In-memory data in a Flask app - flask

Question

I think the correct way to have instance variables in Flask is by adding users and sessions, but I'm trying to test a concept and I don't want to go through all of that just yet. I'm trying to have a web app load an image into a variable that can then have different image operations performed on it. Obviously you don't want to have to keep performing a list of operations on the image on each new request because that would be horribly inefficient.
Is there a way of having an app.var in Flask that I can access from different routes? I've tried using the global context and Flask's current_app, but I get the impression that's not what they're for.
The code for my blueprint is:
import os
from flask import Flask, url_for, render_template, \
g, send_file, Blueprint
from io import BytesIO
from PIL import Image, ImageDraw, ImageOps
home = Blueprint('home', __name__)
#home.before_request
def before_request():
g.img = None
g.user = None
#home.route('/')
def index():
return render_template('home/index.html')
#home.route('/image')
def image():
if g.img is None:
root = os.path.dirname(os.path.abspath(__file__))
filename = os.path.join(root, '../static/images/lena.jpg')
g.img = Image.open(filename)
img_bytes = BytesIO()
g.img.save(img_bytes, 'jpeg')
img_bytes.seek(0)
return send_file(img_bytes, mimetype='image/jpg')
#home.route('/grayscale', methods=['POST'])
def grayscale():
if g.img:
print('POST grayscale request')
g.img = ImageOps.grayscale(img)
return "Grayscale operation successful"
else:
print('Grayscale called with no image loaded')
return "Grayscale operation failed"
The /image route returns the image correctly, but I'd like to be able to call /grayscale, perform the operation, and be able to make another call to /image and have it return the image from memory without loading it.

Answer 1

You could save a key in your session variable and use that to identify the image in a global dictionary. However this might lead to some trouble if you use multiple Flask application instances. But with one it would be fine. Otherwise you could use Redis when working with multiple workers. I haven't tried the following code but it should show the concept.
from flask import session
import uuid
app.config['SECRET_KEY'] = 'your secret key'
img_dict = {}
#route('/image')
def image():
key = session.get('key')
if key is None:
session['key'] = key = uuid.uuid1()
img_dict[key] = yourimagedata
#home.route('/grayscale', methods=['POST'])
def grayscale():
key = session.get('key')
if key is None:
print('Grayscale called with no image loaded')
return "Grayscale operation failed"
else:
img = img_dict[key]
print('POST grayscale request')
g.img = ImageOps.grayscale(img)
return "Grayscale operation successful"