Basic introduction to python sys module

1: sys is a built-in module of python.

Use the import statement to enter the sys module.

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When import sys is executed, python is listed in the directory in the sys.path variable Find the sys module file. Then run the statements in the main block of this module to initialize it, and then you can use the module.

2: Common functions of the sys module

You can use the dir() method to view the methods available in the module. The results are as follows. I have not used many of them, so I just keep them simple. Introduce several methods that I have used.

$ python
Python 2.7.6 (default, Oct 26 2016, 20:30:19) 
[GCC 4.8.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> dir(sys)
['__displayhook__', '__doc__', '__excepthook__', '__name__', '__package__', '__stderr__', '__stdin__', '__stdout__', '_clear_type_cache', '_current_frames', '_getframe', '_mercurial', '_multiarch', 'api_version', 'argv', 'builtin_module_names', 'byteorder', 'call_tracing', 'callstats', 'copyright', 'displayhook', 'dont_write_bytecode', 'exc_clear', 'exc_info', 'exc_type', 'excepthook', 'exec_prefix', 'executable', 'exit', 'flags', 'float_info', 'float_repr_style', 'getcheckinterval', 'getdefaultencoding', 'getdlopenflags', 'getfilesystemencoding', 'getprofile', 'getrecursionlimit', 'getrefcount', 'getsizeof', 'gettrace', 'hexversion', 'long_info', 'maxint', 'maxsize', 'maxunicode', 'meta_path', 'modules', 'path', 'path_hooks', 'path_importer_cache', 'platform', 'prefix', 'ps1', 'ps2', 'py3kwarning', 'pydebug', 'setcheckinterval', 'setdlopenflags', 'setprofile', 'setrecursionlimit', 'settrace', 'stderr', 'stdin', 'stdout', 'subversion', 'version', 'version_info', 'warnoptions']

(1) sys.argv implements passing parameters from outside the program to the program

sys.argv variable is a string containing command line parameters List, use the command line to pass parameters to the program. Among them, the name of the script is always the first parameter in the sys.argv list.

(2) sys.path contains a list of directory names of input modules.

Get the string collection of the specified module search path. You can put the written module under a certain path, and you can find it correctly when importing in the program. When importing module_name, module.name is searched based on the path of sys.path. You can also customize the module path.

sys.path.append("Custom module path")

(3) sys.exit([arg]) Exit in the middle of the program, arg=0 means normal exit

Generally, the interpreter automatically exits when the execution reaches the end of the main program. However, if you need to exit the program midway, you can call the sys.exit function with an optional integer parameter returned to the calling program, indicating that you can The call to sys.exit is captured in the main program. (0 means normal exit, others are exceptions) Of course, you can also use string parameters to indicate unsuccessful error messages.

(4) sys.modules

sys.modules is a global dictionary, which is loaded in memory after python is started. Whenever a programmer imports a new module, sys.modules will automatically record the module. When the module is imported for the second time, Python will directly look it up in the dictionary, thus speeding up the program. It has all the methods that the dictionary has.

(5) sys.getdefaultencoding() / sys.setdefaultencoding() / sys.getfilesystemencoding()

sys.getdefaultencoding()

Get the current encoding of the system, which generally defaults to ascii.

sys.setdefaultencoding()

Set the system default encoding. You will not see this method when executing dir (sys). If the execution fails in the interpreter, you can execute reload (sys) first. , after executing setdefaultencoding('utf8'), the system default encoding is set to utf8. (See setting the system default encoding)

sys.getfilesystemencoding()

Get the encoding used by the file system. It returns 'mbcs' under Windows and 'utf-8' under mac

(6) sys.stdin, sys.stdout, sys.stderr

The stdin, stdout, and stderr variables contain stream objects corresponding to standard I/O streams. If you need better control over the output, and print doesn't meet your requirements, they are what you need. You can also replace them, and then you can redirect output and input to other devices (device), or process them in non-standard ways.

(7) sys.platform

Get the current system platform. Such as: win32, Linux, etc.

