Try the following:

self.env.user.name

current user is nothing but uid in python code Eg

def buttton_click(cr,uid,ids,context=None):

rec=self.pool.get('res.user').browse(cr,uid,uid)

print rec.login

This little function will get you the username you're logged in with: 

def _get_user_name(self, cr, uid, *args):
        user_obj = self.pool.get('res.users')
        user_value = user_obj.browse(cr, uid, uid)
        return user_value.login or False

And you can then call it from Python code and do with it whatever you want. For example to print it in a field as default value:

defaults = {
     'username_print': _get_user_name,
    }