In Python the indentation of your code is critical to the compiler to understand you...In your case def __init__ not indented so this is no part of class - the default creator takes no parameter...
The code should look like this:
class Employee
""" Notice the indentation here!!! """
def __init__(self, name, StaffNumber, YearsWorked, Salary, HolidayEntitlement):
self._name=name
self._StaffNumber=StaffNumber
self._YearsWorked=[]
self._Salary=[]
self._HolidayEntitlement=[]