python标准库介绍——2 os.path模块详解

== os.path 模块 ==

``os.path`` 模块包含了各种处理长文件名(路径名)的函数. 先导入 (import) ``os``
模块, 然后就可以以 ``os.path`` 访问该模块. 

=== 处理文件名===

``os.path`` 模块包含了许多与平台无关的处理长文件名的函数.
也就是说, 你不需要处理前后斜杠, 冒号等. 我们可以看看 [Example 1-42 #eg-1-42] 中的样例代码. 

====Example 1-42. 使用 os.path 模块处理文件名====[eg-1-42]

```
File: os-path-example-1.py

import os

filename = "my/little/pony"

print "using", os.name, "..."
print "split", "=>", os.path.split(filename)
print "splitext", "=>", os.path.splitext(filename)
print "dirname", "=>", os.path.dirname(filename)
print "basename", "=>", os.path.basename(filename)
print "join", "=>", os.path.join(os.path.dirname(filename),
                                 os.path.basename(filename))

*B*using nt ...
split => ('my/little', 'pony')
splitext => ('my/little/pony', '')
dirname => my/little
basename => pony
join => my/little\pony*b*
```

注意这里的 ``split`` 只分割出最后一项(不带斜杠).

``os.path`` 模块中还有许多函数允许你简单快速地获知文件名的一些特征，如 [Example 1-43 #eg-1-43] 所示。 

====Example 1-43. 使用 os.path 模块检查文件名的特征====[eg-1-43]

```
File: os-path-example-2.py

import os

FILES = (
    os.curdir,
    "/",
    "file",
    "/file",
    "samples",
    "samples/sample.jpg",
    "directory/file",
    "../directory/file",
    "/directory/file"
    )

for file in FILES:
    print file, "=>",
    if os.path.exists(file):
        print "EXISTS",
    if os.path.isabs(file):
        print "ISABS",
    if os.path.isdir(file):
        print "ISDIR",
    if os.path.isfile(file):
        print "ISFILE",
    if os.path.islink(file):
        print "ISLINK",
    if os.path.ismount(file):
        print "ISMOUNT",
    print

*B*. => EXISTS ISDIR
/ => EXISTS ISABS ISDIR ISMOUNT
file =>
/file => ISABS
samples => EXISTS ISDIR
samples/sample.jpg => EXISTS ISFILE
directory/file =>
../directory/file =>
/directory/file => ISABS*b*
```

``expanduser`` 函数以与大部分Unix shell相同的方式处理用户名快捷符号(~,
不过在 Windows 下工作不正常), 如 [Example 1-44 #eg-1-44] 所示. 

====Example 1-44. 使用 os.path 模块将用户名插入到文件名====[eg-1-44]

```
File: os-path-expanduser-example-1.py

import os

print os.path.expanduser("~/.pythonrc")

# /home/effbot/.pythonrc
```

``expandvars`` 函数将文件名中的环境变量替换为对应值, 如 [Example 1-45 #eg-1-45] 所示. 

====Example 1-45. 使用 os.path 替换文件名中的环境变量====[eg-1-45]

```
File: os-path-expandvars-example-1.py

import os

os.environ["USER"] = "user"

print os.path.expandvars("/home/$USER/config")
print os.path.expandvars("$USER/folders")

*B*/home/user/config
user/folders*b*
```

=== 搜索文件系统===

``walk`` 函数会帮你找出一个目录树下的所有文件 (如 [Example 1-46 #eg-1-46]
 所示). 它的参数依次是目录名, 回调函数, 以及传递给回调函数的数据对象. 

====Example 1-46. 使用 os.path 搜索文件系统====[eg-1-46]

```
File: os-path-walk-example-1.py

import os

def callback(arg, directory, files):
    for file in files:
        print os.path.join(directory, file), repr(arg)

os.path.walk(".", callback, "secret message")

*B*./aifc-example-1.py 'secret message'
./anydbm-example-1.py 'secret message'
./array-example-1.py 'secret message'
...
./samples 'secret message'
./samples/sample.jpg 'secret message'
./samples/sample.txt 'secret message'
./samples/sample.zip 'secret message'
./samples/articles 'secret message'
./samples/articles/article-1.txt 'secret message'
./samples/articles/article-2.txt 'secret message'
...*b*
```

``walk`` 函数的接口多少有点晦涩 (也许只是对我个人而言, 我总是记不住参数的顺序).
[Example 1-47 #eg-1-47] 中展示的 ``index`` 函数会返回一个文件名列表,
你可以直接使用 ``for-in`` 循环处理文件. 

