lib/django-1.5/docs/topics/files.txt

==============
Managing files
==============

This document describes Django's file access APIs for files such as those
uploaded by a user. The lower level APIs are general enough that you could use
them for other purposes. If you want to handle "static files" (JS, CSS, etc),
see :doc:`/howto/static-files/index`.

By default, Django stores files locally, using the :setting:`MEDIA_ROOT` and
:setting:`MEDIA_URL` settings. The examples below assume that you're using these
defaults.

However, Django provides ways to write custom `file storage systems`_ that
allow you to completely customize where and how Django stores files. The
second half of this document describes how these storage systems work.

.. _file storage systems: `File storage`_

Using files in models
=====================

When you use a :class:`~django.db.models.FileField` or
:class:`~django.db.models.ImageField`, Django provides a set of APIs you can use
to deal with that file.

Consider the following model, using an :class:`~django.db.models.ImageField` to
store a photo::

    class Car(models.Model):
        name = models.CharField(max_length=255)
        price = models.DecimalField(max_digits=5, decimal_places=2)
        photo = models.ImageField(upload_to='cars')

Any ``Car`` instance will have a ``photo`` attribute that you can use to get at
the details of the attached photo::

    >>> car = Car.objects.get(name="57 Chevy")
    >>> car.photo
    <ImageFieldFile: chevy.jpg>
    >>> car.photo.name
    u'cars/chevy.jpg'
    >>> car.photo.path
    u'/media/cars/chevy.jpg'
    >>> car.photo.url
    u'http://media.example.com/cars/chevy.jpg'

This object -- ``car.photo`` in the example -- is a ``File`` object, which means
it has all the methods and attributes described below.

.. note::
    The file is saved as part of saving the model in the database, so the actual
    file name used on disk cannot be relied on until after the model has been
    saved.


The ``File`` object
===================

Internally, Django uses a :class:`django.core.files.File` instance any time it
needs to represent a file. This object is a thin wrapper around Python's
`built-in file object`_ with some Django-specific additions.

.. _built-in file object: http://docs.python.org/library/stdtypes.html#bltin-file-objects

Most of the time you'll simply use a ``File`` that Django's given you (i.e. a
file attached to a model as above, or perhaps an uploaded file).

If you need to construct a ``File`` yourself, the easiest way is to create one
using a Python built-in ``file`` object::

    >>> from django.core.files import File

    # Create a Python file object using open()
    >>> f = open('/tmp/hello.world', 'w')
    >>> myfile = File(f)

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)
    ...     myfile.write('Hello World')
    ...
    >>> 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
============

Behind the scenes, Django delegates decisions about how and where to store files
to a file storage system. This is the object that actually understands things
like file systems, opening and reading files, etc.

Django's default file storage is given by the :setting:`DEFAULT_FILE_STORAGE`
setting; if you don't explicitly provide a storage system, this is the one that
will be used.

See below for details of the built-in default file storage system, and see
:doc:`/howto/custom-file-storage` for information on writing your own file
storage system.

Storage objects
---------------

Though most of the time you'll want to use a ``File`` object (which delegates to
the proper storage for that file), you can use file storage systems directly.
You can create an instance of some custom file storage class, or -- often more
useful -- you can use the global default storage system::

    >>> from django.core.files.storage import default_storage
    >>> from django.core.files.base import ContentFile

    >>> path = default_storage.save('/path/to/file', ContentFile('new content'))
    >>> path
    u'/path/to/file'

    >>> default_storage.size(path)
    11
    >>> default_storage.open(path).read()
    'new content'

    >>> default_storage.delete(path)
    >>> default_storage.exists(path)
    False

See :doc:`/ref/files/storage` for the file storage API.

.. _builtin-fs-storage:

The built-in filesystem storage class
-------------------------------------

Django ships with a built-in ``FileSystemStorage`` class (defined in
``django.core.files.storage``) which implements basic local filesystem file
storage. Its initializer takes two arguments:

======================  ===================================================
Argument                Description
======================  ===================================================
``location``            Optional. Absolute path to the directory that will
                        hold the files. If omitted, it will be set to the
                        value of your :setting:`MEDIA_ROOT` setting.
``base_url``            Optional. URL that serves the files stored at this
                        location. If omitted, it will default to the value
                        of your :setting:`MEDIA_URL` setting.
======================  ===================================================

For example, the following code will store uploaded files under
``/media/photos`` regardless of what your :setting:`MEDIA_ROOT` setting is::

    from django.db import models
    from django.core.files.storage import FileSystemStorage

    fs = FileSystemStorage(location='/media/photos')

    class Car(models.Model):
        ...
        photo = models.ImageField(storage=fs)

:doc:`Custom storage systems </howto/custom-file-storage>` work the same way:
you can pass them in as the ``storage`` argument to a
:class:`~django.db.models.FileField`.