As baseball's second half commences, USA TODAY's Jake Kaplan andGreg Rosenstein break down the National League's contenders andpretenders: Arizona Diamondbacks Schedule: The Diamondbacks open with a 10-gamehomestand, including matchups against divisional opponents, the LosAngeles Dodgers and Colorado Rockies. Their toughest stretchappears in late August with a 10-game, East Coast road trip againstthe Philadelphia Phillies, Atlanta Braves and WashingtonNationals. Keep an eye on: OF Justin Upton, 23, who is on pace to have acareer year. He leads Arizona in average (.293), hits (102) andon-base-percentage (.375) and is second in home runs (15) and RBI(46). Buy, sell or wait: Buy. Arizona ranks 21st in baseball in ERA,19th in quality starts and 24th in batting average against. Look