Flask uploaded file size is not correct

May 6, 2023

Why flask’s uploaded file byte length is shorter than what it is actually? I’m seeing same behavior for several images.

from flask import Flask

app = Flask(__name__)

@app.route('/', methods=['GET', 'POST'])
def home():
    if request.method == 'POST':
        f = file.read()
        print(len(f))  # 45825

However,

f = skimage.io.imread('file.jpg')
len(f.tobytes())  # 1102080

>Solution :

When you upload an image using Flask, it reads the file’s content as a bytestring, which represents the size of the compressed image file. However, when reading the image with skimage.io.imread(‘file.jpg’), it’s read into a NumPy array, representing the uncompressed image data. So I think you’re seeing the compression rate.

To compare the sizes fairly, you can convert the uploaded bytestring to a NumPy array without saving it to disk:

import io
from PIL import Image
import numpy as np
from flask import Flask, request

app = Flask(__name__)

@app.route('/', methods=['GET', 'POST'])
def home():
    if request.method == 'POST':
        file = request.files['file']
        f = file.read()
        print(len(f))

        # Convert the bytestring to a NumPy array
        image = Image.open(io.BytesIO(f))
        image_np = np.array(image)
        print(len(image_np.tobytes()))