Mac 自带的python 没有pip， 用brew 安装pip时，会报错如下：
Error: An unexpected error occurred during the `brew link` step The formula built, but is not symlinked into /usr/local Permission denied @ dir_s_mkdir - /usr/local/Frameworks Error: Permission denied @ dir_s_mkdir - /usr/local/Frameworks
pip is already installed if you are using Python 2 >=2.7.9 or Python 3 >=3.4 downloaded from python.org or if you are working in a Virtual Environment created by virtualenv or pyvenv. Just make sure to upgrade pip.
Installing with get-pip.py
To install pip, securely download get-pip.py. :
curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py
Then run the following:
sudo python get-pip.py
Warning Be cautious if you are using a Python install that is managed by your operating system or another package manager. get-pip.py does not coordinate with those tools, and may leave your system in an inconsistent state.
get-pip.py also installs setuptools  and wheel if they are not already. setuptools is required to install source distributions. Both are required in order to build a Wheel Cache (which improves installation speed), although neither are required to install pre-built wheels.
Note The get-pip.py script is supported on the same python version as pip. For the now unsupported Python 2.6, alternate script is available here.
If set, do not attempt to install setuptools
If set, do not attempt to install wheel
get-pip.py allows pip install options and the general options. Below are some examples:
Install from local copies of pip and setuptools:
python get-pip.py --no-index --find-links=/local/copies
Install to the user site :
python get-pip.py --user
Install behind a proxy:
python get-pip.py --proxy="http://[user:passwd@]proxy.server:port"
get-pip.py can also be used to install a specified combination of pip, setuptools, and wheel using the same requirements syntax as pip:
python get-pip.py pip==9.0.2 wheel==0.30.0 setuptools==28.8.0
See Installing pip/setuptools/wheel with Linux Package Managers in the Python Packaging User Guide.
On Linux or macOS:
pip install -U pip
On Windows :
python -m pip install -U pip
pip works with CPython versions 2.7, 3.4, 3.5, 3.6, 3.7 and also pypy.
This means pip works on the latest patch version of each of these minor versions. Previous patch versions are supported on a best effort approach.
pip works on Unix/Linux, macOS, and Windows.