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.
mac 系统，装了 python2 和 python3，但是执行 python2.7.10 的 pip 命令会报错。
Traceback (most recent call last):
File "/usr/local/bin/pip", line 11, in <module>
load_entry_point('pip==18.0', 'console_scripts', 'pip')()
File "/usr/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 484, in load_entry_point
return get_distribution(dist).load_entry_point(group, name)
File "/usr/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2714, in load_entry_point
return ep.load()
File "/usr/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2332, in load
return self.resolve()
File "/usr/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2338, in resolve
module = __import__(self.module_name, fromlist=['__name__'], level=0)
File "/usr/local/lib/python2.7/site-packages/pip/_internal/__init__.py", line 20, in <module>
from pip._vendor.urllib3.exceptions import DependencyWarning
File "/usr/local/lib/python2.7/site-packages/pip/_vendor/urllib3/__init__.py", line 8, in <module>
from .connectionpool import (
File "/usr/local/lib/python2.7/site-packages/pip/_vendor/urllib3/connectionpool.py", line 36, in <module>
from .response import HTTPResponse
File "/usr/local/lib/python2.7/site-packages/pip/_vendor/urllib3/response.py", line 3, in <module>
import zlib
ImportError: No module named zlib
zlib: stable 1.2.11 (bottled) [keg-only]
General-purpose lossless data-compression library
https://zlib.net/
/usr/local/Cellar/zlib/1.2.11 (12 files, 373KB)
Poured from bottle on 2018-11-26 at 18:31:13
From: https://github.com/Homebrew/homebrew-core/blob/master/Formula/zlib.rb
==> Caveats
zlib is keg-only, which means it was not symlinked into /usr/local,
because macOS already provides this software and installing another version in
parallel can cause all kinds of trouble.
