I'm trying to deploy a large django project to heroku. I installed Heroku CLI, logged in, created an app and ran:

git push heroku master

I have a Pipfile and requirements.txt already set up. I added a runtime.txt to specify that I need python 2.7. This is also in the Pipfile. This is what I get from pushing to heroku:

$ git push heroku master
Counting objects: 12159, done.
Delta compression using up to 2 threads.
Compressing objects: 100% (4853/4853), done.
Writing objects: 100% (12159/12159), 20.94 MiB | 1.82 MiB/s, done.
Total 12159 (delta 6859), reused 12036 (delta 6751)
remote: Compressing source files... done.
remote: Building source:
remote: 
remote: -----> Python app detected
remote: -----> Installing python-3.6.4
remote: -----> Installing pip
remote: -----> Installing dependencies with Pipenv 11.8.2…
remote:        Your Pipfile.lock (3b2ba9) is out of date. Expected: (83a5b4).
remote:        Aborting deploy.
remote:  !     Push rejected, failed to compile Python app.
remote: 
remote:  !     Push failed
remote: Verifying deploy....
remote: 
remote: !   Push rejected to camp-infinity.
remote: 
To https://git.heroku.com/camp-infinity.git! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/camp-infinity.git'

For some reason it tries to install python 3, and it also doesn't like my Pipfile.lock file. I've tried deleting it and generating it again with pipenv install but that didn't change anything.

Experienced the same problem while working on a project, in the branch you are pushing to Heroku, run

pipenv lock

and it will update the Pipfile.lock file. :)