Josh Brolin All Smiles Despite Divorce Filing

2/22/2013 1:10 PM PST

Josh Brolin -- All Smiles After Diane Lane Files for Divorce

5:00 PM PT
-- Sources tell us ... the woman in the photo is only Josh's (hot) assistant.

Josh Brolin
doesn't seem too crushed in the wake of his impending divorce from Diane Lane -- stepping out in L.A. earlier today with a beautiful mystery girl ... sporting his trademark squinty smirk thing.

TMZ broke the story ... Lane filed for divorce from Brolin on February 15th -- the day after Valentine's Day -- citing "irreconcilable differences."

Lane listed the legal date of separation as February 13th but we know Lane claims they've actually been separated longer.

Diane wants to restore her last name to just "Lane."