Re: Callus on left handIt has to be a bad grip. I play almost everyday, year round, and hit tons of practice balls, and wear a glove -- and I've played for decades. There are no calluses on my left hand, other than thicker skin on my finger tips near the nails from a lifetime of playing classical guitar. Nothing from golf. You have to be regripping or allowing the grip to move someway.
I'm not claiming anything special, only the result of years and years of being almost fanatical about a proper grip. At the risk of sounding like a broken record -- read Hogan's Five Fundamentals. You will be glad you did. BTW, there is a slight thickening of the skin on the right side of the right thumb and the left side of the right pointing finger -- very slight.
As a kid, before learning to grip properly and with less pressure, I would practice until calluses would tear and my hands would actually bleed. Then, a guy who took lessons from Hogan saw my grip and hands and changed my grip... no more problems.