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Diffstat (limited to 'app/static/doc/flask-docs/_sources/patterns/streaming.txt')
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diff --git a/app/static/doc/flask-docs/_sources/patterns/streaming.txt b/app/static/doc/flask-docs/_sources/patterns/streaming.txt new file mode 100644 index 0000000..8393b00 --- /dev/null +++ b/app/static/doc/flask-docs/_sources/patterns/streaming.txt @@ -0,0 +1,61 @@ +Streaming Contents +================== + +Sometimes you want to send an enormous amount of data to the client, much +more than you want to keep in memory. When you are generating the data on +the fly though, how do you send that back to the client without the +roundtrip to the filesystem? + +The answer is by using generators and direct responses. + +Basic Usage +----------- + +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:: + + from flask import Response + + @app.route('/large.csv') + def generate_large_csv(): + def generate(): + for row in iter_all_rows(): + yield ','.join(row) + '\n' + return Response(generate(), mimetype='text/csv') + +Each ``yield`` expression is directly sent to the browser. Now though +that some WSGI middlewares might break streaming, so be careful there in +debug environments with profilers and other things you might have enabled. + +Streaming from Templates +------------------------ + +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:: + + from flask import Response + + def stream_template(template_name, **context): + app.update_template_context(context) + t = app.jinja_env.get_template(template_name) + rv = t.stream(context) + rv.enable_buffering(5) + return rv + + @app.route('/my-large-page.html') + def render_large_template(): + rows = iter_all_rows() + return Response(stream_template('the_template.html', rows=rows)) + +The trick here is to get the template object from the Jinja2 environment +on the application and to call :meth:`~jinja2.Template.stream` instead of +:meth:`~jinja2.Template.render` 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 :meth:`~flask.Flask.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 +``rv.enable_buffering(size)``. ``5`` is a sane default. |