python在安装的时候，就自带了很多模块，我们把这些模块称之为标准库，其中，有一个是使用频率比较高的，就是 os 。这个库中方法和属性众多，有兴趣的看官可以参考官方文档：https://docs.python.org/2/library/os.html，或者在交互模式中，用dir(os)看一看。
>>> import os #这个动作很重要，不能缺少
>>> dir(os)
["EX_CANTCREAT", "EX_CONFIG", "EX_DATAERR", "EX_IOERR", "EX_NOHOST", "EX_NOINPUT", "EX_NOPERM", "EX_NOUSER", "EX_OK", "EX_OSERR", "EX_OSFILE", "EX_PROTOCOL", "EX_SOFTWARE", "EX_TEMPFAIL", "EX_UNAVAILABLE", "EX_USAGE", "F_OK", "NGROUPS_MAX", "O_APPEND", "O_ASYNC", "O_CREAT", "O_DIRECT", "O_DIRECTORY", "O_DSYNC", "O_EXCL", "O_LARGEFILE", "O_NDELAY", "O_NOATIME", "O_NOCTTY", "O_NOFOLLOW", "O_NONBLOCK", "O_RDONLY", "O_RDWR", "O_RSYNC", "O_SYNC", "O_TRUNC", "O_WRONLY", "P_NOWAIT", "P_NOWAITO", "P_WAIT", "R_OK", "SEEK_CUR", "SEEK_END", "SEEK_SET", "TMP_MAX", "UserDict", "WCONTINUED", "WCOREDUMP", "WEXITSTATUS", "WIFCONTINUED", "WIFEXITED", "WIFSIGNALED", "WIFSTOPPED", "WNOHANG", "WSTOPSIG", "WTERMSIG", "WUNTRACED", "W_OK", "X_OK", "_Environ", "__all__", "__builtins__", "__doc__", "__file__", "__name__", "__package__", "_copy_reg", "_execvpe", "_exists", "_exit", "_get_exports_list", "_make_stat_result", "_make_statvfs_result", "_pickle_stat_result", "_pickle_statvfs_result", "_spawnvef", "abort", "access", "altsep", "chdir", "chmod", "chown", "chroot", "close", "closerange", "confstr", "confstr_names", "ctermid", "curdir", "defpath", "devnull", "dup", "dup2", "environ", "errno", "error", "execl", "execle", "execlp", "execlpe", "execv", "execve", "execvp", "execvpe", "extsep", "fchdir", "fchmod", "fchown", "fdatasync", "fdopen", "fork", "forkpty", "fpathconf", "fstat", "fstatvfs", "fsync", "ftruncate", "getcwd", "getcwdu", "getegid", "getenv", "geteuid", "getgid", "getgroups", "getloadavg", "getlogin", "getpgid", "getpgrp", "getpid", "getppid", "getresgid", "getresuid", "getsid", "getuid", "initgroups", "isatty", "kill", "killpg", "lchown", "linesep", "link", "listdir", "lseek", "lstat", "major", "makedev", "makedirs", "minor", "mkdir", "mkfifo", "mknod", "name", "nice", "open", "openpty", "pardir", "path", "pathconf", "pathconf_names", "pathsep", "pipe", "popen", "popen2", "popen3", "popen4", "putenv", "read", "readlink", "remove", "removedirs", "rename", "renames", "rmdir", "sep", "setegid", "seteuid", "setgid", "setgroups", "setpgid", "setpgrp", "setregid", "setresgid", "setresuid", "setreuid", "setsid", "setuid", "spawnl", "spawnle", "spawnlp", "spawnlpe", "spawnv", "spawnve", "spawnvp", "spawnvpe", "stat", "stat_float_times", "stat_result", "statvfs", "statvfs_result", "strerror", "symlink", "sys", "sysconf", "sysconf_names", "system", "tcgetpgrp", "tcsetpgrp", "tempnam", "times", "tmpfile", "tmpnam", "ttyname", "umask", "uname", "unlink", "unsetenv", "urandom", "utime", "wait", "wait3", "wait4", "waitpid", "walk", "write"]
在这么多的东西中，本讲只关注os.path，真所谓“弱水三千，只取一瓢”，为什么这么偏爱它呢？因为它和前面已经讲过的文件操作进行配合，就能够随心所欲操作各个地方的文件了（关于文件，请参考：不要红头文件(1)、不要红头文件(2)）
关于os.path的属性也不少，依然可以用dir(os.path)查看：
>>> dir(os.path)
["__all__", "__builtins__", "__doc__", "__file__", "__name__", "__package__", "_joinrealpath", "_unicode", "_varprog", "abspath", "altsep", "basename", "commonprefix", "curdir", "defpath", "devnull", "dirname", "exists", "expanduser", "expandvars", "extsep", "genericpath", "getatime", "getctime", "getmtime", "getsize", "isabs", "isdir", "isfile", "islink", "ismount", "join", "lexists", "normcase", "normpath", "os", "pardir", "pathsep", "realpath", "relpath", "samefile", "sameopenfile", "samestat", "sep", "split", "splitdrive", "splitext", "stat", "supports_unicode_filenames", "sys", "walk", "warnings"]
这么多属性，看官可以用help()逐个查看有关信息，并了解其使用方法。下面列出常见的几个使用方法，为看官减轻一点阅读英文的障碍，不过，如果看官英语足够好，请直接看原文档。就像这样：
>>> help(os.path.split)
split(p)
Split a pathname. Returns tuple "(head, tail)" where "tail" is
everything after the final slash. Either part may be empty.
