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Running a script with large input on Heroku

2020-08-04

Running a script on Heroku is easy:

heroku run -a <your app> python my_script.py

This runs your script in an environment with access to your production database, Redis and all the other stuff that.

This can be great for manual data migrations – say adding a demo course for all of your users.

However, in some cases you may want to run your script only for say 5,000 out of 20,000 users. Let’s say you’ve run some data analytics and you now have the IDs of the 5,000 users that the script should run on.

You don’t want to dirty up the repository with a custom file with the IDs of exactly those 5,000 users. What if you want to run the script again with 200 more users in a week?

The first thing you need to do is to allow your script to accept an input file: python my_script.py <input file>.

Now how do you get those 5,000 IDs into Heroku without adding them directly to the repository?

On Heroku you don’t have access to convienient tools like curl or wget. But if your app includes the requests-library (assuming you have a Python app) you can do

python -c "import requests; print(requests.get('https://google.com').text)" > test.txt

and then

python my_script.py test.txt

The full session would look something like:

heroku run -a eduflow bash
Running bash on ⬢ <your app>... up, run.9942 (Standard-1X)
~ $ python -c "import requests; print(requests.get('https://google.com').text)" > test.txt
~ $ python my_script.py test.txt

If you don’t have access to requests, you can use the built-in urllib-library:

python -c 'from urllib.request import urlopen; print(urlopen("https://google.com").read())'

Here, you’ll need to strip the beginning b' and ending ' since it returns a bytes-object.

For a Node app you can do something similar with fetch or axios.