使用pip3 install scrapy命令之后，会出现如下问题：
Collecting scrapy
Downloading Scrapy-1.4.0-py2.py3-none-any.whl (248kB)
100% |████████████████████████████████| 256kB 1.2MB/s
Collecting PyDispatcher>=2.0.5 (from scrapy)
Downloading PyDispatcher-2.0.5.tar.gz
Collecting Twisted>=13.1.0 (from scrapy)
Downloading Twisted-17.9.0.tar.bz2 (3.0MB)
100% |████████████████████████████████| 3.0MB 323kB/s
Complete output from command python setup.py egg_info:
Download error on https://pypi.python.org/simple/incremental/: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:777) -- Some packages may not be found!
Couldn't find index page for 'incremental' (maybe misspelled?)
Download error on https://pypi.python.org/simple/: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:777) -- Some packages may not be found!
No local packages or working download links found for incremental>=16.10.1
Traceback (most recent call last):
File "<string>", line 1, in <module>
File "/private/var/folders/z5/9myf58gd0q5cj6hh8pnsfm800000gn/T/pip-build-do6a4sxu/Twisted/setup.py", line 21, in <module>
setuptools.setup(**_setup["getSetupArgs"]())
File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/distutils/core.py", line 108, in setup
_setup_distribution = dist = klass(attrs)
File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/setuptools/dist.py", line 315, in __init__
self.fetch_build_eggs(attrs['setup_requires'])
File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/setuptools/dist.py", line 361, in fetch_build_eggs
replace_conflicting=True,
File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pkg_resources/__init__.py", line 850, in resolve
dist = best[req.key] = env.best_match(req, ws, installer)
File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pkg_resources/__init__.py", line 1122, in best_match
return self.obtain(req, installer)
File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pkg_resources/__init__.py", line 1134, in obtain
return installer(requirement)
File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/setuptools/dist.py", line 429, in fetch_build_egg
return cmd.easy_install(req)
File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/setuptools/command/easy_install.py", line 659, in easy_install
raise DistutilsError(msg)
distutils.errors.DistutilsError: Could not find suitable distribution for Requirement.parse('incremental>=16.10.1')
----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /private/var/folders/z5/9myf58gd0q5cj6hh8pnsfm800000gn/T/pip-build-do6a4sxu/Twisted/
以上原因主要是由于证书问题导致的，解决办法如下：
$ wget http://curl.haxx.se/ca/cacert.pem
$ mv cacert.pem ca-bundle.crt
$ sudo cp ca-bundle.crt /etc/pki/tls/certs/
注意：在/etc 下，可能并没有/pki/tls/certs/等目录，此时需要手动创建
cd /etc
sudo mkdir pki
cd pki
sudo mkdir tls
cd tls
sudo mkdir certs
返回含有ca-bundle.crt文件的目录，重新执行 $ sudo cp ca-bundle.crt /etc/pki/tls/certs/
然后执行 命令 pip3 install scrapy
安装完成