[SPARK-8698] partitionBy in Python DataFrame reader/writer interface should not default to empty tuple.
[SPARK-8698] partitionBy in Python DataFrame reader/writer interface should not default to empty tuple.

Author: Reynold Xin <rxin@databricks.com>

Closes #7079 from rxin/SPARK-8698 and squashes the following commits:

8513e1c [Reynold Xin] [SPARK-8698] partitionBy in Python DataFrame reader/writer interface should not default to empty tuple.

saltstar/spark,rikima/spark,jkbradley/spark,ahnqirage/spark,dbtsai/spark,saturday-shi/spark,bOOm-X/spark,actuaryzhang/spark,nchammas/spark,darionyaphet/spark,saltstar/spark,esi-mineset/spark,wangmiao1981/spark,SHASHANKB/spark,metamx/spark,adrian-ionescu/apache-spark,ibm-research-ireland/sparkoscope,wangyum/spark,caneGuy/spark,ajaysaini725/spark,debugger87/spark,jianran/spark,witgo/spark,lxsmnv/spark,LantaoJin/spark,dongjoon-hyun/spark,gengliangwang/spark,debugger87/spark,milliman/spark,xuanyuanking/spark,techaddict/spark,LantaoJin/spark,dongjoon-hyun/spark,darionyaphet/spark,brad-kaiser/spark,rekhajoshm/spark,metamx/spark,huang1900/spark,jrshust/spark,shuangshuangwang/spark,ptkool/spark,tejasapatil/spark,guoxiaolongzte/spark,zzcclp/spark,aray/spark,kevinyu98/spark,szhem/spark,ioana-delaney/spark,techaddict/spark,jrshust/spark,mzl9039/spark,tejasapatil/spark,publicRoman/spark,xflin/spark,SnappyDataInc/spark,tejasapatil/spark,janewangfb/spark,rednaxelafx/apache-spark,wzhfy/spark,apache/spark,jlopezmalla/spark,yanboliang/spark,taroplus/spark,rezasafi/spark,sachintyagi22/spark,WindCanDie/spark,kiszk/spark,kiszk/spark,ioana-delaney/spark,andrewor14/spark,SnappyDataInc/spark,xuanyuanking/spark,ddna1021/spark,zero323/spark,cin/spark,stanzhai/spark,akopich/spark,caneGuy/spark,highfei2011/spark,mahak/spark,xuanyuanking/spark,mzl9039/spark,WindCanDie/spark,ioana-delaney/spark,rekhajoshm/spark,sahilTakiar/spark,guoxiaolongzte/spark,kevinyu98/spark,mike0sv/spark,akopich/spark,ajaysaini725/spark,hvanhovell/spark,ron8hu/spark,kissmetrics/spark,maropu/spark,shubhamchopra/spark,aray/spark,ddna1021/spark,hvanhovell/spark,szhem/spark,michalsenkyr/spark,saturday-shi/spark,chuckchen/spark,LantaoJin/spark,adrian-ionescu/apache-spark,Aegeaner/spark,1haodian/spark,BryanCutler/spark,nlalevee/spark,sahilTakiar/spark,andrewor14/spark,someorz/spark,aosagie/spark,aray/spark,patrick-nicholson/spark,aokolnychyi/spark,cloud-fan/spark,ahnqirage/spark,jlopezmalla/spark,holdenk/spark,milliman/spark,JoshRosen/spark,joseph-torres/spark,rikima/spark,apache/spark,apache-spark-on-k8s/spark,michalsenkyr/spark,techaddict/spark,nlalevee/spark,rikima/spark,witgo/spark,cin/spark,WindCanDie/spark,caneGuy/spark,big-pegasus/spark,ron8hu/spark,vinodkc/spark,aokolnychyi/spark,sachintyagi22/spark,aokolnychyi/spark,skonto/spark,sureshthalamati/spark,spark-test/spark,hvanhovell/spark,icexelloss/spark,wangmiao1981/spark,ddna1021/spark,SnappyDataInc/spark,spark-test/spark,loneknightpy/spark,ueshin/apache-spark,highfei2011/spark,pgandhi999/spark,apache/spark,rednaxelafx/apache-spark,UndeadBaneGitHub/spark,pgandhi999/spark,haowu80s/spark,pgandhi999/spark,facaiy/spark,SHASHANKB/spark,JoshRosen/spark,shuangshuangwang/spark,JoshRosen/spark,bOOm-X/spark,jkbradley/spark,kimoonkim/spark,apache-spark-on-k8s/spark,JerryLead/spark,vinodkc/spark,ueshin/apache-spark,nlalevee/spark,ueshin/apache-spark,cloud-fan/spark,alunarbeach/spark,WindCanDie/spark,WeichenXu123/spark,byakuinss/spark,haowu80s/spark,matthewfranglen/spark,JerryLead/spark,liyichao/spark,janewangfb