Fabric task to create requirements from setup.py's `install_requires` and (if existing) `dependency_links`

fabfile.py

# requires that the `setup()` call in setup.py is guarded with the if __name__ idiom

import imp
import os
from rex import rex  # python-rex on PyPI

@task()
def update_req():
    setup = imp.load_source("setup", "setup.py")
    dep_re = rex("/^(.*?)#egg=(?P<package_name>.*)$/")
    req_re = rex("/^(?P<package_name>.*?)(?:==|<|<=|>|>=)(?P<version>.*?)$/")
    foreign_packages = dict(
        (dep['package_name'], dep[0]) 
        for dep 
        in (dep_re == dep for dep in getattr(setup, 'dependency_links', []))
    )
    with open("requirements.txt.new", "w") as out:
        for req in setup.requirements:
            parsed = (req_re == req)
            package_name = parsed['package_name']
            out.write(foreign_packages.get(package_name, req) + "\n")
    os.rename("requirements.txt.new", "requirements.txt")