以下将一些典型举例说明：
特别说明，下面的所有操作，均是进入到如下的目录中进行的。
qw@qw-Latitude-E4300:~/Documents/ITArticles/BasicPython/codes$ pwd
/home/qw/Documents/ITArticles/BasicPython/codes #当前目录
qw@qw-Latitude-E4300:~/Documents/ITArticles/BasicPython/codes$ python
Python 2.7.6 (default, Nov 13 2013, 19:24:16)
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>
文件的绝对路径
>>> import os.path
>>> os.path.abspath("225.py")
"/home/qw/Documents/ITArticles/BasicPython/codes/225.py"
文件 225.py 是真实存在上述路径中的，得到了该文件的绝对路径。但是，如果随便提供一个不在这个目录中的文件，又如何？
>>> os.path.isfile("225.py")
True
>>> os.path.isfile("2222.py")
False
>>> os.path.abspath("2222.py")
"/home/qw/Documents/ITArticles/BasicPython/codes/2222.py"
os.path.isfile(path)，可以判断path中是否是文件，其实是判断在该路径中，是否存在那个文件，如果存在则返回True，否则False。上面的操作发现 2222.py 这个文件在当前目录下是不存在的，但是，用os.path.abspaht("2222.py")能够返回一个绝对路径并带有这个不存在的文件的文件名。这里不妨理解为，如果要建立这个文件，它即将被放在那个位置。
按照这样理解，还可：
>>> os.path.abspath("/home/qw/kkkkkkkk.kk")
"/home/qw/kkkkkkkk.kk"
分开目录和文件名
>>> pn = os.path.abspath("225.py")
>>> pn
"/home/qw/Documents/ITArticles/BasicPython/codes/225.py"
>>> os.path.split(pn)
("/home/qw/Documents/ITArticles/BasicPython/codes", "225.py")
>>> path, filename = os.path.split(pn)[0], os.path.split(pn)[1]
>>> path
"/home/qw/Documents/ITArticles/BasicPython/codes"
>>> filename
"225.py"
os.paht.split(),参数是目录加文件名，就可以将路径和文件名分开。其实，我看这个功能不是很智能，你看这样
>>> os.path.split("/home/qw")
("/home", "qw")
>>> os.path.split("/home/qw/")
("/home/qw", "")
它就是将最后一组认为是文件名了，即最后一个/后面的就是文件名，所以第二个实验中，文件名是空了。是不是有点傻呢？
同样，参数中的文件或者目录，不一定是你的电脑中真实存在的，请看：
>>> os.path.split("/foo/python/qiwsir/git.git")
("/foo/python/qiwsir", "git.git")
只要符合目录书写结构，就可以分解了。
有另外两个属性，是os.path.split()的分别执行，即可以分别获得路径和文件名，这样让操作更简单了。
>>> os.path.dirname("/foo/python/qiwsir/git.git")
"/foo/python/qiwsir"
>>> os.path.basename("foo/python/qiwsir/git.git")
"git.git"
判断
前面稍微提到了os.path.isfile()可以用来判断一个文件是否存在，那么判断目录路径是否存在，可否？可：
>>> os.path.exists("/foo/python/qiwsir")
False
>>> os.path.exists("/home/qw/Documents")
True
判断相关的属性还有：
os.path.isabs(path)：判断path是否为绝对路径os.paht.isdir(path)：判断path是否为存在的目录
组合路径
将两个或多个对象组合起来，是常见的事情，那么如何将多个路径组合呢？如下：
>>> os.path.join("/home/python","/BasicsPython","226.md")
"/BasicsPython/226.md"
特别提醒，这个属性的返回值中，将第一个绝对路径忽略。
>>> os.path.join("/","/home/qw","learnpython.md")
"/home/qw/learnpython.md"