:`TemporaryFile`, :class:`NamedTemporaryFile`,
:class:`TemporaryDirectory`, and :class:`SpooledTemporaryFile` are high-level
interfaces which provide automatic cleanup and can be used as
:term:`context managers <context manager>`. :func:`mkstemp` and
:func:`mkdtemp` are lower-level functions which require manual cleanup.

All the user-callable functions and constructors take additional arguments which
allow direct control over the location and name of temporary files and
directories. Files names used by this module include a string of
random characters which allows those files to be securely created in
shared temporary directories.
To maintain backward compatibility, the argument order is somewhat odd; it
is recommended to use keyword arguments for clarity.

The module defines the following user-callable items:

.. function:: TemporaryFile(mode='w+b', buffering=-1, encoding=None, newline=None, suffix=None, prefix=None, dir=None, *, errors=None)

   Return a :term:`file-like object` that can be used as a temporary storage area.
   The file is created securely, using the same rules as :func:`mkstemp`. It will be destroyed as soon
   as it is closed (including an implicit close when the object is garbage
   collected).  Under Unix, the directory entry for the file is either not created at all or is removed
   immediately after the file is created.  Other platforms do not support
   this; your code should not rely on a temporary file created using this
   function having or not having a visible name in the file system.

   The resulting object can be used as a :term:`context manager` (see
   :ref:`tempfile-examples`).  On completion of the context or
   destruction of the file object the temporary file will be removed
   from the filesystem.

   The *mode* parameter defaults to ``'w+b'`` so that the file created can
   be read and written without being closed.  Binary mode is used so that it
   behaves consistently on all platforms without regard for the data that is
   stored.  *buffering*, *encoding*, *errors* and *newline* are interpreted as for
   :func:`open`.

   The *dir*, *prefix* and *suffix* parameters have the same meaning and
   defaults as with :func:`mkstemp`.

   The returned object is a true file object on POSIX platforms.  On other
   platforms, it is a file-like object whose :attr:`!file` attribute is the
   underlying true file object.

   The :py:const:`os.O_TMPFILE` flag is used if it is available and works
   (Linux-specific, requires Linux kernel 3.11 or later).

   On platforms that are neither Posix nor Cygwin, TemporaryFile is an alias
   for NamedTemporaryFile.

   .. audit-event:: tempfile.mkstemp fullpath tempfile.TemporaryFile

   .. versionchanged:: 3.5

      The :py:const:`os.O_TMPFILE` flag is now used if available.

   .. versionchanged:: 3.8
      Added *errors* parameter.


.. function:: NamedTemporaryFile(mode='w+b', buffering=-1, encoding=None, newline=None, suffix=None, prefix=None, dir=None, delete=True, *, errors=None, delete_on_close=True)

   This function operates exactly as :func:`TemporaryFile` does, except the
   following differences:

   * This function returns a file that is guaranteed to have a visible name in
     the file system.
   * To manage the named file, it extends the parameters of
     :func:`TemporaryFile` with *delete* and *delete_on_close* parameters that
     determine whether and how the named file should be automatically deleted.

   The returned object is always a :term:`file-like object` whose :attr:`!file`
   attribute is the underlying true file object. This file-like object
   can be used in a :keyword:`with` statement, just like a normal file.  The
   name of the temporary file can be retrieved from the :attr:`!name` attribute
   of the returned file-like object. On Unix, unlike with the
   :func:`TemporaryFile`, the directory entry does not get unlinked immediately
   after the file creation.

   If *delete* is true (the default) and *delete_on_close* is true (the
   default), the file is deleted as soon as it is closed. If *delete* is true
   and *delete_on_close* is false, the file is deleted on context manager exit
   only, or else when the :term:`file-like object` is finalized. Deletion is not
   always guaranteed in this case (see :meth:`object.__del__`). If *delete* is
   false, the value of *delete_on_close* is ignored.

   Therefore to use the name of the temporary file to reopen the file after
   closing it, either make sure not to delete the file upon closure (set the
   *delete* parameter to be false) or, in case the temporary file is created in
   a :keyword:`with` statement, set the *delete_on_close* parameter to be false.
   The latter approach is recommended as it provides assistance in automatic
   cleaning of the temporary file upon the context manager exit.

