[Python] How can I customize a requirements.txt file for multiple environments?

I have two branches, Development and Production. Each has dependencies, some of which are different. Development points to dependencies that are themselves in development. Likewise for Production. I need to deploy to Heroku which expects each branch's dependencies in a single file called 'requirements.txt'.

What is the best way to organize it?

I've thought of:

• Maintain separate requirements files, one in each branch (must survive frequent merges!)
• Tell Heroku which requirements file I want to use (environment variable?)
• Write deploy scripts (create a temporary branch, modify the requirements file, commit, deploy, and delete the temporary branch)

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