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David Lord 2026-03-04 07:36:21 -08:00
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15
docs/patterns/streaming.rst
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@ -8,6 +8,21 @@ roundtrip to the filesystem?
The answer is by using generators and direct responses.
HTTP Response Behavior
----------------------
**Headers cannot be changed after the streaming response starts.**
When using streaming, it's important to be aware of the order than an HTTP
response is sent. All headers must be sent first, then the body. More headers
cannot be sent after the body has begun. Therefore, you must make sure all
headers are set before starting the response, outside the generator.
In particular, if the generator will access ``session``, be sure to do so in the
view as well so that the ``Vary: cookie`` header will be set. Do not modify the
session in the generator, as the ``Set-Cookie`` header will already be sent.
Basic Usage
-----------

10
docs/templating.rst
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@ -251,5 +251,11 @@ functions to make this easier to use.
return stream_template("timeline.html")
These functions automatically apply the
:func:`~flask.stream_with_context` wrapper if a request is active, so
that it remains available in the template.
:func:`~flask.stream_with_context` wrapper if a request is active, so that
:data:`.request`, :data:`.session`, and :data:`.g` remain available in the
template.
More headers cannot be sent after the body has begun. Therefore, you must
make sure all headers are set before starting the response. In particular,
if the template will access ``session``, be sure to do so in the view as
well so that the ``Vary: cookie`` header will be set.

12
src/flask/helpers.py
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@ -68,6 +68,18 @@ def stream_with_context(
available, even though at the point the generator runs the request context
will typically have ended.
.. warning::
Due to the following caveat, it is often safer to pass the data you
need as arguments to the generator, rather than relying on the context
objects.
More headers cannot be sent after the body has begun. Therefore, you must
make sure all headers are set before starting the response. In particular,
if the generator will access ``session``, be sure to do so in the view as
well so that the ``Vary: cookie`` header will be set. Do not modify the
session in the generator, as the ``Set-Cookie`` header will already be sent.
Use it as a decorator on a generator function:
.. code-block:: python