   Opening the temporary file again by its name while it is still open works as
   follows:

   * On POSIX the file can always be opened again.
   * On Windows, make sure that at least one of the following conditions are
     fulfilled:

     * *delete* is false
     * additional open shares delete access (e.g. by calling :func:`os.open`
       with the flag ``O_TEMPORARY``)
     * *delete* is true but *delete_on_close* is false. Note, that in this
       case the additional opens that do not share delete access (e.g.
       created via builtin :func:`open`) must be closed before exiting the
       context manager, else the :func:`os.unlink` call on context manager
       exit will fail with a :exc:`PermissionError`.

   On Windows, if *delete_on_close* is false, and the file is created in a
   directory for which the user lacks delete access, then the :func:`os.unlink`
   call on exit of the context manager will fail with a :exc:`PermissionError`.
   This cannot happen when *delete_on_close* is true because delete access is
   requested by the open, which fails immediately if the requested access is not
   granted.

   On POSIX (only), a process that is terminated abruptly with SIGKILL
   cannot automatically delete any NamedTemporaryFiles it created.

   .. audit-event:: tempfile.mkstemp fullpath tempfile.NamedTemporaryFile

   .. versionchanged:: 3.8
      Added *errors* parameter.

   .. versionchanged:: 3.12
      Added *delete_on_close* parameter.


.. class:: SpooledTemporaryFile(max_size=0, mode='w+b', buffering=-1, encoding=None, newline=None, suffix=None, prefix=None, dir=None, *, errors=None)

   This class operates exactly as :func:`TemporaryFile` does, except that
   data is spooled in memory until the file size exceeds *max_size*, or
   until the file's :func:`~io.IOBase.fileno` method is called, at which point the
   contents are written to disk and operation proceeds as with
   :func:`TemporaryFile`.

   .. method:: SpooledTemporaryFile.rollover

      The resulting file has one additional method, :meth:`!rollover`, which
      causes the file to roll over to an on-disk file regardless of its size.

   The returned object is a file-like object whose :attr:`!_file` attribute
   is either an :class:`io.BytesIO` or :class:`io.TextIOWrapper` object
   (depending on whether binary or text *mode* was specified) or a true file
   object, depending on whether :meth:`rollover` has been called.  This
   file-like object can be used in a :keyword:`with` statement, just like
   a normal file.

   .. versionchanged:: 3.3
      the truncate method now accepts a *size* argument.

   .. versionchanged:: 3.8
      Added *errors* parameter.

   .. versionchanged:: 3.11
      Fully implements the :class:`io.BufferedIOBase` and
      :class:`io.TextIOBase` abstract base classes (depending on whether binary
      or text *mode* was specified).


.. class:: TemporaryDirectory(suffix=None, prefix=None, dir=None, ignore_cleanup_errors=False, *, delete=True)

   This class securely creates a temporary directory using the same rules as :func:`mkdtemp`.
   The resulting object can be used as a :term:`context manager` (see
   :ref:`tempfile-examples`).  On completion of the context or destruction
   of the temporary directory object, the newly created temporary directory
   and all its contents are removed from the filesystem.

   .. attribute:: TemporaryDirectory.name

      The directory name can be retrieved from the :attr:`!name` attribute of the
      returned object.  When the returned object is used as a :term:`context manager`, the
      :attr:`!name` will be assigned to the target of the :keyword:`!as` clause in
      the :keyword:`with` statement, if there is one.

   .. method:: TemporaryDirectory.cleanup

      The directory can be explicitly cleaned up by calling the
      :meth:`!cleanup` method. If *ignore_cleanup_errors* is true, any unhandled
      exceptions during explicit or implicit cleanup (such as a
      :exc:`PermissionError` removing open files on Windows) will be ignored,
      and the remaining removable items deleted on a "best-effort" basis.
      Otherwise, errors will be raised in whatever context cleanup occurs
      (the :meth:`!cleanup` call, exiting the context manager, when the object
      is garbage-collected or during interpreter shutdown).

   The *delete* parameter can be used to disable cleanup of the directory tree
   upon exiting the context.  While it may seem unusual for a context manager
   to disable the action taken when exiting the context, it can be useful during
   debugging or when you need your cleanup behavior to be conditional based on
   other logic.

   .. audit-event:: tempfile.mkdtemp fullpath tempfile.TemporaryDirectory

   .. versionadded:: 3.2

   .. versionchanged:: 3.10
      Added *ignore_cleanup_errors* parameter.

