class Publication: #_registry = [] #_citations = [] #_references = [] def __init__(self, title, publication_date, contributors, doi_url, subjects, num_citations): #self._registry.append(self) self.title = title self.publication_date = publication_date self.contributors = contributors self.doi_url = doi_url self.subjects = subjects self.num_citations = num_citations self.num_references = num_references self._citations = [] self._references = [] class Citation: def __init__(self, title, journal, contributors, doi_url): self.title = title self.journal = journal self.contributors = contributors self.doi_url = doi_url class References: def __init__(self, title, journal, contributors, doi_url): self.title = title self.journal = journal self.contributors = contributors self.doi_url = doi_url