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 stream() instead of +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 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.

+

+ + +

Streaming Contents¶

Basic Usage¶

Streaming from Templates¶

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