California Code, Code of Civil Procedure - CCP § 2018.030

(a) A writing that reflects an attorney's impressions, conclusions, opinions, or legal research or theories is not discoverable under any circumstances.

(b) The work product of an attorney, other than a writing described in subdivision (a), is not discoverable unless the court determines that denial of discovery will unfairly prejudice the party seeking discovery in preparing that party's claim or defense or will result in an injustice.