====Example 1-47. 使用 os.listdir 搜索文件系统====[eg-1-47]

```
File: os-path-walk-example-2.py

import os

def index(directory):
    # like os.listdir, but traverses directory trees
    stack = [directory]
    files = []
    while stack:
        directory = stack.pop()
        for file in os.listdir(directory):
            fullname = os.path.join(directory, file)
            files.append(fullname)
            if os.path.isdir(fullname) and not os.path.islink(fullname):
                stack.append(fullname)
    return files

for file in index("."):
    print file

*B*.\aifc-example-1.py
.\anydbm-example-1.py
.\array-example-1.py
...*b*
```

如果你不想列出所有的文件 (基于性能或者是内存的考虑) , [Example 1-48 #eg-1-48] 展示了另一种方法.
这里 //DirectoryWalker// 类的行为与序列对象相似, 一次返回一个文件. (generator?) 

====Example 1-48. 使用 DirectoryWalker 搜索文件系统====[eg-1-48]

```
File: os-path-walk-example-3.py

import os

class DirectoryWalker:
    # a forward iterator that traverses a directory tree

    def _ _init_ _(self, directory):
        self.stack = [directory]
        self.files = []
        self.index = 0

    def _ _getitem_ _(self, index):
        while 1:
            try:
                file = self.files[self.index]
                self.index = self.index + 1
            except IndexError:
                # pop next directory from stack
                self.directory = self.stack.pop()
                self.files = os.listdir(self.directory)
                self.index = 0
            else:
                # got a filename
                fullname = os.path.join(self.directory, file)
                if os.path.isdir(fullname) and not os.path.islink(fullname):
                    self.stack.append(fullname)
                return fullname

for file in DirectoryWalker("."):
    print file

*B*.\aifc-example-1.py
.\anydbm-example-1.py
.\array-example-1.py
...*b*
```

注意 //DirectoryWalker// 类并不检查传递给 ``_ _getitem_ _`` 方法的索引值.
这意味着如果你越界访问序列成员(索引数字过大)的话, 这个类将不能正常工作. 

最后, 如果你需要处理文件大小和时间戳, [Example 1-49 #eg-1-49] 给出了一个类,
它返回文件名和它的 ``os.stat`` 属性(一个元组). 这个版本在每个文件上都能节省一次或两次
``stat`` 调用( ``os.path.isdir`` 和 ``os.path.islink`` 内部都使用了 ``stat`` ),
并且在一些平台上运行很快. 

====Example 1-49. 使用 DirectoryStatWalker 搜索文件系统====[eg-1-49]

```
File: os-path-walk-example-4.py

import os, stat

class DirectoryStatWalker:
    # a forward iterator that traverses a directory tree, and
    # returns the filename and additional file information

    def _ _init_ _(self, directory):
        self.stack = [directory]
        self.files = []
        self.index = 0

    def _ _getitem_ _(self, index):
        while 1:
            try:
                file = self.files[self.index]
                self.index = self.index + 1
            except IndexError:
                # pop next directory from stack
                self.directory = self.stack.pop()
                self.files = os.listdir(self.directory)
                self.index = 0
            else:
                # got a filename
                fullname = os.path.join(self.directory, file)
                st = os.stat(fullname)
                mode = st[stat.ST_MODE]
                if stat.S_ISDIR(mode) and not stat.S_ISLNK(mode):
                    self.stack.append(fullname)
                return fullname, st

for file, st in DirectoryStatWalker("."):
    print file, st[stat.ST_SIZE]

*B*.\aifc-example-1.py 336
.\anydbm-example-1.py 244
.\array-example-1.py 526*b*
```