.. _howto-custom-file-storage:

Writing a custom storage system
===============================

.. currentmodule:: django.core.files.storage

If you need to provide custom file storage -- a common example is storing files
on some remote system -- you can do so by defining a custom storage class.
You'll need to follow these steps:

#. Your custom storage system must be a subclass of
   ``django.core.files.storage.Storage``::

        from django.core.files.storage import Storage

        class MyStorage(Storage):
            ...

#. Django must be able to instantiate your storage system without any arguments.
   This means that any settings should be taken from ``django.conf.settings``::

        from django.conf import settings
        from django.core.files.storage import Storage

        class MyStorage(Storage):
            def __init__(self, option=None):
                if not option:
                    option = settings.CUSTOM_STORAGE_OPTIONS
                ...

#. Your storage class must implement the ``_open()`` and ``_save()`` methods,
   along with any other methods appropriate to your storage class. See below for
   more on these methods.

   In addition, if your class provides local file storage, it must override
   the ``path()`` method.

Your custom storage system may override any of the storage methods explained in
:ref:`ref-files-storage`, but you **must** implement the following methods:

    * :meth:`Storage.delete`
    * :meth:`Storage.exists`
    * :meth:`Storage.listdir`
    * :meth:`Storage.size`
    * :meth:`Storage.url`

You'll also usually want to use hooks specifically designed for custom storage
objects. These are:

``_open(name, mode='rb')``
~~~~~~~~~~~~~~~~~~~~~~~~~~

**Required**.

Called by ``Storage.open()``, this is the actual mechanism the storage class
uses to open the file. This must return a ``File`` object, though in most cases,
you'll want to return some subclass here that implements logic specific to the
backend storage system.

``_save(name, content)``
~~~~~~~~~~~~~~~~~~~~~~~~

Called by ``Storage.save()``. The ``name`` will already have gone through
``get_valid_name()`` and ``get_available_name()``, and the ``content`` will be a
``File`` object itself. 

Should return the actual name of name of the file saved (usually the ``name``
passed in, but if the storage needs to change the file name return the new name
instead).

``get_valid_name(name)``
------------------------

Returns a filename suitable for use with the underlying storage system. The
``name`` argument passed to this method is the original filename sent to the
server, after having any path information removed. Override this to customize
how non-standard characters are converted to safe filenames.

The code provided on ``Storage`` retains only alpha-numeric characters, periods
and underscores from the original filename, removing everything else.

``get_available_name(name)``
----------------------------

Returns a filename that is available in the storage mechanism, possibly taking
the provided filename into account. The ``name`` argument passed to this method
will have already cleaned to a filename valid for the storage system, according
to the ``get_valid_name()`` method described above.

The code provided on ``Storage`` simply appends underscores to the filename
until it finds one that's available in the destination directory.
