Post image to Django REST API

Question

I have a Django project that receives an image, process it and return a response. I am writing a script to test my API, but the bytes that client sends is not the same that the server receives.

Client code:

# client.py
from urllib.parse import urlencode
from urllib.request import Request, urlopen
import cv2

img = cv2.imread(image_file)
data = {'image': img.tobytes(), 'shape': img.shape}
data = urlencode(data).encode("utf-8")
req = Request(service_url, data)
response = urlopen(req)
print(response.read().decode('utf-8'))

Views code:

# service/app/views.py
import ast
import numpy as np
from django.http import HttpResponse, JsonResponse
from django.views.decorators.csrf import csrf_exempt

@csrf_exempt
def process_image(request):
    if request.method == 'POST':
        # Converts string to tuple
        shape = ast.literal_eval(request.POST.get('shape'))
        img_bytes = request.POST.get('image')
        # Reconstruct the image
        img = np.fromstring(img_bytes, dtype=np.uint8).reshape(shape)
        # Process image
        return JsonResponse({'result': 'Hello'})

When i run cliente code i get ValueError: total size of new array must be unchanged. I did the following checks, with a 8x8 RGB image:

# client.py
>> print(img.shape)
(8, 8, 3)
>> print(img.dtype)
uint8
>> print(len(img.tobytes()))
192

# service/app/views.py
>> print(shape)
(8, 8, 3)
>> print(len(img_bytes))
187

The shape field is ok, but the image filed has different size. As the image is small, i printed the bytes from client and server, and i did not get the same. I think that this is an encoding problem.

I want to send image as bytes, because i think this is a compact way to send this kind of data. If anyone know a better approach to send image via HTTP, let me know.

Thanks!

@DanielRoseman literal_eval converts shape from string to tuple. — Jhonata Antunes
– Jhonata Antunes, Commented Jul 12, 2018 at 18:02
By no means as compact as you want, I have a rest service where I just base64 encode an image before posting, and decode it on the server. I think your problem is likely putting the raw bytes in the JSON request. I don't think you can put raw bytes in JSON. For example the byte for a quote character would close the value string. — John Morris
– John Morris, Commented Jul 12, 2018 at 23:09

Jhonata Antunes · Accepted Answer · 2018-07-13 00:50:33Z

Inspired by John Morris's commentary, i found the answer to my question in Numpy Array to base64 and back to Numpy Array post. If anyone has the same doubt, here is the solution:

Client code:

# client.py
import base64
from urllib.parse import urlencode
from urllib.request import Request, urlopen
import cv2

img = cv2.imread(image_file)
img_b64 = base64.b64encode(img)
data = {'image': img_b64, 'shape': img.shape}
data = urlencode(data).encode("utf-8")
req = Request(service_url, data)
response = urlopen(req)
print(response.read().decode('utf-8'))

Views code:

# service/app/views.py
import ast
import base64
import numpy as np
from django.http import HttpResponse, JsonResponse
from django.views.decorators.csrf import csrf_exempt

@csrf_exempt
def process_image(request):
    if request.method == 'POST':
        shape = ast.literal_eval(request.POST.get('shape'))
        buffer = base64.b64decode(request.POST.get('image'))
        # Reconstruct the image
        img = np.frombuffer(buffer, dtype=np.uint8).reshape(shape)
        # Process image
        return JsonResponse({'result': 'Hello'})

Thank you all!

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