Ah yes, the good old problem of file uploads. The basic idea of fileuploads is actually quite simple. It basically works like this:
- A
<form>
tag is marked withenctype=multipart/form-data
and an<inputtype=file>
is placed in that form. - The application accesses the file from the
files
dictionary on the request object. - use the
save()
method of the file to savethe file permanently somewhere on the filesystem.
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A Gentle Introduction¶
Let’s start with a very basic application that uploads a file to aspecific upload folder and displays a file to the user. Let’s look at thebootstrapping code for our application:
So first we need a couple of imports. Most should be straightforward, the
werkzeug.secure_filename()
is explained a little bit later. TheUPLOAD_FOLDER
is where we will store the uploaded files and theALLOWED_EXTENSIONS
is the set of allowed file extensions.Why do we limit the extensions that are allowed? You probably don’t wantyour users to be able to upload everything there if the server is directlysending out the data to the client. That way you can make sure that usersare not able to upload HTML files that would cause XSS problems (seeCross-Site Scripting (XSS)). Also make sure to disallow
.php
files if the serverexecutes them, but who has PHP installed on their server, right? Mori seiki nt operator manual. :)Next the functions that check if an extension is valid and that uploadsthe file and redirects the user to the URL for the uploaded file:
So what does that
secure_filename()
function actually do?Now the problem is that there is that principle called “never trust userinput”. This is also true for the filename of an uploaded file. Allsubmitted form data can be forged, and filenames can be dangerous. Forthe moment just remember: always use that function to secure a filenamebefore storing it directly on the filesystem.Information for the Pros
So you’re interested in what that
secure_filename()
function does and what the problem is if you’re not using it? So justimagine someone would send the following information as filename toyour application:Assuming the number of
./
is correct and you would join this withthe UPLOAD_FOLDER
the user might have the ability to modify a file onthe server’s filesystem he or she should not modify. This does require someknowledge about how the application looks like, but trust me, hackersare patient :)Now let’s look how that function works:
Now one last thing is missing: the serving of the uploaded files. In the
upload_file()
we redirect the user tourl_for('uploaded_file',filename=filename)
, that is, /uploads/filename
.So we write the uploaded_file()
function to return the file of that name. Asof Flask 0.5 we can use a function that does that for us:Alternatively you can register uploaded_file as build_only rule anduse the
SharedDataMiddleware
. This also works witholder versions of Flask:If you now run the application everything should work as expected.
Improving Uploads¶
Changelog
So how exactly does Flask handle uploads? Well it will store them in thewebserver’s memory if the files are reasonable small otherwise in atemporary location (as returned by
tempfile.gettempdir()
). But howdo you specify the maximum file size after which an upload is aborted? Bydefault Flask will happily accept file uploads to an unlimited amount ofmemory, but you can limit that by setting the MAX_CONTENT_LENGTH
config key:The code above will limit the maximum allowed payload to 16 megabytes.If a larger file is transmitted, Flask will raise a
RequestEntityTooLarge
exception.Connection Reset Issue
When using the local development server, you may get a connectionreset error instead of a 413 response. You will get the correctstatus response when running the app with a production WSGI server.
This feature was added in Flask 0.6 but can be achieved in older versionsas well by subclassing the request object. For more information on thatconsult the Werkzeug documentation on file handling.
Upload Progress Bars¶
A while ago many developers had the idea to read the incoming file insmall chunks and store the upload progress in the database to be able topoll the progress with JavaScript from the client. Long story short: theclient asks the server every 5 seconds how much it has transmittedalready. Do you realize the irony? The client is asking for something itshould already know.
An Easier Solution¶
Now there are better solutions that work faster and are more reliable. Thereare JavaScript libraries like jQuery that have form plugins to ease theconstruction of progress bar.
Because the common pattern for file uploads exists almost unchanged in allapplications dealing with uploads, there is also a Flask extension calledFlask-Uploads that implements a full fledged upload mechanism with white andblacklisting of extensions and more.
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