fix typing on HttpResponse and StreamingHttpResponse (#712)

While the documentation for `HttpResponse` and `StreamingHttpResponse`
*says* `content` and `streaming_content` should be bytestrings [1] or an
iterable of bytestrings respectively [2], this is not what the API
supports [3] [4] and there are tests which make sure the API supports
more than bytestrings [5] [6] [etc]. Before assigning `content` or
`streaming_content` the code paths will call  `self.make_bytes` to
coerce the value to bytes.

[1]: ecf87ad513/django/http/response.py (L324-L327)
[2]: 0a28b42b15/django/http/response.py (L395-L399)
[3]: ecf87ad513/django/http/response.py (L342-L362)
[4]: 0a28b42b15/django/http/response.py (L415-L427)
[5]: 0a28b42b15/tests/cache/tests.py (L2250)
[6]: 0a28b42b15/tests/i18n/urls.py (L8)

django-stubs/http/response.pyi

@@ -1,7 +1,7 @@
 import datetime
 from io import BytesIO
 from json import JSONEncoder
-from typing import Any, Dict, Iterable, Iterator, List, Optional, Tuple, Type, Union, overload
+from typing import Any, Dict, Generic, Iterable, Iterator, List, Optional, Tuple, Type, TypeVar, Union, overload
 
 from django.core.handlers.wsgi import WSGIRequest
 from django.http.cookie import SimpleCookie
@@ -10,6 +10,31 @@ from django.test.client import Client
 from django.urls import ResolverMatch
 from django.utils.datastructures import CaseInsensitiveMapping
 
+_T = TypeVar("_T")
+_U = TypeVar("_U")
+
+class _PropertyDescriptor(Generic[_T, _U]):
+    """
+    This helper property descriptor allows defining asynmetric getter/setters
+    which mypy currently doesn't support with either:
+
+        class HttpResponse:
+            @property
+            def content(...): ...
+            @property.setter
+            def content(...): ...
+
+    or:
+
+        class HttpResponse:
+            def _get_content(...): ...
+            def _set_content(...): ...
+            content = property(_get_content, _set_content)
+    """
+
+    def __get__(self, instance: Any, owner: Optional[Any]) -> _U: ...
+    def __set__(self, instance: Any, value: _T) -> None: ...
+
 class BadHeaderError(ValueError): ...
 
 class ResponseHeaders(CaseInsensitiveMapping):
@@ -21,7 +46,7 @@ class ResponseHeaders(CaseInsensitiveMapping):
     def pop(self, key: str, default: Optional[str] = ...) -> str: ...
     def setdefault(self, key: str, value: str) -> None: ...
 
-class HttpResponseBase(Iterable[Any]):
+class HttpResponseBase:
     status_code: int = ...
     streaming: bool = ...
     cookies: SimpleCookie = ...
@@ -72,10 +97,9 @@ class HttpResponseBase(Iterable[Any]):
     def seekable(self) -> bool: ...
     def writable(self) -> bool: ...
     def writelines(self, lines: Iterable[object]): ...
-    def __iter__(self) -> Iterator[Any]: ...
 
-class HttpResponse(HttpResponseBase):
-    content: Any
+class HttpResponse(HttpResponseBase, Iterable[bytes]):
+    content = _PropertyDescriptor[object, bytes]()
     csrf_cookie_set: bool
     redirect_chain: List[Tuple[str, int]]
     sameorigin: bool
@@ -85,6 +109,7 @@ class HttpResponse(HttpResponseBase):
     def __init__(self, content: object = ..., *args: Any, **kwargs: Any) -> None: ...
     def serialize(self) -> bytes: ...
     __bytes__ = serialize
+    def __iter__(self) -> Iterator[bytes]: ...
     @property
     def url(self) -> str: ...
     # Attributes assigned by monkey-patching in test client ClientHandler.__call__()
@@ -96,13 +121,12 @@ class HttpResponse(HttpResponseBase):
     context: Context
     resolver_match: ResolverMatch
     def json(self) -> Any: ...
-    def __iter__(self): ...
     def getvalue(self) -> bytes: ...
 
