Get required fields from Document in mongoengine?

I want to be able to get a list or some sort of set of fields that are required by the document i've created. For instance, here is my document:

nickName        = StringField(required=True)
password        = StringField(required=True)
firstName       = StringField()
lastName        = StringField()
joinDate        = DateTimeField(required=True)
lastVisited     = DateTimeField(required=True)
subscriptions   = DictField(field=ObjectIdField())
isActivated     = BooleanField(default=True)
profileImage    = FileField()
isModerator     = BooleanField(default=False)
description     = StringField()
location        = GeoPointField()
email           = EmailField()
settings        = DictField()

^From this document I should receive:

["nickName","password","joinDate","lastVisited"]

as results for being required fields. Is this possible? If so, how can I achieve the desired result.

Thanks in advance!