/spark,chuckchen/spark,setjet/spark,narahari92/spark,chuckchen/spark,mahak/spark,aray/spark,ericvandenbergfb/spark,wzhfy/spark,alunarbeach/spark,mdespriee/spark,kissmetrics/spark,ibm-research-ireland/sparkoscope,MLnick/spark,zero323/spark,SHASHANKB/spark,skonto/spark,guoxiaolongzte/spark,bravo-zhang/spark,aray/spark,kimoonkim/spark,wzhfy/spark,kimoonkim/spark,tengpeng/spark,ibm-research-ireland/sparkoscope,big-pegasus/spark,big-pegasus/spark,liutang123/spark,metamx/spark,adrian-ionescu/apache-spark,milliman/spark,ericvandenbergfb/spark,rezasafi/spark,maropu/spark,ueshin/apache-spark,saltstar/spark,jrshust/spark,techaddict/spark,kissmetrics/spark,yanboliang/spark,liutang123/spark,aokolnychyi/spark,1haodian/spark,map222/spark,techaddict/spark,ericvandenbergfb/spark,liyichao/spark,shubhamchopra/spark,szhem/spark,ddna1021/spark,BryanCutler/spark,HyukjinKwon/spark,adrian-ionescu/apache-spark,stanzhai/spark,kevinyu98/spark,MLnick/spark,ueshin/apache-spark,wangmiao1981/spark,esi-mineset/spark,zzcclp/spark,zzcclp/spark,poffuomo/spark,ibm-research-ireland/sparkoscope,ptkool/spark,holdenk/spark,milliman/spark,ConeyLiu/spark,metamx/spark,saltstar/spark,nlalevee/spark,mike0sv/spark,tejasapatil/spark,witgo/spark,ron8hu/spark,patrick-nicholson/spark,jkbradley/spark,loneknightpy/spark,narahari92/spark,zero323/spark,ahnqirage/spark,jianran/spark,facaiy/spark,darionyaphet/spark,WindCanDie/spark,poffuomo/spark,bdrillard/spark,ioana-delaney/spark,lvdongr/spark,taroplus/spark,rezasafi/spark,tengpeng/spark,jlopezmalla/spark,byakuinss/spark,mahak/spark,spark-test/spark,dbtsai/spark,sachintyagi22/spark,setjet/spark,map222/spark,eyalfa/spark,hhbyyh/spark,maropu/spark,nchammas/spark,pronix/spark,spark-test/spark,dhruve/spark,guoxiaolongzte/spark,JoshRosen/spark,SnappyDataInc/spark,aosagie/spark,JerryLead/spark,tengpeng/spark,loneknightpy/spark,narahari92/spark,1haodian/spark,SHASHANKB/spark,caneGuy/spark,yanboliang/spark,huang1900/spark,rikima/spark,dbtsai/spark,nilsgrabbert/spark,Panos-Bletsos/spark-cost-model-optimizer,kevinyu98/spark,zero323/spark,dbtsai/spark,WeichenXu123/spark,jkbradley/spark,nlalevee/spark,aokolnychyi/spark,actuaryzhang/spark,ajaysaini725/spark,pronix/spark,kimoonkim/spark,lvdongr/spark,kiszk/spark,wangyum/spark,xflin/spark,dhruve/spark,WindCanDie/spark,jkbradley/spark,publicRoman/spark,LantaoJin/spark,mzl9039/spark,holdenk/spark,big-pegasus/spark,holdenk/spark,yanboliang/spark,SnappyDataInc/spark,alunarbeach/spark,hhbyyh/spark,stanzhai/spark,zzcclp/spark,map222/spark,nchammas/spark,jiangxb1987/spark,minixalpha/spark,gengliangwang/spark,skonto/spark,tengpeng/spark,sahilTakiar/spark,ueshin/apache-spark,huang1900/spark,map222/spark,xflin/spark,shaneknapp/spark,apache-spark-on-k8s/spark,BryanCutler/spark,sahilTakiar/spark,skonto/spark,akopich/spark,joseph-torres/spark,gengliangwang/spark,SHASHANKB/spark,ericvandenbergfb/spark,mahak/spark,markhamstra/spark,patrick-nicholson/spark,pronix/spark,shubhamchopra/spark,techaddict/spark,zero323/spark,Panos-Bletsos/spark-cost-model-optimizer,byakuinss/spark,saltstar/spark,alunarbeach/spark,someorz/spark,loneknightpy/spark,MLnick/spark,patrick-nicholson/spark,rikima/spark,someorz/spark,dbtsai/spark,tengpeng/spark,ptkool/spark,rednaxelafx/apache-spark,big-pegasus/spark,zhouyejoe/spark,bdrillard/spark,gioenn/xSpark,jianran/spark,jiangxb1987/spark,BryanCutler/spark,lvdongr/spark,andrewor14/spark,hvanhovell/spark,xuanyuanking/spark,JoshRosen/spark,aosagie/spark,ahnqirage/spark,Aegeaner/spark,bravo-zhang/spark