hg.stevelosh.com > hg-review

.. _uploading-files:

Uploading Files
===============

Ah yes, the good old problem of file uploads.  The basic idea of file
uploads is actually quite simple.  It basically works like this:

1. A ``<form>`` tag is marked with ``enctype=multipart/form-data``
   and an ``<input type=file>`` is placed in that form.
2. The application accesses the file from the :attr:`~flask.request.files`
   dictionary on the request object.
3. use the :meth:`~werkzeug.FileStorage.save` method of the file to save
   the file permanently somewhere on the filesystem.

A Gentle Introduction
---------------------

Let's start with a very basic application that uploads a file to a
specific upload folder and displays a file to the user.  Let's look at the
bootstrapping code for our application::

    import os
    from flask import Flask, request, redirect, url_for
    from werkzeug import secure_filename

    UPLOAD_FOLDER = '/path/to/the/uploads'
    ALLOWED_EXTENSIONS = set(['txt', 'pdf', 'png', 'jpg', 'jpeg', 'gif'])

    app = Flask(__name__)

So first we need a couple of imports.  Most should be straightforward, the
:func:`werkzeug.secure_filename` is explained a little bit later.  The
`UPLOAD_FOLDER` is where we will store the uploaded files and the
`ALLOWED_EXTENSIONS` is the set of allowed file extensions.  Then we add a
URL rule by hand to the application.  Now usually we're not doing that, so
why here?  The reasons is that we want the webserver (or our development
server) to serve these files for us and so we only need a rule to generate
the URL to these files.

Why do we limit the extensions that are allowed?  You probably don't want
your users to be able to upload everything there if the server is directly
sending out the data to the client.  That way you can make sure that users
are not able to upload HTML files that would cause XSS problems (see
:ref:`xss`).  Also make sure to disallow `.php` files if the server
executes them, but who has PHP installed on his server, right?  :)

Next the functions that check if an extension is valid and that uploads
the file and redirects the user to the URL for the uploaded file::

    def allowed_file(filename):
        return '.' in filename and \
               filename.rsplit('.', 1)[1] in ALLOWED_EXTENSIONS

    @app.route('/', methods=['GET', 'POST'])
    def upload_file():
        if request.method == 'POST':
            file = request.files['file']
            if file and allowed_file(file.filename):
                filename = secure_filename(file.filename)
                file.save(os.path.join(UPLOAD_FOLDER, filename))
                return redirect(url_for('uploaded_file',
                                        filename=filename))
        return '''
        <!doctype html>
        <title>Upload new File</title>
        <h1>Upload new File</h1>
        <form action="" method=post enctype=multipart/form-data>
          <p><input type=file name=file>
             <input type=submit value=Upload>
        </form>
        '''

So what does that :func:`~werkzeug.secure_filename` function actually do?
Now the problem is that there is that principle called "never trust user
input".  This is also true for the filename of an uploaded file.  All
submitted form data can be forged, and filenames can be dangerous.  For
the moment just remember: always use that function to secure a filename
before storing it directly on the filesystem.

.. admonition:: Information for the Pros

   So you're interested in what that :func:`~werkzeug.secure_filename`
   function does and what the problem is if you're not using it?  So just
   imagine someone would send the following information as `filename` to
   your application::

      filename = "../../../../home/username/.bashrc"

   Assuming the number of ``../`` is correct and you would join this with
   the `UPLOAD_FOLDER` the user might have the ability to modify a file on
   the server's filesystem he should not modify.  This does require some
   knowledge about how the application looks like, but trust me, hackers
   are patient :)

   Now let's look how that function works:

   >>> secure_filename('../../../../home/username/.bashrc')
   'home_username_.bashrc'

Now one last thing is missing: the serving of the uploaded files.  As of
Flask 0.5 we can use a function that does that for us::

    from flask import send_from_directory

    @app.route('/uploads/<filename>')
    def uploaded_file(filename):
        return send_from_directory(app.config['UPLOAD_FOLDER'],
                                   filename)

Alternatively you can register `uploaded_file` as `build_only` rule and
use the :class:`~werkzeug.SharedDataMiddleware`.  This also works with
older versions of Flask::

    from werkzeug import SharedDataMiddleware
    app.add_url_rule('/uploads/<filename>', 'uploaded_file',
                     build_only=True)
    app.wsgi_app = SharedDataMiddleware(app.wsgi_app, {
        '/uploads':  UPLOAD_FOLDER
    })

If you now run the application everything should work as expected.


Improving Uploads
-----------------

.. versionadded:: 0.6

So how exactly does Flask handle uploads?  Well it will store them in the
webserver's memory if the files are reasonable small otherwise in a
temporary location (as returned by :func:`tempfile.gettempdir`).  But how
do you specify the maximum file size after which an upload is aborted?  By
default Flask will happily accept file uploads to an unlimited amount of
memory, but you can limit that by setting the ``MAX_CONTENT_LENGTH``
config key::

    from flask import Flask, Request

    app = Flask(__name__)
    app.config['MAX_CONTENT_LENGTH'] = 16 * 1024 * 1024

The code above will limited the maximum allowed payload to 16 megabytes.
If a larger file is transmitted, Flask will raise an
:exc:`~werkzeug.exceptions.RequestEntityTooLarge` exception.

This feature was added in Flask 0.6 but can be achieved in older versions
as well by subclassing the request object.  For more information on that
consult the Werkzeug documentation on file handling.


Upload Progress Bars
--------------------

A while ago many developers had the idea to read the incoming file in
small chunks and store the upload progress in the database to be able to
poll the progress with JavaScript from the client.  Long story short: the
client asks the server every 5 seconds how much he has transmitted
already.  Do you realize the irony?  The client is asking for something he
should already know.

Now there are better solutions to that work faster and more reliable.  The
web changed a lot lately and you can use HTML5, Java, Silverlight or Flash
to get a nicer uploading experience on the client side.  Look at the
following libraries for some nice examples how to do that:

-   `Plupload <http://www.plupload.com/>`_ - HTML5, Java, Flash
-   `SWFUpload <http://www.swfupload.org/>`_ - Flash
-   `JumpLoader <http://jumploader.com/>`_ - Java


An Easier Solution
------------------

Because the common pattern for file uploads exists almost unchanged in all
applications dealing with uploads, there is a Flask extension called
`Flask-Uploads`_ that implements a full fledged upload mechanism with
white and blacklisting of extensions and more.

.. _Flask-Uploads: http://packages.python.org/Flask-Uploads/
author	Christophe de Vienne <christophe@cdevienne.info>
date	Fri, 19 Aug 2016 18:21:28 +0200
parents	f33efe14bff1
children	(none)