Fixed #18154 -- Documentation on closing File objects and best practices

Now you can use any of the documented attributes and methods

of the :class:`~django.core.files.File` class.

Be aware that files created in this way are not automatically closed.

The following approach may be used to close files automatically::

    >>> from django.core.files import File

    # Create a Python file object using open() and the with statement

    >>> with open('/tmp/hello.world', 'w') as f:

    >>>     myfile = File(f)

    >>>     for line in myfile:

    >>>         print line

    >>> myfile.closed

    True

    >>> f.closed

    True

Closing files is especially important when accessing file fields in a loop

over a large number of objects:: If files are not manually closed after

accessing them, the risk of running out of file descriptors may arise. This

may lead to the following error:

    IOError: [Errno 24] Too many open files

File storage

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