安装pyspider时，需要安装pycurl，但是报错specify the SSL backend manually

Curl is configured to use SSL, but we have not been able to determine which SSL backend it is using. Please see PycURL documentation for how to specify the SSL backend manually.

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我是从官方文档中找到答案的：官方文档地址

specify the SSL backend manually

# 安装方式easy_install / pip都可以
export PYCURL_SSL_LIBRARY=[openssl|gnutls|nss]
easy_install pycurl

完整的命令如下：

# upgrade pip if necessary
pip install --upgrade pip

# remove current pycurl
pip uninstall pycurl

# set PYCURL_SSL_LIBRARY
export PYCURL_SSL_LIBRARY=nss

# recompile and install pycurl
pip install --compile pycurl