Read more about sending files in Flask over at the official documentation, linked here. Drop the app. Using the sass, rcssmin and rjsmin Python packages to optimize web assets, including Bootstrap. Using Python decorators to add another layer of functionality to Flask routes. Home Articles Categories Series. Recommended learning Pluralsight Treehouse. Sending files with Flask Learning Flask Ep. Let's get started.
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We will be creating back-end to remove watermark from pdf files. For that we need to get user file, process remove watermark from pdf file and provide processed file to user for download. We will create a simple HTML page that provides a button to select file and another button to upload that file.
Let the HTML page be index. 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.
The client asks the server every 5 seconds how much it has transmitted, but this is something it should already know. Now there are better solutions that work faster and are more reliable. There are JavaScript libraries like jQuery that have form plugins to ease the construction of progress bar. Because the common pattern for file uploads exists almost unchanged in all applications dealing with uploads, there are also some Flask extensions that implement a full fledged upload mechanism that allows controlling which file extensions are allowed to be uploaded.
Navigation index modules next previous Flask Documentation 2. We can't really trust that the filenames provided by the client are valid and safe to use, so filenames coming with uploaded files have to be validated. A very simple validation to perform is to make sure that the file extension is one that the application is willing to accept, which is similar to what the FileAllowed validator does when using Flask-WTF. Let's say the application accepts images, then it can configure the list of approved file extensions:.
For every uploaded file, the application can make sure that the file extension is one of the allowed ones:. With this logic, any filenames that do not have one of the approved file extensions is going to be responded with a error. In addition to the file extension, it is also important to validate the filename, and any path given with it.
If your application does not care about the filename provided by the client, the most secure way to handle the upload is to ignore the client provided filename and generate your own filename instead, that you pass to the save method. An example use case where this technique works well is with avatar image uploads. Each user's avatar can be saved with the user id as filename, so the filename provided by the client can be discarded.
If your application uses Flask-Login, you could implement the following save call:. In other cases it may be better to preserve the filenames provided by the client, so the filename must be sanitized first.
Let's see how this function works by running a few tests in a Python session:. Here is the complete app. The third layer of validation that I'm going to discuss is the most complex. If your application accepts uploads of a certain file type, it should ideally perform some form of content validation and reject any files that are of a different type. How you achieve content validation largely depends on the file types your application accepts.
For the example application in this article I'm using images, so I can use the imghdr package from the Python standard library to validate that the header of the file is, in fact, an image.
This function takes a byte stream as an argument. It starts by reading bytes from the stream, and then resetting the stream pointer back, because later when the save function is called we want it to see the entire stream.
The first bytes of the image data are going to be sufficient to identify the format of the image. The imghdr. The FileStorage object gives us a stream, so the most convenient option is to read a safe amount of data from it and pass it as a byte sequence in the second argument.
The return value of imghdr. The function supports a variety of formats, among them the popular jpeg , png and gif. If not known image format is detected, then the return value is None.
If a format is detected, the name of the format is returned. This is as simple as adding a dot as prefix for all image formats except jpeg , which normally uses the.
This expanded check first makes sure that the file extension is in the allowed list, and then ensures that the detected file extension from looking at the data stream is the same as the file extension. You now know how to handle file uploads.
For some applications this is all that is needed, as the files are used for some internal process. But for a large number of applications, in particular those with social features such as avatars, the files that are uploaded by users have to be integrated with the application.
Using the example of avatars, once a user uploads their avatar image, any mention of the username requires the uploaded image to appear to the side. I divide file uploads into two large groups, depending on whether the files uploaded by users are intended for public use, or they are private to each user. The avatar images discussed several times in this article are clearly in the first group, as these avatars are intended to be publicly shared with other users. On the other side, an application that performs editing operations on uploaded images would probably be in the second group, because you'd want each user to only have access to their own images.
When images are of a public nature, the easiest way to make the images available for use by the application is to put the upload directory inside the application's static folder. For example, an avatars subdirectory can be created inside static , and then avatar images can be saved in that location using the user id as name.
I previously suggested using the user id as a filename, when saving an uploaded avatar image.
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