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URL generation rule explained for file uploads
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Sourya Vatsyayan
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.. _uploading-files:
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Uploading Files
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===============
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Ah yes, the good old problem of file uploads. The basic idea of file
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uploads is actually quite simple. It basically works like this:
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1. A ``<form>`` tag is marked with ``enctype=multipart/form-data``
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and an ``<input type=file>`` is placed in that form.
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2. The application accesses the file from the :attr:`~flask.request.files`
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dictionary on the request object.
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3. use the :meth:`~werkzeug.datastructures.FileStorage.save` method of the file to save
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the file permanently somewhere on the filesystem.
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A Gentle Introduction
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---------------------
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Let's start with a very basic application that uploads a file to a
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specific upload folder and displays a file to the user. Let's look at the
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bootstrapping code for our application::
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import os
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from flask import Flask, request, redirect, url_for
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from werkzeug.utils import secure_filename
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UPLOAD_FOLDER = '/path/to/the/uploads'
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ALLOWED_EXTENSIONS = set(['txt', 'pdf', 'png', 'jpg', 'jpeg', 'gif'])
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app = Flask(__name__)
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app.config['UPLOAD_FOLDER'] = UPLOAD_FOLDER
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So first we need a couple of imports. Most should be straightforward, the
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:func:`werkzeug.secure_filename` is explained a little bit later. The
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``UPLOAD_FOLDER`` is where we will store the uploaded files and the
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``ALLOWED_EXTENSIONS`` is the set of allowed file extensions. Then we add a
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URL rule by hand to the application. Now usually we're not doing that, so
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why here? The reasons is that we want the webserver (or our development
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server) to serve these files for us and so we only need a rule to generate
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the URL to these files.
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Why do we limit the extensions that are allowed? You probably don't want
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your users to be able to upload everything there if the server is directly
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sending out the data to the client. That way you can make sure that users
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are not able to upload HTML files that would cause XSS problems (see
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:ref:`xss`). Also make sure to disallow ``.php`` files if the server
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executes them, but who has PHP installed on his server, right? :)
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Next the functions that check if an extension is valid and that uploads
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the file and redirects the user to the URL for the uploaded file::
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def allowed_file(filename):
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return '.' in filename and \
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filename.rsplit('.', 1)[1] in ALLOWED_EXTENSIONS
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@app.route('/', methods=['GET', 'POST'])
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def upload_file():
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if request.method == 'POST':
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# check if the post request has the file part
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if 'file' not in request.files:
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flash('No file part')
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return redirect(request.url)
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file = request.files['file']
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# if user does not select file, browser also
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# submit a empty part without filename
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if file.filename == '':
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flash('No selected file')
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return redirect(request.url)
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if file and allowed_file(file.filename):
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filename = secure_filename(file.filename)
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file.save(os.path.join(app.config['UPLOAD_FOLDER'], filename))
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return redirect(url_for('uploaded_file',
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filename=filename))
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return '''
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<!doctype html>
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<title>Upload new File</title>
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<h1>Upload new File</h1>
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<form action="" method=post enctype=multipart/form-data>
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<p><input type=file name=file>
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<input type=submit value=Upload>
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</form>
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'''
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So what does that :func:`~werkzeug.utils.secure_filename` function actually do?
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Now the problem is that there is that principle called "never trust user
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input". This is also true for the filename of an uploaded file. All
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submitted form data can be forged, and filenames can be dangerous. For
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the moment just remember: always use that function to secure a filename
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before storing it directly on the filesystem.
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.. admonition:: Information for the Pros
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So you're interested in what that :func:`~werkzeug.utils.secure_filename`
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function does and what the problem is if you're not using it? So just
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imagine someone would send the following information as `filename` to
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your application::
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filename = "../../../../home/username/.bashrc"
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Assuming the number of ``../`` is correct and you would join this with
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the ``UPLOAD_FOLDER`` the user might have the ability to modify a file on
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the server's filesystem he or she should not modify. This does require some
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knowledge about how the application looks like, but trust me, hackers
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are patient :)
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Now let's look how that function works:
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>>> secure_filename('../../../../home/username/.bashrc')
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'home_username_.bashrc'
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Now one last thing is missing: the serving of the uploaded files. As of
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Flask 0.5 we can use a function that does that for us::
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from flask import send_from_directory
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@app.route('/uploads/<filename>')
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def uploaded_file(filename):
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return send_from_directory(app.config['UPLOAD_FOLDER'],
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filename)
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Alternatively you can register `uploaded_file` as `build_only` rule and
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use the :class:`~werkzeug.wsgi.SharedDataMiddleware`. This also works with
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older versions of Flask::
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from werkzeug import SharedDataMiddleware
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app.add_url_rule('/uploads/<filename>', 'uploaded_file',
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build_only=True)
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app.wsgi_app = SharedDataMiddleware(app.wsgi_app, {
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'/uploads': app.config['UPLOAD_FOLDER']
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})
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If you now run the application everything should work as expected.
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Improving Uploads
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-----------------
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.. versionadded:: 0.6
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So how exactly does Flask handle uploads? Well it will store them in the
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webserver's memory if the files are reasonable small otherwise in a
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temporary location (as returned by :func:`tempfile.gettempdir`). But how
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do you specify the maximum file size after which an upload is aborted? By
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default Flask will happily accept file uploads to an unlimited amount of
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memory, but you can limit that by setting the ``MAX_CONTENT_LENGTH``
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config key::
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from flask import Flask, Request
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app = Flask(__name__)
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app.config['MAX_CONTENT_LENGTH'] = 16 * 1024 * 1024
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The code above will limited the maximum allowed payload to 16 megabytes.
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If a larger file is transmitted, Flask will raise an
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:exc:`~werkzeug.exceptions.RequestEntityTooLarge` exception.
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This feature was added in Flask 0.6 but can be achieved in older versions
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as well by subclassing the request object. For more information on that
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consult the Werkzeug documentation on file handling.
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Upload Progress Bars
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--------------------
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A while ago many developers had the idea to read the incoming file in
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small chunks and store the upload progress in the database to be able to
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poll the progress with JavaScript from the client. Long story short: the
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client asks the server every 5 seconds how much it has transmitted
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already. Do you realize the irony? The client is asking for something it
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should already know.
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Now there are better solutions to that work faster and more reliable. The
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web changed a lot lately and you can use HTML5, Java, Silverlight or Flash
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to get a nicer uploading experience on the client side. Look at the
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following libraries for some nice examples how to do that:
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- `Plupload <http://www.plupload.com/>`_ - HTML5, Java, Flash
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- `SWFUpload <http://www.swfupload.org/>`_ - Flash
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- `JumpLoader <http://jumploader.com/>`_ - Java
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An Easier Solution
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------------------
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Because the common pattern for file uploads exists almost unchanged in all
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applications dealing with uploads, there is a Flask extension called
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`Flask-Uploads`_ that implements a full fledged upload mechanism with
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white and blacklisting of extensions and more.
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.. _Flask-Uploads: http://pythonhosted.org/Flask-Uploads/
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