3: Example

(1) sys.argv sys.path

$ cat sys-test.py
 #!/usr/bin/python
import sys
print 'The command line arguments are:'
for i in sys.argv:
    print i
print '\n\nThe PYTHONPATH is', sys.path, '\n'

Run result:

$ python sys-test.py my name is ubuntu 
The command line arguments are:
sys-test.py
my
name
is
ubuntu
The PYTHONPATH is ['/work/python-practice', '/usr/lib/python2.7', '/usr/lib/python2.7/plat-x86_64-linux-gnu', '/usr/lib/python2.7/lib-tk', '/usr/lib/python2.7/lib-old', '/usr/lib/python2.7/lib-dynload', '/usr/local/lib/python2.7/dist-packages', '/usr/lib/python2.7/dist-packages', '/usr/lib/python2.7/dist-packages/PILcompat', '/usr/lib/python2.7/dist-packages/gtk-2.0', '/usr/lib/pymodules/python2.7', '/usr/lib/python2.7/dist-packages/ubuntu-sso-client']

(2) sys.exit

import sys
def exitfunc(value):
    print (value)
    sys.exit(0)
print("hello")
try:
    sys.exit(90)
except SystemExit as value:
    exitfunc(value)   
print("come?")

Running results:

hello
90

The program first prints hello, then executes exit(90), throws an exception and passes 90 to values. Values ​​is executed in the passed function and prints 90. The program exits. The following "come?" will not be printed because it has already exited. If you remove sys.exit(0) in the exitfunc function at this time, the program will continue to execute until "come?" is output.

(3) sys.modules

sys.modules.keys() returns a list of all imported modules

keys is the module name

values ​​is the module

modules return path

import sys
print(sys.modules.keys())
print("**************************************************************************")
print(sys.modules.values())
print("**************************************************************************")
print(sys.modules["os"])

Run result:

