sys.argv

功能：在外部向程序内部传递参数
示例：sys.py

import sys
print sys.argv[0]
print sys.argv[1]

运行：

>>>python sys.py argv1
sys.py
argv1

sys.exit(n)

功能：执行到主程序末尾，解释器自动退出，但是如果需要中途退出程序，可以调用sys.exit函数，带有一个可选的整数参数返回给调用它的程序，表示你可以在主程序中捕获对sys.exit的调用。（0是正常退出，其他为异常）

示例：exit.py

#!/usr/bin/env python
import sys

def exitfunc(value):
    print value
    sys.exit(0)

print "hello"
try:
    sys.exit(1)
except SystemExit,value:
    exitfunc(value)
print "come?"

运行：

>>> python exit.py
hello
1

sys.exitfunc （回调函数）

通过sys.exitfunc来注册程序退出时的回调函数，我们可以在这个回调函数中做一些资源清理的操作，
但通过它只能注册一个回调，而且还不支持参数（可以带默认参数）。
【1】exit_handler.py文件：

#!usr/bin/env python
#coding:utf-8
import sys
def exitfunc():
    print 'exit is done!'
sys.exitfunc=exitfunc #注册回调函数
if __name__=='__main__':
    print 'hello world'
　　#程序运行结束，在退出解释器之前，会自动调用回调函数。。

>>> python exit_handler.py
hello world
exit is done!

【2】exit_handler1.py文件：

#!usr/bin/env python  
#coding:utf-8  
import sys  
def exitfunc():  
    print 'exit is done!'  
sys.exitfunc=exitfunc  #通过sys.exitfunc来注册程序退出时的回调函数  
print 'hello world'  
sys.exit(1)#执行至主程序的末尾时,解释器会自动退出。 我们也可以调用sys.exit函数使程序中途退出解释器！  
print 'ok'#该语句不执行

>>> python exit_handler1.py
hello world
exit is done!

【3】exit_handler2.py文件：

#!usr/bin/env python
#coding:utf-8
import sys
def exitfunc():
    print 'exit is done!'
sys.exitfunc=exitfunc
if __name__=='__main__':
    import os
    print '回调函数没有执行！'
    os._exit(0) #通过os._exit()退出，注册的回调函数将不会被调用

>>> python exit_handler3.py
回调函数没有执行！

通过os._exit()退出，注册的回调函数将不会被调用

关于回调函数，sys.exitfunc有其自身的缺点，比如，只能注册一个回调函数，不能传递参数。。。为了更加方便的注册回调，我们一般使用atexit模块。。。

关于atexit模块，请参考：点击打开链接

sys.path

本着下定义开头吧：python中import某个A模块时，首先会从python的内置模块中查找是否含义该模块的定义若未查询到会从sys.path对应的模块路径查询是否含有对应模块的定义，如果搜索完成依然没有对应A模块时则抛出import的异常

接着说明下python的两种加载py文件的方式：

• python xxx.py
• python -m xxx.py

第一种方式是直接运行方式

第二种方式是把模块当做脚本来启动

可能看起来说的python运行方式和sys.path有点大相径庭，但实际上两种不同方式的运行导致sys.path[0]的值是有差异的

第一种方式：sys.path[0]是当前脚本的运行目录

第二种方式：sys.path[0]是空值字符串，也就是当前执行python的目录

sys.path是一个python搜索模块的路径列表：
 eg、下边的X.py文件中打印出sys.path内容：

import sys
print sys.path

python X.py运行

其中sys.path[0]是 I:\restful_code\tester，对应调用python解释器的脚本所在的目录。 其实就是存放需要运行的代码的路径

python -m x.py运行

 使用场景：

 在实际开发中，默认包含了当前目录为搜索路径，所以，当前目录下的模块和子模块均可以正常访问。但是若一个模块需要import平级的不同目录的模块，或者上级目录里面的模块，就可以通过修改path来实现

