How to send multipart text and form parts with FastAPI TestClient?

Question

I'm working against an API specification outside my control. It expects to POST multipart/form-data to my server. Some parts are sent as files, some are sent as text.

I want to write a test using TestClient that sends both a file and text parts.

I don't have access to the exact body that is posted, so I don't know exactly how the multipart form data looks. But I can parse the input from the external system with FastAPI's Request. Some parts are of type UploadFile and some parts are of type str.

The Starlette source for FormData is a ImmutableMultiDict[str, Union[UploadFile, str]]. So this looks like a valid use case.

The following code can handle the POST:

@router.post("/test")
async def test_multipart(
    request: Request,
):
    content_type = request.headers.get("content-type")
    if content_type.startswith("multipart/form-data"):
        async with request.form() as form:
            for (filename, form_file) in form.items():
                logger.info("Form file type: %s", str(type(form_file)))

                # This is the line I want to test:
                if type(form_file) is str:
                    print("Got data as string")

                elif type(form_file) is UploadFile:
                    print("Got data as UploadFile")

                    if form_file.content_type == "application/octet-stream":
                      print("Got octet")

I can test the UploadFile parts easily:

with TestClient(app) as client:
  client.post(
    "/test",
    files={
      "file1": ("upload.json", "STRING CONTENT"),
      "file2": ("file2.pdf", "FILE CONTENT", "application/octet-stream")
    }
)

But I want that first entry to appear as str not UploadFile. How can I do this?

Joe · Accepted Answer · 2025-08-14 14:45:32Z

0

Looks like data can be used at the same time as files:

with TestClient(app) as client:
  client.post(
    "/test",
    data={"upload.json": "STRING CONTENT"},
    files={
      "file2": ("file2.pdf", "FILE CONTENT", "application/octet-stream")
    }
)

This sends the parts as both str and FileUpload.

answered Aug 14 at 14:45

Joe

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