In python using Flask, how can I write out an object for download?

Question

I'm using Flask and running foreman. I data that I've constructed in memory and I want the user to be able to download this data in a text file. I don't want write out the data to a file on the local disk and make that available for download.

I'm new to python. I thought I'd create some file object in memory and then set response header, maybe?

Solution

Streaming files to the client without saving them to disk is covered in the "pattern" section of Flask's docs - specifically, in the section on streaming. Basically, what you do is return a fully-fledged Response object wrapping your iterator:

from flask import Response

# construct your app

@app.route("/get-file")
def get_file():
    results = generate_file_data()
    generator = (cell for row in results
                    for cell in row)

    return Response(generator,
                       mimetype="text/plain",
                       headers={"Content-Disposition":
                                    "attachment;filename=test.txt"})

Comments

[swidnikk]

I have no idea what cell for row in results... is doing, can you explain?

[Sean Vieira]

@swidnikk - that is a generator expression - it is like the list comprehension expression [x for x in range(10)] except it produces a generator object rather than a list. (x for x in range(10)) does not generate the whole list at once. Instead it lazily evaluates the next value of x each time __next__ (next in Python 2.X) is called. The docs show you a different way of creating generators using yield (def generator_func(): for x in range(10): yield x) The nested for expressions are there because I assumed a list of lists type of data structure. Does that make sense?

[swidnikk]

Thanks. I wasn't sure how to look it up for more information. I understand now and found it in the docs here: docs.python.org/tutorial/classes.html#generators I suppose this will be especially useful if the file I'm creating becomes too large to keep in memory.