Error message:

“…Library not loaded: libmysqlclient.18.dylib

Referenced from: …ython2.7/site-packages/_mysql.so
  Reason: image not found”

the reason:

After upgrading MySQL, libmysqlclient.18.dylib becomes libmysqlclient.20.dylib.

Solution:

Find mysql lib under the installation path of libmysqlclient.20.dylibestablishing a soft connection.

sudo ln -s /usr/local/mysql/lib/libmysqlclient.20.dylib /usr/local/mysql/lib/libmysqlclient.18.dylib