在其他类内创建类的实例

我得像下面这样上课。

class Reports()
_name = 'reports'
def fetch_transaction_invoice():
    #any logic here

 class Bar():
 _name = 'bar'
 def test_method():
     # here i want to access fetch_transaction_invoice of Reports class.

但当我尝试跟随

class Bar():
 _name = 'bar'
 def test_method():
     Reports.fetch_transaction_invoice ()

它给了我以下的错误。

TypeError: unbound method fetch_transaction_invoice() must be called with Reports instance as first argument (got nothing instead)