python的返回值

时间：2008-05-16 来源：yk325

今天在试验<<PYTHON编程金典>>上的例程，发现了一个小问题，列举如下：

下面是创建的一个类

#! /usr/bin/python
# FileName: time1.py
# Description: Simple definition of class Time
# Author: yk
# Date: Sat May 17 04:07:34 CST 2008

class Time:
    """Time abstract definition of class Time."""

    def __init__( self ):
        """Initializes hour,minute and second to zero"""

        self.hour = 0
        self.minute = 0
        self.second = 0

    def printMilitary( self ):
        """Prints object of class Time in military format"""

        print "%.2d:%.2d:%.2d" % ( self.hour,self.minute,self.second )

    def printStandard( self ):
        """Prints object of class Time in standard format"""

        standardTime = ""

        if self.hour == 0 or self.hour == 12:
            standardTime += "12:"
        else:
            standardTime += "%d:" % ( self.hour )

        standardTime += "%.2d:%.2d" % ( self.minute,self.second )

        if self.hour < 12:
            standardTime += "AM"
        else:
            standardTime += "PM"

        print standardTime

下面是测试这个类的一个小程序

#! /usr/bin/python
# FileName: t702.py
# Description: Creating and manipulating objects of class Time.
# Author: yk
# Date: Sat May 17 04:25:03 CST 2008

from time1 import Time

time = Time()

print "The attributes of time are: "
print "time1.hour:",time.hour
print "time1.minute:",time.minute
print "time1.second:",time.second

print "\nCalling method printMilitary:",
time.printMilitary()
print "\nCalling method printStandard:",
time.printStandard()

print "\n\nChanging time1's hour attribute..."
time.hour = 25
print "Calling method printMilitary after alteration:",
time.printMilitary()

当程序如上所示的时候，程序的输出为:

The attributes of time are:
time1.hour: 0
time1.minute: 0
time1.second: 0

Calling method printMilitary: 00:00:00

Calling method printStandard: 12:00:00AM

Changing time1

但是，如果把类的函数放在print语句的同一行的时候，如把下边的语句

print "\nCalling method printMilitary:",
time.printMilitary()

改为

print "\nCalling method printMilitary:",time.printMilitary()

后，程序的输出如下所示：

The attributes of time are:
time1.hour: 0
time1.minute: 0
time1.second: 0

Calling method printMilitary: 00:00:00
None

Calling method printStandard: 12:00:00AM

None

Changing time1's hour attribute...
Calling method printMilitary after alteration: 25:00:00

None

    在网上查了一下，有如下的说明，在PYTHON中，函数没有定义返回的数据类型。Python 不需要指定返回值的数据类型；甚至不需要指定是否有返回值。实际上，每个 Python 函数都返回一个值；如果函数执行过 return 语句，它将返回指定的值，否则将返回 None (Python 的空值)

    但是，我现在还是不明白，为什么换行的话就不输出None。