python中的继承和抽象类的实现

#!/usr/local/bin/python
# Fig 9.9: fig09_09.py
# Creating a class hierarchy with an abstract base class.

class Employee:
   """Abstract base class Employee"""

   def __init__(self, first, last):
      """Employee constructor, takes first name and last name.
      NOTE: Cannot create object of class Employee."""

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

      self.firstName = first
      self.lastName = last

   def __str__(self):
      """String representation of Employee"""

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

   def _checkPositive(self, value):
      """Utility method to ensure a value is positive"""

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

   def earnings(self):
      """Abstract method; derived classes must override"""

      raise NotImplementedError, "Cannot call abstract method"

class Boss(Employee):
   """Boss class, inherits from Employee"""

   def __init__(self, first, last, salary):
      """Boss constructor, takes first and last names and salary"""

      Employee.__init__(self, first, last)
      self.weeklySalary = self._checkPositive(float(salary))

   def earnings(self):
      """Compute the Boss's pay"""

      return self.weeklySalary

   def __str__(self):
      """String representation of Boss"""

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

class CommissionWorker(Employee):
   """CommissionWorker class, inherits from Employee"""

   def __init__(self, first, last, salary, commission, quantity):
      """CommissionWorker constructor, takes first and last names,
      salary, commission and quantity"""

      Employee.__init__(self, first, last)
      self.salary = self._checkPositive(float(salary))
      self.commission = self._checkPositive(float(commission))
      self.quantity = self._checkPositive(quantity)

   def earnings(self):
      """Compute the CommissionWorker's pay"""

      return self.salary + self.commission * self.quantity

   def __str__(self):
      """String representation of CommissionWorker"""

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

class PieceWorker(Employee):
   """PieceWorker class, inherits from Employee"""

   def __init__(self, first, last, wage, quantity):
      """PieceWorker constructor, takes first and last names, wage
      per piece and quantity"""

      Employee.__init__(self, first, last)
      self.wagePerPiece = self._checkPositive(float(wage))
      self.quantity = self._checkPositive(quantity)

   def earnings(self):
      """Compute PieceWorker's pay"""

      return self.quantity * self.wagePerPiece

   def __str__(self):
      """String representation of PieceWorker"""

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

class HourlyWorker(Employee):
   """HourlyWorker class, inherits from Employee"""

   def __init__(self, first, last, wage, hours):
      """HourlyWorker constructor, takes first and last names,
      wage per hour and hours worked"""

      Employee.__init__(self, first, last)
      self.wage = self._checkPositive(float(wage))
      self.hours = self._checkPositive(float(hours))

   def earnings(self):
      """Compute HourlyWorker's pay"""

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

   def __str__(self):
      """String representation of HourlyWorker"""

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

# main program

# create list of Employees
employees = [ Boss("John", "Smith", 800.00),
              CommissionWorker("Sue", "Jones", 200.0, 3.0, 150),
              PieceWorker("Bob", "Lewis", 2.5, 200),
              HourlyWorker("Karen", "Price", 13.75, 40) ]

# print Employee and compute earnings
for employee in employees:
   print "%s earned $%.2f" % (employee, employee.earnings())

　　输出结果如下：

             Boss: John Smith earned $800.00

Commission Worker: Sue Jones earned $650.00

     Piece Worker: Bob Lewis earned $500.00

    Hourly Worker: Karen Price earned $550.00