   .. versionchanged:: 3.12
      Added the *delete* parameter.


.. function:: mkstemp(suffix=None, prefix=None, dir=None, text=False)

   Creates a temporary file in the most secure manner possible.  There are
   no race conditions in the file's creation, assuming that the platform
   properly implements the :const:`os.O_EXCL` flag for :func:`os.open`.  The
   file is readable and writable only by the creating user ID.  If the
   platform uses permission bits to indicate whether a file is executable,
   the file is executable by no one.

   The file descriptor is :ref:`not inherited by child processes <fd_inheritance>`.

   Unlike :func:`TemporaryFile`, the user of :func:`mkstemp` is responsible
   for deleting the temporary file when done with it.

   If *suffix* is not ``None``, the file name will end with that suffix,
   otherwise there will be no suffix.  :func:`mkstemp` does not put a dot
   between the file name and the suffix; if you need one, put it at the
   beginning of *suffix*.

   If *prefix* is not ``None``, the file name will begin with that prefix;
   otherwise, a default prefix is used.  The default is the return value of
   :func:`gettempprefix` or :func:`gettempprefixb`, as appropriate.

   If *dir* is not ``None``, the file will be created in that directory;
   otherwise, a default directory is used.  The default directory is chosen
   from a platform-dependent list, but the user of the application can
   control the directory location by setting the *TMPDIR*, *TEMP* or *TMP*
   environment variables.  There is thus no guarantee that the generated
   filename will have any nice properties, such as not requiring quoting
   when passed to external commands via ``os.popen()``.

   If any of *suffix*, *prefix*, and *dir* are not
   ``None``, they must be the same type.
   If they are bytes, the returned name will be bytes instead of str.
   If you want to force a bytes return value with otherwise default behavior,
   pass ``suffix=b''``.

   If *text* is specified and true, the file is opened in text mode.
   Otherwise, (the default) the file is opened in binary mode.

   :func:`mkstemp` returns a tuple containing an OS-level handle to an open
   file (as would be returned by :func:`os.open`) and the absolute pathname
   of that file, in that order.

   .. audit-event:: tempfile.mkstemp fullpath tempfile.mkstemp

   .. versionchanged:: 3.5
      *suffix*, *prefix*, and *dir* may now be supplied in bytes in order to
      obtain a bytes return value.  Prior to this, only str was allowed.
      *suffix* and *prefix* now accept and default to ``None`` to cause
      an appropriate default value to be used.

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


.. function:: mkdtemp(suffix=None, prefix=None, dir=None)

   Creates a temporary directory in the most secure manner possible. There
   are no race conditions in the directory's creation.  The directory is
   readable, writable, and searchable only by the creating user ID.

   The user of :func:`mkdtemp` is responsible for deleting the temporary
   directory and its contents when done with it.

   The *prefix*, *suffix*, and *dir* arguments are the same as for
   :func:`mkstemp`.

   :func:`mkdtemp` returns the absolute pathname of the new directory.

   .. audit-event:: tempfile.mkdtemp fullpath tempfile.mkdtemp

   .. versionchanged:: 3.5
      *suffix*, *prefix*, and *dir* may now be supplied in bytes in order to
      obtain a bytes return value.  Prior to this, only str was allowed.
      *suffix* and *prefix* now accept and default to ``None`` to cause
      an appropriate default value to be used.

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

   .. versionchanged:: 3.12
      :func:`mkdtemp` now always returns an absolute path, even if *dir* is relative.


.. function:: gettempdir()

   Return the name of the directory used for temporary files. This
   defines the default value for the *dir* argument to all functions
   in this module.

   Python searches a standard list of directories to find one which
   the calling user can create files in.  The list is:

   #. The directory named by the :envvar:`TMPDIR` environment variable.

   #. The directory named by the :envvar:`TEMP` environment variable.

   #. The directory named by the :envvar:`TMP` environment variable.

   #. A platform-specific location:

      * On Windows, the directories :file:`C:\\TEMP`, :file:`C:\\TMP`,
        :file:`\\TEMP`, and :file:`\\TMP`, in that order.

      * On all other platforms, the directories :file:`/tmp`, :file:`/var/tmp`, and
        :file:`/usr/tmp`, in that order.

   #. As a last resort, the current working directory.

   The result of this search is cached, see the description of
   :data:`tempdir` below.

