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