Streaming Contents

The answer is by using generators and direct responses.

This is a basic view function that generates a lot of CSV data on the fly. The trick is to have an inner function that uses a generator to generate data and to then invoke that function and pass it to a response object:

The Jinja2 template engine also supports rendering templates piece by piece. This functionality is not directly exposed by Flask because it is quite uncommon, but you can easily do it yourself:

The trick here is to get the template object from the Jinja2 environment on the application and to call stream() instead of which returns a stream object instead of a string. Since we’re bypassing the Flask template render functions and using the template object itself we have to make sure to update the render context ourselves by calling update_template_context(). The template is then evaluated as the stream is iterated over. Since each time you do a yield the server will flush the content to the client you might want to buffer up a few items in the template which you can do with . 5 is a sane default.

New in version 0.9.

Note that when you stream data, the request context is already gone the moment the function executes. Flask 0.9 provides you with a helper that can keep the request context around during the execution of the generator: