编译安装完Python3之后，使用pip来安装python库，发现了如下报错：
$ pip install numpy
pip is configured with locations that require TLS/SSL, however the ssl module in Python is not available.
Collecting numpy
Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError("Can't connect to HTTPS URL because the SSL module is not available.",)': /simple/numpy/
Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError("Can't connect to HTTPS URL because the SSL module is not available.",)': /simple/numpy/
Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError("Can't connect to HTTPS URL because the SSL module is not available.",)': /simple/numpy/
Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError("Can't connect to HTTPS URL because the SSL module is not available.",)': /simple/numpy/
Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'SSLError("Can't connect to HTTPS URL because the SSL module is not available.",)': /simple/numpy/
Could not fetch URL https://pypi.python.org/simple/numpy/: There was a problem confirming the ssl certificate: HTTPSConnectionPool(host='pypi.python.org', port=443): Max retries exceeded with url: /simple/numpy/ (Caused by SSLError("Can't connect to HTTPS URL because the SSL module is not available.",)) - skipping
Could not find a version that satisfies the requirement numpy (from versions: )
No matching distribution found for numpy
网上说了一种解决方案，是在./configure 的时候，加上--with-ssl选项，然后重新编译安装，尝试了下：
$ ./configure --with-ssl ... ... configure: WARNING: unrecognized options: --with-ssl ... ...
出了个警告：不可识别的--with-ssl选项。
./configure --help看了下确实也没发现这个选项，估计是版本不一致，不大想折腾这个版本问题了，决定换个思路。
尝试安装openssl:
$ sudo yum install openssl
安装成功之后，重新编译安装，依旧报这个错，但是在make的时候有了一些发现：
$ make ... ... Python build finished successfully! The necessary bits to build these optional modules were not found: _bz2 _curses _curses_panel _dbm _gdbm _lzma _sqlite3 _ssl _tkinter readline To find the necessary bits, look in setup.py in detect_modules() for the module's name.
可以看到，这里虽然make成功了，但是报了很多模块缺失，查看下编译安装目录下的setup.py，搜索_ssl，可以定位到如下代码：
843 # Detect SSL support for the socket module (via _ssl)
844 search_for_ssl_incs_in = [
845 '/usr/local/ssl/include',
846 '/usr/contrib/ssl/include/'
847 ]
848 ssl_incs = find_file('openssl/ssl.h', inc_dirs,
849 search_for_ssl_incs_in
850 )
851 if ssl_incs is not None:
852 krb5_h = find_file('krb5.h', inc_dirs,
853 ['/usr/kerberos/include'])
854 if krb5_h:
855 ssl_incs += krb5_h
856 ssl_libs = find_library_file(self.compiler, 'ssl',lib_dirs,
857 ['/usr/local/ssl/lib',
858 '/usr/contrib/ssl/lib/'
859 ] )
860
861 if (ssl_incs is not None and
862 ssl_libs is not None):
863 exts.append( Extension('_ssl', ['_ssl.c'],
864 include_dirs = ssl_incs,
865 library_dirs = ssl_libs,
866 libraries = ['ssl', 'crypto'],
867 depends = ['socketmodule.h']), )
868 else:
869 missing.append('_ssl')
可以看到，setup.py会在'/usr/local/ssl/include', '/usr/contrib/ssl/include/' 这两个目录里面搜索'openssl/ssl.h' 这个头文件，然后会在 '/usr/local/ssl/lib' 和 '/usr/contrib/ssl/lib/' 之中搜索 ssl 的 lib文件，搜索不到，会将_ssl加入missing这个数组里面，然后寻找missing调用的地方：
313 if missing:
314 print()
315 print("Python build finished successfully!")
316 print("The necessary bits to build these optional modules were not "
317 "found:")
318 print_three_column(missing)
319 print("To find the necessary bits, look in setup.py in"
320 " detect_modules() for the module's name.")
321 print()
找到了上面报错时候的输出，很明显，是由于搜索不到ssl.h头文件或者搜索不到lib文件而导致的报错，但是我刚刚明明是装了openssl的啊，为啥还会报找不到呢？手动搜索下：
$ sudo find / -name ssl.h
没找到ssl.h，折腾了一番之后，找到了如下命令：
$ sudo yum install zlib-devel bzip2-devel openssl-devel ncurses-devel sqlite-devel readline-devel tk-devel
可以看到，这个命令安装的是openssl-devel，与我之前安装的openssl有所不同，查阅资料之后发现，openssl只包含了可执行部分，openssl-devel才包含了头文件、头文件参考、某些库文件等跟开发相关的东西。所以只安装openssl是找不到相应的头文件的，安装完之后，再次编译：
$ make clean $ make ... ... Python build finished successfully! The necessary bits to build these optional modules were not found: _dbm _gdbm _lzma To find the necessary bits, look in setup.py in detect_modules() for the module's name.
果然发现，缺失的模块少了很多。
继续安装：
$ sudo make install
安装完之后，重新执行pip:
$ pip install numpy
Collecting numpy
Downloading https://files.pythonhosted.org/packages/68/1e/116ad560de97694e2d0c1843a7a0075cc9f49e922454d32f49a80eb6f1f2/numpy-1.14.5-cp36-cp36m-manylinux1_x86_64.whl (12.2MB)
6% |██ | 747kB 10kB/s eta 0:17:52
至此，pip安装报错的问题解决。