os.walk目录遍历
每个月都有那么几天想划水，又到划水的日子了，今天分享的是刚在处理遍历目录相关用到的相关方法。
os.walk
os.walk的参数如下:
os.walk(top, topdown=True, onerror=None, followlinks=False)
其中：
top是要遍历的目录。
topdown是代表要从上而下遍历还是从下往上遍历。
onerror可以用来设置当便利出现错误的处理函数(该函数接受一个OSError的实例作为参数)，设置为空则不作处理。
followlinks表示是否要跟随目录下的链接去继续遍历，要注意的是，os.walk不会记录已经遍历的目录，所以跟随链接遍历的话有可能一直循环调用下去。
os.walk返回的是一个3个元素的元组 (root, dirs, files) ，分别表示遍历的路径名，该路径下的目录列表和该路径下文件列表。注意目录列表和文件列表不是具体路径，需要具体路径(从root开始的路径)的话可以用 os.path.join(root,dir) 和 os.path.join(root,dir) 。
例子
假设现在存在如下的文件和目录结构:
➜ test_os_walk git:(master) ✗ tree
.
├── a.py
├── b.py
├── c.py
├── dir1
│ ├── dir4
│ │ ├── g.py
│ │ └── h.py
│ ├── dirx
│ │ ├── diry
│ │ │ └── k.py
│ │ └── z.py
│ ├── e.py
│ ├── f.py
│ └── g.py
├── dir2
│ ├── dira
│ │ └── dirb
│ │ └── dirc
│ │ └── aha.py
│ ├── k.py
│ ├── l.py
│ └── m.py
└── dir3
├── dir5
│ └── z.py
├── x.py
└── y.py
10 directories, 17 files
测试topdown
当我用 os.walk 遍历这个目录时，程序和输出如下:
import os
path = '/Users/nisen/Projects/python_advanced_class/test/test_os_walk'
for root, dirs, files in os.walk(path, True):
print 'root: %s' % root
print 'dirs: %s' % dirs
print 'files: %s' % files
print ''
结果如下，从root的路径可以看出遍历是自上而下的：
➜ test git:(master) ✗ python test11.py
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk
dirs: ['dir1', 'dir2', 'dir3']
files: ['a.py', 'b.py', 'c.py']
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir1
dirs: ['dir4', 'dirx']
files: ['e.py', 'f.py', 'g.py']
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir1/dir4
dirs: []
files: ['g.py', 'h.py']
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir1/dirx
dirs: ['diry']
files: ['z.py']
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir1/dirx/diry
dirs: []
files: ['k.py']
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir2
dirs: ['dira']
files: ['k.py', 'l.py', 'm.py']
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir2/dira
dirs: ['dirb']
files: []
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir2/dira/dirb
dirs: ['dirc']
files: []
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir2/dira/dirb/dirc
dirs: []
files: ['aha.py']
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir3
dirs: ['dir5']
files: ['x.py', 'y.py']
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir3/dir5
dirs: []
files: ['z.py']
而当设置os.walk的topdown为False时，结果如下, 可以看出他是自上而下遍历的：
➜ test git:(master) ✗ python test11.py
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir1/dir4
dirs: []
files: ['g.py', 'h.py']
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir1/dirx/diry
dirs: []
files: ['k.py']
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir1/dirx
dirs: ['diry']
files: ['z.py']
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir1
dirs: ['dir4', 'dirx']
files: ['e.py', 'f.py', 'g.py']
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir2/dira/dirb/dirc
dirs: []
files: ['aha.py']
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir2/dira/dirb
dirs: ['dirc']
files: []
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir2/dira
dirs: ['dirb']
files: []
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir2
dirs: ['dira']
files: ['k.py', 'l.py', 'm.py']
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir3/dir5
dirs: []
files: ['z.py']
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk/dir3
dirs: ['dir5']
files: ['x.py', 'y.py']
root: /Users/nisen/Projects/python_advanced_class/test/test_os_walk
dirs: ['dir1', 'dir2', 'dir3']
files: ['a.py', 'b.py', 'c.py']
运行时修改遍历目录
当topdown设置为True时，可以在处理时修改返回的 dirs 列表，这样可以遍历下面的目录时会根据修改后的 dirs 来遍历。比如下面的例子，在遍历的时候不把”CSV”目录包括在内:
import os
from os.path import join, getsize
for root, dirs, files in os.walk('python/Lib/email'):
print root, "consumes",
print sum(getsize(join(root, name)) for name in files),
print "bytes in", len(files), "non-directory files"
if 'CVS' in dirs:
dirs.remove('CVS') # don't visit CVS directories