,ahnqirage/spark,esi-mineset/spark,cloud-fan/spark,setjet/spark,eyalfa/spark,goldmedal/spark,milliman/spark,lxsmnv/spark,jianran/spark,poffuomo/spark,srowen/spark,publicRoman/spark,bdrillard/spark,map222/spark,bravo-zhang/spark,lxsmnv/spark,icexelloss/spark,jlopezmalla/spark,map222/spark,lxsmnv/spark,BryanCutler/spark,zhouyejoe/spark,szhem/spark,jianran/spark,liyichao/spark,haowu80s/spark,rekhajoshm/spark,ibm-research-ireland/sparkoscope,minixalpha/spark,bravo-zhang/spark,rezasafi/spark,markhamstra/spark,goldmedal/spark,mdespriee/spark,panfengfeng/spark,zuotingbing/spark,apache-spark-on-k8s/spark,dongjoon-hyun/spark,srowen/spark,dhruve/spark,zuotingbing/spark,UndeadBaneGitHub/spark,SnappyDataInc/spark,xuanyuanking/spark,MLnick/spark,adrian-ionescu/apache-spark,tejasapatil/spark,dhruve/spark,poffuomo/spark,debugger87/spark,srowen/spark,minixalpha/spark,ibm-research-ireland/sparkoscope,liutang123/spark,darionyaphet/spark,kissmetrics/spark,srowen/spark,shaneknapp/spark,ahnqirage/spark,eyalfa/spark,poffuomo/spark,esi-mineset/spark,caneGuy/spark,nchammas/spark,taroplus/spark,markhamstra/spark,kiszk/spark,aokolnychyi/spark,ddna1021/spark,MLnick/spark,aosagie/spark,hhbyyh/spark,rekhajoshm/spark,nchammas/spark,JerryLead/spark,chuckchen/spark,jlopezmalla/spark,icexelloss/spark,ConeyLiu/spark,sureshthalamati/spark,apache-spark-on-k8s/spark,cin/spark,shaneknapp/spark,rikima/spark,nilsgrabbert/spark,shaneknapp/spark,cloud-fan/spark,shubhamchopra/spark,sureshthalamati/spark,saturday-shi/spark,aosagie/spark,wzhfy/spark,HyukjinKwon/spark,liutang123/spark,techaddict/spark,akopich/spark,zzcclp/spark,mzl9039/spark,WeichenXu123/spark,brad-kaiser/spark,janewangfb/spark,rikima/spark,dongjoon-hyun/spark,ron8hu/spark,mdespriee/spark,kissmetrics/spark,bravo-zhang/spark,hvanhovell/spark,alunarbeach/spark,nchammas/spark,haowu80s/spark,shubhamchopra/spark,nlalevee/spark,janewangfb/spark,Panos-Bletsos/spark-cost-model-optimizer,pgandhi999/spark,ron8hu/spark,kevinyu98/spark,joseph-torres/spark,1haodian/spark,goldmedal/spark,Aegeaner/spark,bdrillard/spark,cin/spark,BryanCutler/spark,saturday-shi/spark,andrewor14/spark,ptkool/spark,andrewor14/spark,jrshust/spark,HyukjinKwon/spark,chuckchen/spark,pronix/spark,cloud-fan/spark,saltstar/spark,bOOm-X/spark,jkbradley/spark,ioana-delaney/spark,guoxiaolongzte/spark,brad-kaiser/spark,JerryLead/spark,zzcclp/spark,panfengfeng/spark,actuaryzhang/spark,apache/spark,xflin/spark,andrewor14/spark,tengpeng/spark,ioana-delaney/spark,mahak/spark,someorz/spark,maropu/spark,brad-kaiser/spark,WeichenXu123/spark,vinodkc/spark,zhouyejoe/spark,WeichenXu123/spark,sureshthalamati/spark,gioenn/xSpark,cin/spark,esi-mineset/spark,LantaoJin/spark,mzl9039/spark,huang1900/spark,skonto/spark,big-pegasus/spark,highfei2011/spark,srowen/spark,rednaxelafx/apache-spark,lxsmnv/spark,wangmiao1981/spark,Panos-Bletsos/spark-cost-model-optimizer,holdenk/spark,mdespriee/spark,map222/spark,yanboliang/spark,stanzhai/spark,mdespriee/spark,mike0sv/spark,shuangshuangwang/spark,spark-test/spark,jrshust/spark,lvdongr/spark,poffuomo/spark,vinodkc/spark,rednaxelafx/apache-spark,mike0sv/spark,ConeyLiu/spark,wzhfy/spark,ConeyLiu/spark,ConeyLiu/spark,huang1900/spark,milliman/spark,zuotingbing/spark,gengliangwang/spark,ptkool/spark,JoshRosen/spark,1haodian/spark,cin/spark,shaneknapp/spark,facaiy/spark,loneknightpy/spark,Aegeaner/spark,guoxiaolongzte/spark,highfei2011/spark,ron8hu/spark,dongjoon-hyun/spark,actuaryzhang/sp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