['copy_reg', 'sre_compile', '_sre', 'encodings', 'site', '__builtin__', 'sysconfig', '__main__', 'encodings.encodings', 'abc', 'posixpath', '_weakrefset', 'errno', 'encodings.codecs', 'sre_constants', 're', '_abcoll', 'types', '_codecs', 'encodings.__builtin__', '_warnings', 'genericpath', 'stat', 'zipimport', '_sysconfigdata', 'warnings', 'UserDict', 'encodings.ascii', 'sys', 'codecs', '_sysconfigdata_nd', 'os.path', 'sitecustomize', 'signal', 'traceback', 'linecache', 'posix', 'encodings.aliases', 'exceptions', 'sre_parse', 'os', '_weakref']
*******************************************************************************
[<module &#39;copy_reg&#39; from &#39;/usr/lib/python2.7/copy_reg.pyc&#39;>, <module &#39;sre_compile&#39; from &#39;/usr/lib/python2.7/sre_compile.pyc&#39;>, <module &#39;_sre&#39; (built-in)>, <module &#39;encodings&#39; from &#39;/usr/lib/python2.7/encodings/__init__.pyc&#39;>, <module &#39;site&#39; from &#39;/usr/lib/python2.7/site.pyc&#39;>, <module &#39;__builtin__&#39; (built-in)>, <module &#39;sysconfig&#39; from &#39;/usr/lib/python2.7/sysconfig.pyc&#39;>, <module &#39;__main__&#39; from &#39;sys-test1.py&#39;>, None, <module &#39;abc&#39; from &#39;/usr/lib/python2.7/abc.pyc&#39;>, <module &#39;posixpath&#39; from &#39;/usr/lib/python2.7/posixpath.pyc&#39;>, <module &#39;_weakrefset&#39; from &#39;/usr/lib/python2.7/_weakrefset.pyc&#39;>, <module &#39;errno&#39; (built-in)>, None, <module &#39;sre_constants&#39; from &#39;/usr/lib/python2.7/sre_constants.pyc&#39;>, <module &#39;re&#39; from &#39;/usr/lib/python2.7/re.pyc&#39;>, <module &#39;_abcoll&#39; from &#39;/usr/lib/python2.7/_abcoll.pyc&#39;>, <module &#39;types&#39; from &#39;/usr/lib/python2.7/types.pyc&#39;>, <module &#39;_codecs&#39; (built-in)>, None, <module &#39;_warnings&#39; (built-in)>, <module &#39;genericpath&#39; from &#39;/usr/lib/python2.7/genericpath.pyc&#39;>, <module &#39;stat&#39; from &#39;/usr/lib/python2.7/stat.pyc&#39;>, <module &#39;zipimport&#39; (built-in)>, <module &#39;_sysconfigdata&#39; from &#39;/usr/lib/python2.7/_sysconfigdata.pyc&#39;>, <module &#39;warnings&#39; from &#39;/usr/lib/python2.7/warnings.pyc&#39;>, <module &#39;UserDict&#39; from &#39;/usr/lib/python2.7/UserDict.pyc&#39;>, <module &#39;encodings.ascii&#39; from &#39;/usr/lib/python2.7/encodings/ascii.pyc&#39;>, <module &#39;sys&#39; (built-in)>, <module &#39;codecs&#39; from &#39;/usr/lib/python2.7/codecs.pyc&#39;>, <module &#39;_sysconfigdata_nd&#39; from &#39;/usr/lib/python2.7/plat-x86_64-linux-gnu/_sysconfigdata_nd.pyc&#39;>, <module &#39;posixpath&#39; from &#39;/usr/lib/python2.7/posixpath.pyc&#39;>, <module &#39;sitecustomize&#39; from &#39;/usr/lib/python2.7/sitecustomize.pyc&#39;>, <module &#39;signal&#39; (built-in)>, <module &#39;traceback&#39; from &#39;/usr/lib/python2.7/traceback.pyc&#39;>, <module &#39;linecache&#39; from &#39;/usr/lib/python2.7/linecache.pyc&#39;>, <module &#39;posix&#39; (built-in)>, <module &#39;encodings.aliases&#39; from &#39;/usr/lib/python2.7/encodings/aliases.pyc&#39;>, <module &#39;exceptions&#39; (built-in)>, <module &#39;sre_parse&#39; from &#39;/usr/lib/python2.7/sre_parse.pyc&#39;>, <module &#39;os&#39; from &#39;/usr/lib/python2.7/os.pyc&#39;>, <module &#39;_weakref&#39; (built-in)>]
*******************************************************************************
<module &#39;os&#39; from &#39;/usr/lib/python2.7/os.pyc&#39;>

(4) sys.stdin/sys.stdout/sys.stderr

stdin, stdout, stderr are in All file attribute objects in Python are automatically related to the standard input, output, and errors in the shell environment when python is started. The I/O redirection of the python program in the shell is provided by the shell and has nothing to do with python itself. Relationship. The python program internally redirects stdin, stdout, and stderr read and write operations to an internal object.

Standard input

import sys
#print('Hi, %s!' %input('Please enter your name: ')) python3.*版本用input
print('Hi, %s!' %raw_input('Please enter your name: ')) #python2.*版本用raw_input
运行结果:
Please enter your name: er
Hi, er!
等同于:
 #!/usr/bin/python
import sys
print('Please enter your name:')
name=sys.stdin.readline()[:-1]
print('Hi, %s!' %name)
标准输出
print('Hello World!\n')
等同于:
 #!/usr/bin/python
import sys
sys.stdout.write('output resule is good!\n')
其他实验
#!/usr/bin/python
import sys
for i in (sys.stdin, sys.stdout, sys.stderr):
    print(i)

Execution result:

python sys-test1.py
<open file &#39;<stdin>&#39;, mode &#39;r&#39; at 0x7fa4e630f0c0>
<open file &#39;<stdout>&#39;, mode &#39;w&#39; at 0x7fa4e630f150>
<open file &#39;<stderr>&#39;, mode &#39;w&#39; at 0x7fa4e630f1e0>

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