In California, the police can only arrest the "dominant" (physically stronger) partner, regardless of who was the initiator or the aggressor. So a woman can attack her husband, and he goes to jail.
This is not true for California as a whole. It may be a local policy in your county, but I am dubious. In Santa Clara County, police must make an arrest where there is evidence of felony, but that does not mean that they will arrest the stronger partner. The officer will arrest the perpetrator of the felony. (Pg13 of SCC Domestic Violence Protocol)
My anecdote is an incident where a 6'2", 200lbs husband was cut by his 5'4", 130lbs wife. Officers arrived on the scene and arrested the wife, even to the objection of the husband. The blood on the husband's head was evidence of a felony. They were obligated to make an arrest. They arrested the non-dominant partner.
Those who can't write, write manuals.