JSONResponse Wrong Content-Length Value

[accredited-unicorn]

Describe the bug

JSONResponse populates the response header with a non-zero Content-Length whenever the response object is instantiated without any content.

Checklist

• The bug is reproducible against the latest release and/or master.
• There are no similar issues or pull requests to fix it yet.

To reproduce

from starlette.datastructures import MutableHeaders
from starlette.responses import JSONResponse

r1 = JSONResponse('')
r1.headers

r2 = JSONResponse()
r2.headers

Expected behavior

( r1.headers == r2.headers
  and
  r1.headers == MutableHeaders({'content-length': '0', 'content-type': 'application/json'}) )

Actual behavior

( r1.headers != r2.headers
  and
  r1.headers == MutableHeaders({'content-length': '2', 'content-type': 'application/json'})
  and
  r2.headers == MutableHeaders({'content-length': '4', 'content-type': 'application/json'}))

Debugging material

All that is needed is the currently stable release of Starlette, and the trusty terminal.

Environment

• OS: same on all of Linux/Windows/macOS
• Python version: 3.7.x
• Starlette version: 0.14.x

Additional context

I was trying to adhere to the HTTP ref spec when constructing responses to the OPTIONS and HEAD methods.

[accredited-unicorn]

Nota Bene

While the bug still persists, there is a work around for the particular use case which prompted this bug report ...

Implement the patch for an empty response class, from pull request #1270, directly in your own backend code. Then

return EmptyResponse(headers={'Allow': '<comma separated list of allowed HTTP methods>'}, status_code=204)

[iamthen0ise]

@accredited-unicorn Hi!

Looks like I have a suggestion why this is happens.

When you pass just quotes ('') into JSONResponse, then length is counting against the quotes, and 2 looks like reasonable value.

But most interesting part is — Why it is 4 when you pass None or even do not specify value?

Let's take a look at the code:

Here is rendering of empty JSON Response

starlette/starlette/responses.py

Lines 156 to 163 in 42592d6

	def render(self, content: typing.Any) -> bytes:
	return json.dumps(
	content,
	ensure_ascii=False,
	allow_nan=False,
	indent=None,
	separators=(",", ":"),
	).encode("utf-8")

Looks good, but what happens if we try to encode None into UTF-8 with json.dumps? Let's try in your favourite REPL:

In [3]: import json

In [4]: json.dumps(None).encode("utf-8")
Out[4]: 'null'

That's why empty response weights 4.

Personally I do not even sure this is a bug. I suggest such behaviour as expected when you deal with JSON.
Here's good related case at StackOverflow https://stackoverflow.com/questions/3548635/python-json-what-happened-to-none
And one more https://stackoverflow.com/questions/11410896/python-how-json-dumps-none-to-empty-string

So, json.dumps must rewrite None to valid null, that's why it happens.

[declaresub]

Nota Bene

While the bug still persists, there is a work around for the particular use case which prompted this bug report ...

Implement the patch for an empty response class, from pull request #1270, directly in your own backend code. Then

return EmptyResponse(headers={'Allow': '<comma separated list of allowed HTTP methods>'}, status_code=204)

I have just revised that pull request. I eliminated the EmptyResponse class in favor of handling the special cases in the Response class. The unit tests I wrote also check that JSONResponse does the right thing for 204, 205, and 304 status codes.

[tomchristie]

If you want to return an empty response, then return Response().

Or even better, once #1397 is in, then return Response(status_code=204) which is a more semantically correct way to return no-content responses.

The behaviour we currently have does make sense...

• Returning JSONResponse(None) will set the body as json.dumps(None) -> null.
• Returning JSONResponse('') will set the body as json.dumps('') -> "".

Including an application/json content type, but not including any body with the response would be invalid.