初学者一个课本实例的问题
请教坛子里的高手们，本人最近在学习Python，做实例做到如下一个，但是编译总是说抽象方法不能调用，不知道问题处在哪里，请大家帮我看看，谢谢！


'''
EmployeeModule.py
'''
class Employee:
""" """

def __init__(self,first,last):
"""
"""

if self.__class__==Employee:
raise NotImplementedError,"Cannot create object of class Employee"


self.firstName=first
self.lastName=last

def __str__(self):
""" """

return "s% %s"(self.firstName,self.lastName)

def _checkPositive(self,value):
""" """

if value<0:
raise ValueError,"Attribute value (%s) must be positive" % value
else:
return value

def earnings(self):
""" """

raise NotImplementedError,"Cannot call abstract method"

#===============================================

'''
BossModule.py
'''

from EmployeeModule import *

class Boss(Employee):
def __init__(self,first,last,salary):
Employee.__init__(self,first,last)
self.weekSalary=self._checkPositive(float(salary))

def eranings(self):
return self.weekSalary

def __str__(self):
return "%17s,%s"%("Boss",Employee.__str__(self))

#======================================

'''
CommissionWorkerModule.py
'''
from EmployeeModule import *

class CommissionWorker(Employee):
def __init__(self,first,last,salary,commission,quantity):
Employee.__init__(self,first,last)
self.weekSalary=self._checkPositive(float(salary))
self.commission=self._checkPositive(float(commission))
self.quantity=self._checkPositive(quantity)

def eranings(self):
return self.salary+self.commission*self.quantity

def __str__(self):
return "%17s,%s"%("Commission Worker",Employee.__str__(self))

#==================================================
'''
HourlyWorkerModule.py
'''

from EmployeeModule import *
class HourlyWorker(Employee):
def __init__(self,first,last,wage,hours):
Employee.__init__(self, first, last)
self.wage=self._checkPositive(float(wage))
self.hours=self._checkPositive(float(hours))

def earnings(self):
if self.hours<=40:
return self.wage*self.hours
else:
return 40*self.wage+(self.hours-40)*self.wage*1.5

def __str__(self):
return "%17s,%s"%("Hourly Worker",Employee.__str__(self))

#==================================================
'''
Created on 2011-12-15

@author: peter
'''
from EmployeeModule import *

class PieceWorker(Employee):
def __init__(self,first,last,wage,quantity):
Employee.__init__(self, first, last)
self.wagePerPiece=self._checkPositive(float(wage))
self.quantity=self._checkPositive(quantity)

def earnings(self):
return self.quantity*self.wagePerPiece

def __str__(self):
return "%17s,%s"%("Piece Worker",Employee.__str__(self))

#============================================

'''
test.py
'''
import unittest
from BossModule import *
from CommissionWorkerModule import *
from HourlyWorkerModule import *
from PieceWorkerModule import *

employees=[Boss("Peter","Storm",800.00),
CommissionWorker("Sue","Jones",200.0,3.0,150),
PieceWorker("Bob","Lewis",2.5,200),
HourlyWorker("Karen","Price",13.75,40)]
for employee in employees:
print "%s earned $%.2f" % (employee,employee.earnings())

#====================
调试信息：

Traceback (most recent call last):
File "/home/peter/Ubuntu One/workspace/fig09_09_abstract/src/test.py", line 17, in <module>
print "%s earned $%.2f" % (employee,employee.earnings())
File "/home/peter/Ubuntu One/workspace/fig09_09_abstract/src/EmployeeModule.py", line 34, in earnings
raise NotImplementedError,"Cannot call abstract method"
NotImplementedError: Cannot call abstract method


[解决办法]
这大概是个异常的练习示例，或者在test.py里那句报错的代码17行前后用类似这样的语句捕获就可以正常运行了：
try:
print ...
except Exception,ex:
print ex
[解决办法]