如果我创建一个虚拟包——这里是 /tmp/example_package/setup.py （注意要求）：
from distutils.core import setup
setup(name='my_project',
description="Just a test project",
version="1.0",
py_modules=['sample'],
install_requires=['requests > 0.12'])
这是/tmp/example_package/sample.py：
import requests
def get_example():
return requests.get("http://www.example.com")
现在，我创建了一个 virtualenv：
$ virtualenv /tmp/foobar --distribute -p python2.7
Running virtualenv with interpreter /usr/bin/python2.7
New python executable in /tmp/foobar/bin/python2.7
Also creating executable in /tmp/foobar/bin/python
Installing distribute.................................................................................................................................................................................................done.
Installing pip................done.
$ source /tmp/foobar/bin/activate
我创建了一个需求冲突的 requirements.pip：
# this requires requests > 0.12:
file:///tmp/example_package
# but this conflicts:
requests==0.9.0
Pip 愉快地安装了这个：
$ pip install -r requirements.pip [18:40:10]
Unpacking ./example_package
Running setup.py egg_info for package from file:///tmp/example_package
Downloading/unpacking requests==0.9.0 (from -r requirements.pip (line 3))
Downloading requests-0.9.0.tar.gz (55Kb): 55Kb downloaded
Running setup.py egg_info for package requests
Downloading/unpacking certifi>=0.0.4 (from requests==0.9.0->-r requirements.pip (line 3))
Downloading certifi-0.0.8.tar.gz (118Kb): 118Kb downloaded
Running setup.py egg_info for package certifi
Installing collected packages: requests, my-project, certifi
Running setup.py install for requests
Running setup.py install for my-project
Running setup.py install for certifi
Successfully installed requests my-project certifi
Cleaning up...
为什么 Pip 允许这样做？我的example_package不会工作，因为它的要求没有得到满足。