Write test for large file size. Django rest framwork

In my Django application there is an endpoint that handles file upload. In the serializer I define the file size validation like below

extra_kwargs = {
    "image_file": {
        "validators": [
            get_file_size_validator(1e8),
        ],
               
},

The validator function looks like this

def get_file_size_validator(file_size):
    def validator_func(file):
        if file_size < file.size:
            raise ValidationError(f"Maximum file size is {file_size}")

    return validator_func

I would like to write a test where I want to test that user cannot upload large files.

What I have tried so far.

1. SimpleUploadedFile("foo.pdf", b"aaaaa" * (9**10), content_type="image/jpeg")
2. InMemoryUploadedFile(BytesIO(b"large_content"), None, "foo.jpg", "image/jpeg", lambda: 1e9, None, None)
3. Used mock.patch.object(InMemoryUploadedFile, "size", return_value=1e9):
4. Mocked os.path.getsize as recommended here https://stackoverflow.com/a/35246275/1847898

The first solution gives me MemoryError and for all the others file size is returned as 13 in validator_func

Any ideas how I can achieve this?

You should mock the MemoryFileUploadHandler.file_complete method instead of SimpleUploadedFile or InMemoryUploadedFile.

from unittest.mock import patch

from django.core.files.uploadedfile import InMemoryUploadedFile, SimpleUploadedFile
from rest_framework.test import APITestCase


class TestUploadAPI(APITestCase):
    url = "/api/to/the/end-point/"

    @patch("django.core.files.uploadhandler.MemoryFileUploadHandler.file_complete")
    def test_validate_size(self, mock_file_complete):
        file_name = "any-file-name"
        file_content_type = "image/png"
        max_size = 1e8

        mock_file_complete.return_value = InMemoryUploadedFile(
            file=b"",
            field_name=None,
            name=file_name,
            content_type=file_content_type,
            size=max_size + 1,
            charset=None,
        )

        file = SimpleUploadedFile(
            name=file_name,
            content=b"",
            content_type=file_content_type,
        )
        response = self.client.post(self.url, {"file": file})

        self.assertEqual(response.status_code, 400)
        self.assertEqual(
            response.json(),
            {
                "file": ["Maximum file size is 100000000.0"],
            },
        )
        mock_file_complete.assert_called_once()