标题几乎总结了它。输出参考：
❯ pip freeze -r requirements.txt
Django==1.7.10
Pillow==2.9.0
PyYAML==3.11
Requirement file contains argparse==1.2.1, but that package is not installed
betterpath==0.2.2
❯ pip install -r requirements.txt
Requirement already satisfied (use --upgrade to upgrade): Django==1.7.10 in /Users/***/.virtualenvs/***/lib/python2.7/site-packages (from -r requirements.txt (line 1))
Requirement already satisfied (use --upgrade to upgrade): Pillow==2.9.0 in /Users/***/.virtualenvs/***/lib/python2.7/site-packages (from -r requirements.txt (line 2))
Requirement already satisfied (use --upgrade to upgrade): PyYAML==3.11 in /Users/***/.virtualenvs/***/lib/python2.7/site-packages (from -r requirements.txt (line 3))
Requirement already satisfied (use --upgrade to upgrade): argparse==1.2.1 in /Users/***/.virtualenvs/***/lib/python2.7/site-packages (from -r requirements.txt (line 4))
Requirement already satisfied (use --upgrade to upgrade): betterpath==0.2.2 in /Users/***/.virtualenvs/***/lib/python2.7/site-packages (from -r requirements.txt (line 5))