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More and more I'm seeing the "requirements.txt pattern" come up. This generally refers to projects (but not just), and seems to have started around the same time as Heroku adopting Python. I feel like this is something that matters in the Python world, and because I have an opinion on everything, I want to share mine.

requirements.txt

Let's first talk about what this pattern actually is. As you should already be familiar with pip (if you're not, this post is not for you), the idea of this is that whatever you're doing, is installable by pointing pip at a requirements.txt file which contains a list of your projects dependencies. This has some obvious benefits, one being that you can mark repositories as dependencies.

Another benefit of this is when you have a large project (like DISQUS) and your dependencies can vary between environments. For example, we have several various requirements files for disqus-web (our largest package):

requirements/global.txt requirements/production.txt requirements/development.txt

These end up being pretty obvious, and when an app has specific needs there's no reason not to approach the problem this way. That said, you dont need to do things this way, and in every project other than our main repository, including our open source work, all dependencies are specified completely in setup.py. Even in this case, we could just as easily specify our core requirements as part of the package and simply have additional files which label the production and development dependencies.

setup.py is the right choice

A common argument for not using setup.py is that a library is not the same as an app (or larger project). Why not? We employ the same metadata in everything. Each contains a list of dependencies, some various metadata, and possibly a list of extra resources (such as scripts, or documentation). Fundamentally they're identical. Additionally, if pip is your thing, it does not prevent you from using setup.py. Let's take an example setup.py:

from setuptools import setup , find_packages requires = [ 'Flask==0.8' , 'redis==2.4.11' , 'hiredis==0.1.1' , 'nydus==0.8.1' , ] setup ( name = 'something-sexy' , version = '1.0.0' , author = "DISQUS" , author_email = "dev@disqus.com" , package_dir = { '' : 'src' } , packages = find_packages ( "src" ) , install_requires = requires , zip_safe = False , )

Now, in our case, this is probably a service on Disqus, which means we're not listing it as a dependancy. In every single scenario we have, we want our package to be on PYTHONPATH , and this is no different. There's many ways to solve the problem, and generally adjusting sys.path is not what you're going to want. Whether you install the package or you just run it as an editable package (via pip install -e or setuptool's develop command), packaging your app makes it that much easier.

What's even more important is that you stick with standards, especially in our growing ecosystem of open source and widely available libraries. There's absolutely no reason to have to explain to a developer that they need to run some arbitrary command to get your neat tool to install. Following the well defined and adopted standards ensures that is never the case.

Keep it simple. Keep it obvious.

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