FileStorage contains stream field. This object must extend IO or file object, so it must contain read and other similar methods. FileStorage also extend stream field object attributes, so you can just use file.read() instead file.stream.read(). Also you can use save argument with dst parameter as StringIO or other IO or file object to copy FileStorage.stream to another IO or file object.

See documentation: http://flask.pocoo.org/docs/api/#flask.Request.files and http://werkzeug.pocoo.org/docs/datastructures/#werkzeug.datastructures.FileStorage.

Data uploaded via a FileField in a FlaskForm (the data thats in form.photo.data in your example) is represented as a FileStorage object. According to the documentation, instead of doing f.save(...), you also can do f.read() to access that data.

If you want only to read data from file then simply use .read()

uploaded_file = request.files.get('uploaded_file')

data = uploaded_file.read()

print(data)

And if you want to use it with function which needs filename then usually it may work also with file-like object and you can use it directly

ie. Pillo.Image

from PIL import Image

uploaded_file = request.files.get('uploaded_file')

img = Image.open(uploaded_file)
img.save('new_name.jpg')

ie. pandas.DataFrame

import pandas as pd

uploaded_file = request.files.get('uploaded_file')

df = pd.read_csv(uploaded_file)

You can also use io.BytesIO() to create file-like object in memory.

from PIL import Image
import io

uploaded_file = request.files.get('uploaded_file')

data = uploaded_file.read()

file_object = io.BytesIO(data)
#file_object.seek(0)

img = Image.open(file_object)
img.save('new_name.jpg')

import pandas as pd
import io

uploaded_file = request.files.get('uploaded_file')

data = uploaded_file.read()

file_object = io.BytesIO(data)
#file_object.seek(0)

df = pd.read_csv(file_object)

But this is more useful when you want to save/generate data without writing on disk and later send it back to browser.

uploaded_file = request.files.get('uploaded_file')

img = Image.open(uploaded_file)

# generate smaller version
img.thumbnail((200,200))

# write in file-like object as JPG
file_object = io.BytesIO()
img.save(file_object, 'JPEG')

# get data from file
data = file_object.getvalue()  # get data directly
# OR
#file_object.seek(0)           # move to the beginning of file after previous writing/reading
#data = file_object.read()     # read like from normal file

# send to browser as HTML
data = base64.b64encode(data).decode()
html = f'<img src="data:image/jpg;base64, {data}">'

Request.files

A MultiDict with files uploaded as part of a POST or PUT request. Each file is stored as FileStorage object. It basically behaves like a standard file object you know from Python, with the difference that it also has a save() function that can store the file on the filesystem. http://flask.pocoo.org/docs/0.10/api/#flask.Request.files

You don't need use json, just use read(), like this:

if request.method == 'POST':
    file = request.files['file']        
    myfile = file.read()

You already saved it here

file.save(os.path.join(app.config['UPLOAD_FOLDER'], filename))

just open it up and read as you work with any other files, example:

@app.route('/read_file', methods=['GET'])
def read_uploaded_file():
    filename = secure_filename(request.args.get('filename'))
    try:
        if filename and allowed_filename(filename):
            with open(os.path.join(app.config['UPLOAD_FOLDER'], filename)) as f:
                return f.read()
    except IOError:
        pass
    return "Unable to read file"

You need to carefully sanitize user input here, otherwise method could be used to read something unintended (like app source code for example). Best is just not grant user ability to read arbitrary files - for example when you save a file, store it's path in database with some token and give user just this token:

filename = secure_filename(file.filename)
filepath = os.path.join(app.config['UPLOAD_FOLDER'], filename)
file.save(filepath)
token = store_in_db(filepath)
return redirect(url_for('read_uploaded_file',
                                   token=token))

Then accept a token, not a filename when you read a file:

@app.route('/read_file', methods=['GET'])
def read_uploaded_file():
    filepath = get_filepath(request.args.get('token'))
    try:
        if filepath and allowed_filepath(filepath):
            with open(filepath) as f:
                return f.read()
    except IOError:
        pass
    return "Unable to read file"

Tokens need to be random, long, not guessable (uuid4 for example) - otherwise there will be a possibility to easily read other users files. Or you need to store relation between file and user in database and check it too. Finally, you need to control size of file uploads to prevent user from uploading huge files (app.config['MAX_CONTENT_LENGTH']) and control amount of info you read in memory when you display "filtered" file content (f.read(max_allowed_size)).

Thanks to the commenters on this post, I could come up with a fix to this. Initially, I was getting a b" as the only output after using only f.read().

When I used f.seek(0), I got the output of my text file but with a lot of encoding and formatting errors, including a new preceding b` followed by my output.

But adding content = f.read()
content = str(content, 'utf-8') fixed most of that aswell.

So here goes the final solution-

def upload_source():
if request.method == 'POST':
    # check if the post request has the file part
    f = request.files['file']
    if f.filename == "":
        print("No file Name")
        return redirect(request.url)
    if not allowed_file(f.filename):
        print("File extension not allowed!")
        return redirect(request.url)
    else:
        full_filename = secure_filename(f.filename)
        f.save(os.path.join(app.config['UPLOAD_FOLDER'], full_filename))
        f.seek(0)
        print("File saved")
        content = f.read()
        content = str(content, 'utf-8')
        return render_template('source.html', text=content)

and don't forget the same old HTML -

<p> {{ text }} </p>

I hope this can help others as well. Thank you!