 修改path的两种方法：

方法一：函数添加
这是即时生效的方法，就是在模块里面修改sys.path值，这种方法修改的sys.path作用域只是当前进程，进程结束后就失效了。
个人比较推荐这种方法，比较干净， 避免一些冲突问题。
比如现在的代码目录结构：
/src/configs/config.py
/src/common/Database.py
假如Database.py期望导入config. py，则可以增加上级目录到sys.path列表里面：

parent_path = os.path.dirname(sys.path[0]) 
if parent_path not in sys.path: 
    sys.path.append(parent_path) 
import configs.config

方法一：修改环境变量

添加系统环境变量PYTHONPATH，在这个环境变量中输入相关的路径，不同的路径之间用逗号（英文的！)分开。路径会自动加入到sys.path中。

os.path.abspath(__file__) 作用： 获取当前脚本的完整路径
sys.path.append(’引用模块的地址')对于模块和自己写的程序不在同一个目录下，
可以把模块的路径通过sys.path.append(路径)添加到程序中。 

sys.modules

sys.modules是一个全局字典，该字典是python启动后就加载在内存中。每当程序员导入新的模块，sys.modules都将记录这些模块。字典sys.modules对于加载模块起到了缓冲的作用。当某个模块第一次导入，字典sys.modules将自动记录该模块。当第二次再导入该模块时，python会直接到字典中查找，从而加快了程序运行的速度。

字典sys.modules具有字典所拥有的一切方法，可以通过这些方法了解当前的环境加载了哪些模块

import os
import shutil
import sys
print sys.modules

{'code': <module 'code' from 'D:\ana\envs\py27\lib\code.pyc'>, 'numpy.core.info': <module 'numpy.core.info' from 'D:\ana\envs\py27\lib\site-packages\numpy\core\info.pyc'>, '_pydevd_bundle.code': None, 'ctypes.os': None, 'gc': <module 'gc' (built-in)>, '_pydevd_bundle.os': None, 'distutils.sysconfig': <module 'distutils.sysconfig' from 'D:\ana\envs\py27\lib\distutils\sysconfig.pyc'>, 'pkg_resources._vendor.traceback': None, 'email.warnings': None, 'pkg_resources._vendor.packaging.__about__': <module 'pkg_resources._vendor.packaging.__about__' from 'D:\ana\envs\py27\lib\site-packages\pkg_resources\_vendor\packaging\__about__.pyc'>, '_pydev_bundle._pydev_completer': <module '_pydev_bundle._pydev_completer' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\_pydev_bundle\_pydev_completer.py'>, 'pprint': <module 'pprint' from 'D:\ana\envs\py27\lib\pprint.pyc'>, 'pydevd_concurrency_analyser.pydevd_concurrency_logger': <module 'pydevd_concurrency_analyser.pydevd_concurrency_logger' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\pydevd_concurrency_analyser\pydevd_concurrency_logger.py'>, 'unittest.sys': None, 'numpy.core.umath': <module 'numpy.core.umath' from 'D:\ana\envs\py27\lib\site-packages\numpy\core\umath.pyc'>, 'numpy._pytesttester': <module 'numpy._pytesttester' from 'D:\ana\envs\py27\lib\site-packages\numpy\_pytesttester.pyc'>, 'string': <module 'string' from 'D:\ana\envs\py27\lib\string.pyc'>, 'SocketServer': <module 'SocketServer' from 'D:\ana\envs\py27\lib\SocketServer.pyc'>, 'numpy.lib.arraysetops': <module 'numpy.lib.arraysetops' from 'D:\ana\envs\py27\lib\site-packages\numpy\lib\arraysetops.pyc'>, 'pydevd_concurrency_analyser.time': None, 'numpy.core._multiarray_tests': <module 'numpy.core._multiarray_tests' from 'D:\ana\envs\py27\lib\site-packages\numpy\core\_multiarray_tests.pyd'>, '_pydevd_bundle.pydevd_comm_constants': <module '_pydevd_bundle.pydevd_comm_constants' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\_pydevd_bundle\pydevd_comm_constants.py'>, 'email.quoprimime': <module 'email.quoprimime' from 'D:\ana\envs\py27\lib\email\quoprimime.pyc'>, 'json.encoder': <module 'json.encoder' from 'D:\ana\envs\py27\lib\json\encoder.pyc'>, 'subprocess': <module 'subprocess' from 'D:\ana\envs\py27\lib\subprocess.pyc'>, 'numpy.core.machar': <module 'numpy.core.machar' from 'D:\ana\envs\py27\lib\site-packages\numpy\core\machar.pyc'>, '_pydevd_bundle.pydevd_dont_trace_files': <module '_pydevd_bundle.pydevd_dont_trace_files' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\_pydevd_bundle\pydevd_dont_trace_files.py'>, 'unittest.StringIO': None, 'encodings._codecs_cn': None, 'email.codecs': None, 'numpy.ma.extras': <module 'numpy.ma.extras' from 'D:\ana\envs\py27\lib\site-packages\numpy\ma\extras.pyc'>, 'numpy.fft.fftpack_lite': <module 'numpy.fft.fftpack_lite' from 'D:\ana\envs\py27\lib\site-packages\numpy\fft\fftpack_lite.pyd'>, 'math': <module 'math' (built-in)>, 'dis': <module 'dis' from 'D:\ana\envs\py27\lib\dis.pyc'>, 'zlib': <module 'zlib' (built-in)>, '_pydevd_bundle.pydevd_cython_wrapper': <module '_pydevd_bundle.pydevd_cython_wrapper' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\_pydevd_bundle\pydevd_cython_wrapper.py'>, 'pkg_resources._vendor.pprint': None, '_pydevd_bundle.pydevd_extension_api': <module '_pydevd_bundle.pydevd_extension_api' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\_pydevd_bundle\pydevd_extension_api.py'>, '_pydev_bundle.pydev_log': <module '_pydev_bundle.pydev_log' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\_pydev_bundle\pydev_log.py'>, 'numpy.core._ctypes': None, 'matplotlib.cbook.warnings': None, 'unittest.pprint': None, 'encodings.gbk': <module 'encodings.gbk' from 'D:\ana\envs\py27\lib\encodings\gbk.pyc'>, 'matplotlib.cbook.textwrap': None, 'abc': <module 'abc' from 'D:\ana\envs\py27\lib\abc.pyc'>, '_warnings': <module '_warnings' (built-in)>, 'pkg_resources._vendor.collections': None, 'numpy._globals': <module 'numpy._globals' from 'D:\ana\envs\py27\lib\site-packages\numpy\_globals.pyc'>, 'numpy.lib.npyio': <module 'numpy.lib.npyio' from 'D:\ana\envs\py27\lib\site-packages\numpy\lib\npyio.pyc'>, 'pkg_resources._vendor.copy': None, 'matplotlib.sys': None, 'ntpath': <module 'ntpath' from 'D:\ana\envs\py27\lib\ntpath.pyc'>, 'numpy.fft.helper': <module 'numpy.fft.helper' from 'D:\ana\envs\py27\lib\site-packages\numpy\fft\helper.pyc'>, 'pydev_ipython': <module 'pydev_ipython' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\pydev_ipython\__init__.py'>, 'unittest.suite': <module 'unittest.suite' from 'D:\ana\envs\py27\lib\unittest\suite.pyc'>, '_pydevd_bundle._pydev_imps': None, 'matplotlib': <module 'matplotlib' from 'D:\ana\envs\py27\lib\site-packages\matplotlib\__init__.pyc'>, 'exceptions': <module 'exceptions' (built-in)>, 'json.scanner': <module 'json.scanner' from 'D:\ana\envs\py27\lib\json\scanner.pyc'>, 'codecs': <module 'codecs' from 'D:\ana\envs\py27\lib\codecs.pyc'>, 'numpy.os': None, '_pydevd_bundle.types': None, 'email.socket': None, 'StringIO': <module 'StringIO' from 'D:\ana\envs\py27\lib\StringIO.pyc'>, 'pkg_resources': <module 'pkg_resources' from 'D:\ana\envs\py27\lib\site-packages\pkg_resources\__init__.pyc'>, 'weakref': <module 'weakref' from 'D:\ana\envs\py27\lib\weakref.pyc'>, 'numpy.core._internal': <module 'numpy.core._internal' from 'D:\ana\envs\py27\lib\site-packages\numpy\core\_internal.pyc'>, '_pydevd_bundle': <module '_pydevd_bundle' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\_pydevd_bundle\__init__.py'>, 'distutils.sys': None, 'numpy.lib.arraypad': <module 'numpy.lib.arraypad' from 'D:\ana\envs\py27\lib\site-packages\numpy\lib\arraypad.pyc'>, '_pydevd_bundle.abc': None, '_pydevd_bundle.pydevd_plugin_utils': <module '_pydevd_bundle.pydevd_plugin_utils' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\_pydevd_bundle\pydevd_plugin_utils.py'>, 'base64': <module 'base64' from 'D:\ana\envs\py27\lib\base64.pyc'>, '_pydevd_bundle.linecache': None, '_json': <module '_json' (built-in)>, 'email.FeedParser': <email.LazyImporter object at 0x00000000134E9748>, 'pkg_resources._vendor.warnings': None, 'email.charset': <module 'email.charset' from 'D:\ana\envs\py27\lib\email\charset.pyc'>, 'pkg_resources._vendor.datetime': None, '_pydev_imps': <module '_pydev_imps' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\_pydev_imps\__init__.py'>, 'select': <module 'select' from 'D:\ana\envs\py27\DLLs\select.pyd'>, 'ctypes._ctypes': None, '_heapq': <module '_heapq' (built-in)>, 'six.moves.urllib': <module 'six.moves.urllib' (built-in)>, '_pydev_bundle._subprocess': None, 'numpy.lib.financial': <module 'numpy.lib.financial' from 'D:\ana\envs\py27\lib\site-packages\numpy\lib\financial.pyc'>, 'pkg_resources._vendor.packaging.specifiers': <module 'pkg_resources._vendor.packaging.specifiers' from 'D:\ana\envs\py27\lib\site-packages\pkg_resources\_vendor\packaging\specifiers.pyc'>, 'binascii': <module 'binascii' (built-in)>, 'email.MIMEMessage': <email.LazyImporter object at 0x00000000134E7088>, 'email._parseaddr': <module 'email._parseaddr' from 'D:\ana\envs\py27\lib\email\_parseaddr.pyc'>, 'email.sys': None, 'tokenize': <module 'tokenize' from 'D:\ana\envs\py27\lib\tokenize.pyc'>, 'numpy.core.numpy': None, 'numpy.polynomial.chebyshev': <module 'numpy.polynomial.chebyshev' from 'D:\ana\envs\py27\lib\site-packages\numpy\polynomial\chebyshev.pyc'>, 'pkg_resources.extern.six.moves.urllib': <module 'pkg_resources._vendor.six.moves.urllib' (built-in)>, 'cPickle': <module 'cPickle' (built-in)>, 'numpy.polynomial.hermite_e': <module 'numpy.polynomial.hermite_e' from 'D:\ana\envs\py27\lib\site-packages\numpy\polynomial\hermite_e.pyc'>, '_pydevd_bundle.pydevd_signature': <module '_pydevd_bundle.pydevd_signature' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\_pydevd_bundle\pydevd_signature.py'>, 'pydevd_concurrency_analyser.pydevd_thread_wrappers': <module 'pydevd_concurrency_analyser.pydevd_thread_wrappers' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\pydevd_concurrency_analyser\pydevd_thread_wrappers.py'>, '_pydevd_bundle.pydevd_vars': <module '_pydevd_bundle.pydevd_vars' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\_pydevd_bundle\pydevd_vars.py'>, 'pydevd_concurrency_analyser.pydevd_concurrency_analyser': None, '_pydevd_bundle.pydevd_traceproperty': <module '_pydevd_bundle.pydevd_traceproperty' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\_pydevd_bundle\pydevd_traceproperty.py'>, '_pydevd_frame_eval.os': None, 'pydev_ipython.inputhook': <module 'pydev_ipython.inputhook' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\pydev_ipython\inputhook.py'>, 'numpy.core.fromnumeric': <module 'numpy.core.fromnumeric' from 'D:\ana\envs\py27\lib\site-packages\numpy\core\fromnumeric.pyc'>, 'numpy.ctypeslib': <module 'numpy.ctypeslib' from 'D:\ana\envs\py27\lib\site-packages\numpy\ctypeslib.pyc'>, 'matplotlib._version': <module 'matplotlib._version' from 'D:\ana\envs\py27\lib\site-packages\matplotlib\_version.pyc'>, '_ast': <module '_ast' (built-in)>, '_pydev_bundle._pydev_filesystem_encoding': <module '_pydev_bundle._pydev_filesystem_encoding' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\_pydev_bundle\_pydev_filesystem_encoding.py'>, 'pkg_resources.os': None, '_pydev_bundle._pydev_imports_tipper': <module '_pydev_bundle._pydev_imports_tipper' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\_pydev_bundle\_pydev_imports_tipper.py'>, '_bisect': <module '_bisect' (built-in)>, 'encodings.aliases': <module 'encodings.aliases' from 'D:\ana\envs\py27\lib\encodings\aliases.pyc'>, 'fnmatch': <module 'fnmatch' from 'D:\ana\envs\py27\lib\fnmatch.pyc'>, 'sre_parse': <module 'sre_parse' from 'D:\ana\envs\py27\lib\sre_parse.pyc'>, 'pickle': <module 'pickle' from 'D:\ana\envs\py27\lib\pickle.pyc'>, 'numpy.random.warnings': None, 'numpy.lib.polynomial': <module 'numpy.lib.polynomial' from 'D:\ana\envs\py27\lib\site-packages\numpy\lib\polynomial.pyc'>, 'numpy.compat': <module 'numpy.compat' from 'D:\ana\envs\py27\lib\site-packages\numpy\compat\__init__.pyc'>, 'pkg_resources._vendor.os': None, 'pkg_resources.extern.six': <module 'pkg_resources._vendor.six' from 'D:\ana\envs\py27\lib\site-packages\pkg_resources\_vendor\six.pyc'>, 'numbers': <module 'numbers' from 'D:\ana\envs\py27\lib\numbers.pyc'>, '_pydev_bundle.pydev_code_executor': <module '_pydev_bundle.pydev_code_executor' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\_pydev_bundle\pydev_code_executor.py'>, 'numpy.core.records': <module 'numpy.core.records' from 'D:\ana\envs\py27\lib\site-packages\numpy\core\records.pyc'>, 'pydevd_tracing': <module 'pydevd_tracing' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\pydevd_tracing.py'>, '_pydevd_bundle.StringIO': None, 'email.Header': <email.LazyImporter object at 0x00000000134E9A48>, 'shutil': <module 'shutil' from 'D:\ana\envs\py27\lib\shutil.pyc'>, 'strop': <module 'strop' (built-in)>, 'numpy.core.numeric': <module 'numpy.core.numeric' from 'D:\ana\envs\py27\lib\site-packages\numpy\core\numeric.pyc'>, 'six': <module 'six' from 'D:\ana\envs\py27\lib\site-packages\six.pyc'>, 'pydev_ipython.matplotlibtools': <module 'pydev_ipython.matplotlibtools' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\pydev_ipython\matplotlibtools.py'>, 'email.message': <module 'email.message' from 'D:\ana\envs\py27\lib\email\message.pyc'>, '_pydevd_bundle.imp': None, '_pydev_bundle.traceback': None, '_pydevd_bundle.json': None, 'numpy.lib.utils': <module 'numpy.lib.utils' from 'D:\ana\envs\py27\lib\site-packages\numpy\lib\utils.pyc'>, 'email.Charset': <email.LazyImporter object at 0x00000000134E9408>, 'encodings.utf_8': 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'D:\ana\envs\py27\lib\site-packages\numpy\core\defchararray.pyc'>, '_abcoll': <module '_abcoll' from 'D:\ana\envs\py27\lib\_abcoll.pyc'>, 'nt': <module 'nt' (built-in)>, 'genericpath': <module 'genericpath' from 'D:\ana\envs\py27\lib\genericpath.pyc'>, 'stat': <module 'stat' from 'D:\ana\envs\py27\lib\stat.pyc'>, 'urllib2': <module 'urllib2' from 'D:\ana\envs\py27\lib\urllib2.pyc'>, 'unittest.signals': <module 'unittest.signals' from 'D:\ana\envs\py27\lib\unittest\signals.pyc'>, 'pydevd_plugins.django_debug': <module 'pydevd_plugins.django_debug' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\pydevd_plugins\django_debug.py'>, 'numpy.core.warnings': None, 'email.Encoders': <email.LazyImporter object at 0x00000000134E9208>, 'backports.functools_lru_cache': <module 'backports.functools_lru_cache' from 'D:\ana\envs\py27\lib\site-packages\backports\functools_lru_cache.pyc'>, 'ctypes.ctypes': None, 'numpy.lib.format': <module 'numpy.lib.format' from 'D:\ana\envs\py27\lib\site-packages\numpy\lib\format.pyc'>, 'sitecustomize': <module 'sitecustomize' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pycharm_matplotlib_backend\sitecustomize.pyc'>, 'numpy.testing._private.decorators': <module 'numpy.testing._private.decorators' from 'D:\ana\envs\py27\lib\site-packages\numpy\testing\_private\decorators.pyc'>, 'xmlrpclib': <module 'xmlrpclib' from 'D:\ana\envs\py27\lib\xmlrpclib.pyc'>, 'encodings.mbcs': <module 'encodings.mbcs' from 'D:\ana\envs\py27\lib\encodings\mbcs.pyc'>, 'pkg_resources._vendor.types': None, '_pydev_bundle.pydev_monkey': <module '_pydev_bundle.pydev_monkey' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\_pydev_bundle\pydev_monkey.py'>, '_pydev_imps.thread': None, 'time': <module 'time' (built-in)>, 'pydevd_plugins': <module 'pydevd_plugins' from 'D:\pycharm\PyCharm 2020.1\plugins\python\helpers\pydev\pydevd_plugins\__init__.py'>}