   .. versionchanged:: 3.10

      Always returns a str.  Previously it would return any :data:`tempdir`
      value regardless of type so long as it was not ``None``.

.. function:: gettempdirb()

   Same as :func:`gettempdir` but the return value is in bytes.

   .. versionadded:: 3.5

.. function:: gettempprefix()

   Return the filename prefix used to create temporary files.  This does not
   contain the directory component.

.. function:: gettempprefixb()

   Same as :func:`gettempprefix` but the return value is in bytes.

   .. versionadded:: 3.5

The module uses a global variable to store the name of the directory
used for temporary files returned by :func:`gettempdir`.  It can be
set directly to override the selection process, but this is discouraged.
All functions in this module take a *dir* argument which can be used
to specify the directory. This is the recommended approach that does
not surprise other unsuspecting code by changing global API behavior.

.. data:: tempdir

   When set to a value other than ``None``, this variable defines the
   default value for the *dir* argument to the functions defined in this
   module, including its type, bytes or str.  It cannot be a
   :term:`path-like object`.

   If ``tempdir`` is ``None`` (the default) at any call to any of the above
   functions except :func:`gettempprefix` it is initialized following the
   algorithm described in :func:`gettempdir`.

   .. note::

      Beware that if you set ``tempdir`` to a bytes value, there is a
      nasty side effect: The global default return type of
      :func:`mkstemp` and :func:`mkdtemp` changes to bytes when no
      explicit ``prefix``, ``suffix``, or ``dir`` arguments of type
      str are supplied. Please do not write code expecting or
      depending on this. This awkward behavior is maintained for
      compatibility with the historical implementation.

.. _tempfile-examples:

Examples
--------

Here are some examples of typical usage of the :mod:`tempfile` module::

    >>> import tempfile

    # create a temporary file and write some data to it
    >>> fp = tempfile.TemporaryFile()
    >>> fp.write(b'Hello world!')
    # read data from file
    >>> fp.seek(0)
    >>> fp.read()
    b'Hello world!'
    # close the file, it will be removed
    >>> fp.close()

    # create a temporary file using a context manager
    >>> with tempfile.TemporaryFile() as fp:
    ...     fp.write(b'Hello world!')
    ...     fp.seek(0)
    ...     fp.read()
    b'Hello world!'
    >>>
    # file is now closed and removed

    # create a temporary file using a context manager
    # close the file, use the name to open the file again
    >>> with tempfile.NamedTemporaryFile(delete_on_close=False) as fp:
    ...     fp.write(b'Hello world!')
    ...     fp.close()
    ... # the file is closed, but not removed
    ... # open the file again by using its name
    ...     with open(fp.name, mode='rb') as f:
    ...         f.read()
    b'Hello world!'
    >>>
    # file is now removed

    # create a temporary directory using the context manager
    >>> with tempfile.TemporaryDirectory() as tmpdirname:
    ...     print('created temporary directory', tmpdirname)
    >>>
    # directory and contents have been removed

.. _tempfile-mktemp-deprecated:

Deprecated functions and variables
----------------------------------

A historical way to create temporary files was to first generate a
file name with the :func:`mktemp` function and then create a file
using this name. Unfortunately this is not secure, because a different
process may create a file with this name in the time between the call
to :func:`mktemp` and the subsequent attempt to create the file by the
first process. The solution is to combine the two steps and create the
file immediately. This approach is used by :func:`mkstemp` and the
other functions described above.

.. function:: mktemp(suffix='', prefix='tmp', dir=None)

   .. deprecated:: 2.3
      Use :func:`mkstemp` instead.

   Return an absolute pathname of a file that did not exist at the time the
   call is made.  The *prefix*, *suffix*, and *dir* arguments are similar
   to those of :func:`mkstemp`, except that bytes file names, ``suffix=None``
   and ``prefix=None`` are not supported.

   .. warning::

      Use of this function may introduce a security hole in your program.  By
      the time you get around to doing anything with the file name it returns,
      someone else may have beaten you to the punch.  :func:`mktemp` usage can
      be replaced easily with :func:`NamedTemporaryFile`, passing it the
      ``delete=False`` parameter::

         >>> f = NamedTemporaryFile(delete=False)
         >>> f.name
         '/tmp/tmptjujjt'
         >>> f.write(b"Hello World!\n")
         13
         >>> f.close()
         >>> os.unlink(f.name)
         >>> os.path.exists(f.name)
         False