How to test a FastAPI api endpoint that consumes images?

Question

I am using pytest to test a FastAPI endpoint that gets in input an image in binary format as in

@app.post("/analyse")
async def analyse(file: bytes = File(...)):

    image = Image.open(io.BytesIO(file)).convert("RGB")
    stats = process_image(image)
    return stats

After starting the server, I can manually test the endpoint successfully by running a call with requests

import requests
from requests_toolbelt.multipart.encoder import MultipartEncoder

url = "http://127.0.0.1:8000/analyse"

filename = "./example.jpg"
m = MultipartEncoder(
        fields={'file': ('filename', open(filename, 'rb'), 'image/jpeg')}
    )
r = requests.post(url, data=m, headers={'Content-Type': m.content_type}, timeout = 8000)
assert r.status_code == 200

However, setting up tests in a function of the form:

from fastapi.testclient import TestClient
from requests_toolbelt.multipart.encoder import MultipartEncoder
from app.server import app

client = TestClient(app)

def test_image_analysis():

    filename = "example.jpg"

    m = MultipartEncoder(
        fields={'file': ('filename', open(filename, 'rb'), 'image/jpeg')}
        )

    response = client.post("/analyse",
                           data=m,
                           headers={"Content-Type": "multipart/form-data"}
                           )

    assert response.status_code == 200

when running tests with python -m pytest, that gives me back a

>       assert response.status_code == 200
E       assert 400 == 200
E        +  where 400 = <Response [400]>.status_code

tests\test_server.py:22: AssertionError
-------------------------------------------------------- Captured log call --------------------------------------------------------- 
ERROR    fastapi:routing.py:133 Error getting request body: can't concat NoneType to bytes
===================================================== short test summary info ====================================================== 
FAILED tests/test_server.py::test_image_analysis - assert 400 == 200

what am I doing wrong?
What's the right way to write a test function test_image_analysis() using an image file?

Answer 1

You see a different behavior because requests and TestClient are not exactly same in every aspect as TestClient wraps requests. To dig deeper, refer to the source code: (FastAPI is using TestClient from starlette library, FYI)

https://github.com/encode/starlette/blob/master/starlette/testclient.py

To solve, you can get rid of MultipartEncoder because requests can accept file bytes and encode it by form-data format, with something like

# change it
r = requests.post(url, data=m, headers={'Content-Type': m.content_type}, timeout = 8000)

# to 
r = requests.post(url, files={"file": ("filename", open(filename, "rb"), "image/jpeg")})

and modifying the FastAPI test code:

# change
response = client.post("/analyse",
                       data=m,
                       headers={"Content-Type": "multipart/form-data"}
                       )
# to
response = client.post(
    "/analyse", files={"file": ("filename", open(filename, "rb"), "image/jpeg")}
)