sys.platform

获取当前执行环境的平台，如win32表示是Windows系统，linux2表示是linux平台

sys.getrecursionlimit

python默认支持的递归数量

sys.stdout.write

可以做输出重定向

for i in range(3):
    print("魔降风云变")
import sys
for i in range(3):
    sys.stdout.write('小马过河')
-----------------结果：
魔降风云变
魔降风云变
魔降风云变
小马过河小马过河小马过河

import sys
for i in range(3):
    sys.stderr.write('小马过河')
------------------结果：
小马过河小马过河小马过河

stdout 是一个类文件对象；调用它的 write 函数可以打印出你给定的任何字符串。
实际上，这就是 print 函数真正做的事情；它在你打印的字符串后面加上一个硬回车，然后调用 sys.stdout.write 函数。
在最简单的例子中，stdout 和 stderr 把它们的输出发送到相同的地方
和 stdout 一样，stderr 并不为你添加硬回车；如果需要，要自己加上。
stdout 和 stderr 都是类文件对象，但是它们都是只写的。
它们都没有 read 方法，只有 write 方法。然而，它们仍然是类文件对象，因此你可以将其它任何 (类) 文件对象赋值给它们来重定向其输出

标准流stdin&stdout&stderr

描述:
sys.stdin ---- 标准输入函数，类似于input()
sys.stdput----标准输出函数，类似于print()
sys.stderr----标准错误输出函数
实际上，input()和print()都是通过调用标准流实现，sys.stdin和sys.stdout实质上不过是两个interface接口,用于切换不同i/o

 sys.stdin vs input()

• sys.stdin.readline() 用于读取一行文本输入，直到按回车，注意，使用该函数时，回车符也被读入

>>> import sys
>>> l=sys.stdin.readline()
123
>>>print(l)
# 输出 '123\n'
>>> len(l)
# 输出 4

• sys.stdin.readlines() 用于读取多行文本输入，按下 回车 后，无法退出输入，需要 再 按下 Ctrl+z 然后 再按 回车

>>>import sys
>>>> i=sys.stdin.readlines()
1
2
3
a
b
c
^Z
>>> print(i)
['1\n', '2\n', '3\n', 'a\n', 'b\n', 'c\n']

• input() 用于读取一行文本输入，回车符结束输入，但是不会被包含在输入内

总结：

sys.stdin的两种输入都会包含回车，而input()不会

sys.std和input() 两种输入的返回对象都是字符型，可以通过int()、floadt() 等 进行强制类型转换

• sys.stdin消去换行符：

  • 法一：sys.stdin.readline().strip(’/n’)
  • 法二：sys.stdin.readline()[:-1]

sys.stdout vs print()

• print：python在调用print的过程中，实际上是引用了sys.stdout.write(obj+’/n’)
  即 print()结束时默认换行，若想实现不自动换行，可使用print(param,end=’ ')
• sys.stdout.write() 方法把字符写入到标准输出中，也就是控制台。
  该方法默认不换行，若想实现换行，可使用sys.stdout.write(‘str/n’)
• 区别：print()几乎可以打印所有类型的数据，但是sys.stdout.write()只接受字符型数据
  eg

>>> a=1
>>> type(a)
<class 'int'>
>>> sys.stdout.write(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: write() argument must be str, not int
>>> print(a)
1

重定向

1、重定向流到文件或程序
在Windows和类Unix平台下，可以利用shell语法 < filename 把标准输入流重定向到文件输入
eg：以下是一个示例脚本，命名为 test.py

def interact():
	print('Hello stream world')  #正常输出
	while True:
		try:
			reply=input('Enter a number >') #输入（读取）来自sys.stdin的数据（也就是标准输入流的数据）
		except EOFError: #当文件结束时，抛出异常，结束循环
			break
		else:
			num=int(reply)
			print("%d squared is %d" % (num,num**2))
	print('Bye')

if __name__=='__main__':
	interact()

>>> python test.py  # 路径因人而异
Hello stream world
Enter a number >8  #输入8
8 squared is 64
Enter a number >6 #输入6
6 squared is 36
Enter a number >^Z # ctrl +z 是Windows下的文件结束符
Bye

接下来利用重定向的方式运行该脚本

>>> python test.py < input.txt
Hello stream world
Enter a number >8 squared is 64
Enter a number >6 squared is 36
Bye

利用重定向，无需交互式输入

 同样，可以利用> filename 将输出重定向到文件

>>> python test.py > output.txt  #仍需交互式输入，当outpu.txtt不存在时会在当前工作目录下自动创建
8
6
^Z

output.txt中内容如下 
Hello stream world
Enter a number >8 squared is 64
Enter a number >6 squared is 36
Bye

也可以在一个命令中把输入输出重定向结合起来，如下所示

>>> python test.py < inpu.txt > output.txt
Hello stream world
Enter a number >8 squared is 64
Enter a number >6 squared is 36
Bye

获取调用者的函数名，两种方法

sys._getframe().f_back.f_code.co_name
# 或
sys._getframe(1).f_code.co_name