From 653533566e51d2346236d7e6f8ddfc95b8a0bf07 Mon Sep 17 00:00:00 2001
From: Omer Shamash <38649465+92dev@users.noreply.github.com>
Date: Mon, 15 Apr 2024 13:20:54 +0300
Subject: [PATCH] Allow streamable response classes

Basically classes such as `StreamingResponse` in `starlette` cannot work unless specified directly in the response-type due to this serialization concept (which they always fallback to use JSON)

There is the alternative which I opted out of for now, which is pass a `response_serialize: bool` parameter throughout the entire flow, it might be better but I wanted to avoid API changes (even through defaults) until any maintainer gives his opinion on the change

example usage:
```
# Here `response_class` is only used for documentation
@app.get('/test_before', response_class=StreamingResponse)
async def before_fix(msg: str):
    async def txt_generator():
        for i in range(10):
            yield f'event: something\ndata: {i}\n\n'
    return StreamingResponse(txt_generator())

# Here `response_class` is actually working
@app.get('/test_after', response_class=StreamingResponse)
async def after_fix(msg: str):
    async def txt_generator():
        for i in range(10):
            yield f'event: something\ndata: {i}\n\n'
    returntxt_generator()
```
---
 fastapi/routing.py | 24 +++++++++++++-----------
 1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/fastapi/routing.py b/fastapi/routing.py
index fa1351859..825eda9bf 100644
--- a/fastapi/routing.py
+++ b/fastapi/routing.py
@@ -293,17 +293,19 @@ def get_request_handler(
                             response_args["status_code"] = current_status_code
                         if sub_response.status_code:
                             response_args["status_code"] = sub_response.status_code
-                        content = await serialize_response(
-                            field=response_field,
-                            response_content=raw_response,
-                            include=response_model_include,
-                            exclude=response_model_exclude,
-                            by_alias=response_model_by_alias,
-                            exclude_unset=response_model_exclude_unset,
-                            exclude_defaults=response_model_exclude_defaults,
-                            exclude_none=response_model_exclude_none,
-                            is_coroutine=is_coroutine,
-                        )
+                        # Some response classes handle serialization themselves via iteration
+                        if not hasattr(actual_response_class, 'body_iterator'):
+                            content = await serialize_response(
+                                field=response_field,
+                                response_content=raw_response,
+                                include=response_model_include,
+                                exclude=response_model_exclude,
+                                by_alias=response_model_by_alias,
+                                exclude_unset=response_model_exclude_unset,
+                                exclude_defaults=response_model_exclude_defaults,
+                                exclude_none=response_model_exclude_none,
+                                is_coroutine=is_coroutine,
+                            )
                         response = actual_response_class(content, **response_args)
                         if not is_body_allowed_for_status_code(response.status_code):
                             response.body = b""