[Zope] How to know if a user already exists?

I have a form with two fields, one to write a login and the other for the password. When the anonymous user pushs the submit button I will create a new user with the id and password written. But I want to know, before doing this action, if the login already exists in the folder 'acl_users'. I tried with the function 'has_key' but it didn't work. If anyone has do it before, please send me the answer. Thank you.