I have looked at similar posts but could not find the solution to my problem.  I had installed mySQL 8.0 version using homebrew on MacOS but then needed to downgrade it to mySql 5.6. I uninstalled the 8.0 version completely and deleted any left over files.I then installed the 5.6.1 using the native mySQL dmg package for Mac. On running my python project I get the Library not loaded error for /usr/local/opt/mysql/lib/libmysqlclient.21.dylib referenced from the _mysql.cpython-36m-darwin.so. I am not sure why this location is getting referenced as I have only libmysqlclient.18.dylib on my system under a different folder usr/local/mysql/lib . How can I fix the issue ?
My project with Python 3.6 was lookin for libmysqlclient.21.dylib.
I installed brew install mysql-client. It installed mysql-client 8.0. it has libmysqlclient.21.dylib. Where as i wanted to use mysql@5.6.
So I copied the libmysqlclient.21.dylib from /usr/local/Cellar/mysql-client/8.0.19/lib to  /usr/local/lib/ 
sudo ln -s /usr/local/Cellar/mysql-client/8.0.19/lib/libmysqlclient.21.dylib /usr/local/lib/libmysqlclient.21.dylib
Library not loaded: /usr/local/opt/mysql/lib/libmysqlclient.21.dylib error when it does not exist