clarify that attr_name is a string for [ticket:2176]
clarify that attr_name is a string for [ticket:2176]

pdufour/sqlalchemy,alex/sqlalchemy,alex/sqlalchemy,brianv0/sqlalchemy,wujuguang/sqlalchemy,hsum/sqlalchemy,elelianghh/sqlalchemy,Cito/sqlalchemy,WinterNis/sqlalchemy,alex/sqlalchemy,davidjb/sqlalchemy,bdupharm/sqlalchemy,bdupharm/sqlalchemy,wfxiang08/sqlalchemy,inspirehep/sqlalchemy,epa/sqlalchemy,bootandy/sqlalchemy,Cito/sqlalchemy,monetate/sqlalchemy,276361270/sqlalchemy,robin900/sqlalchemy,halfcrazy/sqlalchemy,WinterNis/sqlalchemy,dstufft/sqlalchemy,EvaSDK/sqlalchemy,zzzeek/sqlalchemy,graingert/sqlalchemy,olemis/sqlalchemy,j5int/sqlalchemy,bootandy/sqlalchemy,276361270/sqlalchemy,brianv0/sqlalchemy,davidjb/sqlalchemy,itkovian/sqlalchemy,robin900/sqlalchemy,davidfraser/sqlalchemy,itkovian/sqlalchemy,sandan/sqlalchemy,dstufft/sqlalchemy,hsum/sqlalchemy,j5int/sqlalchemy,monetate/sqlalchemy,olemis/sqlalchemy,elelianghh/sqlalchemy,sqlalchemy/sqlalchemy,halfcrazy/sqlalchemy,davidfraser/sqlalchemy,ThiefMaster/sqlalchemy,wfxiang08/sqlalchemy,wujuguang/sqlalchemy,EvaSDK/sqlalchemy,ThiefMaster/sqlalchemy,graingert/sqlalchemy,epa/sqlalchemy,sandan/sqlalchemy,Cito/sqlalchemy,pdufour/sqlalchemy,Akrog/sqlalchemy,inspirehep/sqlalchemy,Akrog/sqlalchemy