if(trigger.isBefore){
        if(trigger.isInsert){
            OpportunityTriggerHandler.setPartnerManagerEmail(Trigger.New);
        }
        if(trigger.isUpdate){ handler.onBeforeUpdate(); }
        //if(trigger.isDelete){ handler.onBeforeDelete(); }
    }

public static void setPartnerManagerEmail (List<Opportunity> oppList) {
    Set<Id> partnerAccounts = new Set<Id> ();
    for (Opportunity o : oppList) {
        partnerAccounts.add(o.Partner_Account__c);
    }

    Map <Id, Account> accountMap = new Map<Id,Account>([select id, Owner.Email from Account where id in :partnerAccounts]);
    for (Opportunity o : (List<Opportunity>) oppList) {
        o.EmailPartnerAccountManager__c = accountMap.get(o.Partner_Account__c).Owner.Email;
    }
}