Xiaoxia[PG] Python可变参数与标准输出的重定位

Python可变参数与标准输出的重定位 « Xiaoxia[PG]

Python可变参数与标准输出的重定位

使用Python的内置函数print支持可变参数，也就是说，print的调用参数是不固定的。例如：

# 一个参数
print "Hello China!"
# 两个参数
print "Your name is", name
# 三个参数
print "The number of", what, "is", count, "!"

# 一个参数
print "Hello China!"
# 两个参数
print "Your name is", name
# 三个参数
print "The number of", what, "is", count, "!"

在Python里使用*和**来设置可变参数，它们的区别是*传递一个参数列表（准确来说是参数元组），**传递一个参数字典。二者可以同时混合使用。

>>> def printArgs(*argList, **argDict):
...     print "argList =", argList, ", argDict =", argDict
...
>>> printArgs("The end of the world is", 2012, lastMan = "Xiaoxia")
argList = ('The end of the world is', 2012) , argDict = {'lastMan': 'Xiaoxia'}

>>> def printArgs(*argList, **argDict):
...     print "argList =", argList, ", argDict =", argDict
...
>>> printArgs("The end of the world is", 2012, lastMan = "Xiaoxia")
argList = ('The end of the world is', 2012) , argDict = {'lastMan': 'Xiaoxia'}

下面举一个例子来模仿print的实现，

>>> import sys
>>> def myprint(*argv):
...     sys.stdout.write(" ".join([str(i) for i in argv]) + "\n")
...
>>> print "I believe", 2012, "is the end of the world."
I believe 2012 is the end of the world.
>>> myprint("I believe", 2012, "is the end of the world.")
I believe 2012 is the end of the world.
>>> print "tuple:", (1, 2, 3), "list:", [1, 2, 3], "dict:", {"begin":-2012, "end":2012}
tuple: (1, 2, 3) list: [1, 2, 3] dict: {'begin': -2012, 'end': 2012}
>>> myprint("tuple:", (1, 2, 3), "list:", [1, 2, 3], "dict:", {"begin":-2012, "end":2012})
tuple: (1, 2, 3) list: [1, 2, 3] dict: {'begin': -2012, 'end': 2012}

>>> import sys
>>> def myprint(*argv):
...     sys.stdout.write(" ".join([str(i) for i in argv]) + "\n")
...
>>> print "I believe", 2012, "is the end of the world."
I believe 2012 is the end of the world.
>>> myprint("I believe", 2012, "is the end of the world.")
I believe 2012 is the end of the world.
>>> print "tuple:", (1, 2, 3), "list:", [1, 2, 3], "dict:", {"begin":-2012, "end":2012}
tuple: (1, 2, 3) list: [1, 2, 3] dict: {'begin': -2012, 'end': 2012}
>>> myprint("tuple:", (1, 2, 3), "list:", [1, 2, 3], "dict:", {"begin":-2012, "end":2012})
tuple: (1, 2, 3) list: [1, 2, 3] dict: {'begin': -2012, 'end': 2012}

print默认是输出到stdout中，在终端运行的程序，无重定向的情况下，print输出到控制台。如果要做代码里实现把print的输出写入到log文件中，可以通过修改stdout的文件对象来实现。同理，重定位标准输入和标准错误输出分别修改stdin和stderr的文件对象即可。

下面的例子捕捉所有print的输出，让输出的每一行前增加一个时间的显示：

import sys, time

class MyOutput():
    def __init__(self, fd):
        self.formatTime()
        self.out = fd
        self.newLine = True

    def formatTime(self):
        return time.strftime("%H:%M:%S  ", time.localtime())

    def write(self, s):
        if self.newLine:
            self.out.write(self.formatTime())
            self.newLine = False
        self.out.write(s)
        if s.endswith("\n"):
            self.newLine = True

    def flush(self):
        self.out.flush()

sys.stdout = MyOutput(sys.stdout)

print "Program begin."
mylist = [5, 4, 3, 2, 1]
print "prev:  ", mylist
mylist.sort()
print "after: ", mylist
time.sleep(3)
print "Program end."

import sys, time

class MyOutput():
    def __init__(self, fd):
        self.formatTime()
        self.out = fd
        self.newLine = True

    def formatTime(self):
        return time.strftime("%H:%M:%S  ", time.localtime())

    def write(self, s):
        if self.newLine:
            self.out.write(self.formatTime())
            self.newLine = False
        self.out.write(s)
        if s.endswith("\n"):
            self.newLine = True

    def flush(self):
        self.out.flush()

sys.stdout = MyOutput(sys.stdout)

print "Program begin."
mylist = [5, 4, 3, 2, 1]
print "prev:  ", mylist
mylist.sort()
print "after: ", mylist
time.sleep(3)
print "Program end."

运行效果：

root@xiaoxia-pc:~/桌面# python redirect.py
02:25:41 Program begin.
02:25:41 prev: [5, 4, 3, 2, 1]
02:25:41 after: [1, 2, 3, 4, 5]
02:25:44 Program end.