-class StreamingHttpResponse(HttpResponseBase):
-    content: Any
-    streaming_content: Iterator[bytes]
-    def __init__(self, streaming_content: Iterable[bytes] = ..., *args: Any, **kwargs: Any) -> None: ...
+class StreamingHttpResponse(HttpResponseBase, Iterable[bytes]):
+    streaming_content = _PropertyDescriptor[Iterable[object], Iterator[bytes]]()
+    def __init__(self, streaming_content: Iterable[object] = ..., *args: Any, **kwargs: Any) -> None: ...
+    def __iter__(self) -> Iterator[bytes]: ...
     def getvalue(self) -> bytes: ...
 
 class FileResponse(StreamingHttpResponse):

tests/typecheck/views/test_function_based_views.yml

@@ -0,0 +1,102 @@
+-   case: http_response
+    main: |
+      from django.http.request import HttpRequest
+      from django.http.response import HttpResponse
+      from django.utils.translation import gettext_lazy as _
+
+      def empty_response(request: HttpRequest) -> HttpResponse:
+          return HttpResponse()
+
+      def str_response(request: HttpRequest) -> HttpResponse:
+          return HttpResponse('It works!')
+
+      def bytes_response(request: HttpRequest) -> HttpResponse:
+          return HttpResponse(b'It works!')
+
+      def object_response(request: HttpRequest) -> HttpResponse:
+          return HttpResponse(_('It works!'))
+
+-   case: http_response_content
+    main: |
+      from django.http.request import HttpRequest
+      from django.http.response import HttpResponse
+      from django.utils.translation import gettext_lazy as _
+
+      def empty_response(request: HttpRequest) -> HttpResponse:
+          response = HttpResponse()
+          reveal_type(response.content)  # N: Revealed type is "builtins.bytes*"
+          return response
+
+      def str_response(request: HttpRequest) -> HttpResponse:
+          response = HttpResponse()
+          response.content = 'It works!'
+          reveal_type(response.content)  # N: Revealed type is "builtins.bytes*"
+          return response
+
+      def bytes_response(request: HttpRequest) -> HttpResponse:
+          response = HttpResponse()
+          response.content = b'It works!'
+          reveal_type(response.content)  # N: Revealed type is "builtins.bytes*"
+          return response
+
+      def object_response(request: HttpRequest) -> HttpResponse:
+          response = HttpResponse()
+          response.content = _('It works!')
+          reveal_type(response.content)  # N: Revealed type is "builtins.bytes*"
+          return response
+
+-   case: streaming_http_response
+    main: |
+      from django.http.request import HttpRequest
+      from django.http.response import StreamingHttpResponse
+      from django.utils.translation import gettext_lazy as _
+
+      def empty_response(request: HttpRequest) -> StreamingHttpResponse:
+          return StreamingHttpResponse()
+
+      def str_response(request: HttpRequest) -> StreamingHttpResponse:
+          return StreamingHttpResponse(['It works!'])
+
+      def bytes_response(request: HttpRequest) -> StreamingHttpResponse:
+          return StreamingHttpResponse([b'It works!'])
+
+      def object_response(request: HttpRequest) -> StreamingHttpResponse:
+          return StreamingHttpResponse([_('It works!')])
+
+      def mixed_response(request: HttpRequest) -> StreamingHttpResponse:
+          return StreamingHttpResponse([_('Yes'), '/', _('No')])
+
+-   case: streaming_http_response_streaming_content
+    main: |
+      from django.http.request import HttpRequest
+      from django.http.response import StreamingHttpResponse
+      from django.utils.translation import gettext_lazy as _
+
+      def empty_response(request: HttpRequest) -> StreamingHttpResponse:
+          response = StreamingHttpResponse()
+          reveal_type(response.streaming_content)  # N: Revealed type is "typing.Iterator*[builtins.bytes]"
+          return response
+
+      def str_response(request: HttpRequest) -> StreamingHttpResponse:
+          response = StreamingHttpResponse()
+          response.streaming_content = ['It works!']
+          reveal_type(response.streaming_content)  # N: Revealed type is "typing.Iterator*[builtins.bytes]"
+          return response
+
+      def bytes_response(request: HttpRequest) -> StreamingHttpResponse:
+          response = StreamingHttpResponse()
+          response.streaming_content = [b'It works!']
+          reveal_type(response.streaming_content)  # N: Revealed type is "typing.Iterator*[builtins.bytes]"
+          return response
+
+      def object_response(request: HttpRequest) -> StreamingHttpResponse:
+          response = StreamingHttpResponse()
+          response.streaming_content = [_('It works!')]
+          reveal_type(response.streaming_content)  # N: Revealed type is "typing.Iterator*[builtins.bytes]"
+          return response
+
+      def mixed_response(request: HttpRequest) -> StreamingHttpResponse:
+          response = StreamingHttpResponse()
+          response.streaming_content = [_('Yes'), '/', _('No')]
+          reveal_type(response.streaming_content)  # N: Revealed type is "typing.Iterator*[builtins.bytes]"
+          return response