Read file data without saving it in Flask

I am writing my first flask application. I am dealing with file uploads, and basically what I want is to read the data/content of the uploaded file without saving it and then print it on the resulting page. Yes, I am assuming that the user uploads a text file always.

Here is the simple upload function i am using:

@app.route("/upload/", methods=["GET", "POST"])
def upload():
    if request.method == "POST":
        file = request.files["file"]
        if file:
            filename = secure_filename(file.filename)
            file.save(os.path.join(app.config["UPLOAD_FOLDER"], filename))
            a = "file uploaded"

    return render_template("upload.html", data = a)

Right now, I am saving the file, but what I need is that "a" variable to contain the content/data of the file .. any ideas?