my personal defined snipps doesn't work

I don't know why it happend. I have come throngh the doc file and everything seems ok.
it work fine first with the system snippets. But the system snippets seem ugly. So I redefine some snippets. But find it doesnot work. I try to change the snippets file's position,but it still doesnot work. After all tries, I open a python file, enter the insert mode, press <f7> I can only see the system snippets but not my own. I have search the bugs list find not help. I do not think this is bug. So I am here to ask for some help.
 this is what my conf
1) this is my conf:
" 使用python2.7
let g:UltiSnipsUsePythonVersion = 2
" how window is opened
let g:UltiSnipsEditSplit = "horizontal"
" 定义私有Snips目录
let g:UltiSnipsSnippetsDir = "~/.vim/snippets/ultisnips"
" 定义按键
let g:UltiSnipsExpandTrigger = "<C-e>"
let g:UltiSnipsListSnippets = "<f7>"
let g:UltiSnipsJumpForwardTrigger = "<C-k>"
let g:UltiSnipsJumpBackwardTrigger = "<C-j>"

and this is my personal snippet dirs:
daipeng@daipeng:~/.vim$ ll -R snippets/
snippets/:
total 12
drwxrwxr-x 3 daipeng vboxusers 4096 Jul 17 17:43 ./
drwxrwxr-x 15 daipeng vboxusers 4096 Jul 15 17:56 ../
drwxr-xr-x 3 daipeng vboxusers 4096 Jul 17 17:23 ultisnips/

snippets/ultisnips:
total 20
drwxr-xr-x 3 daipeng vboxusers 4096 Jul 17 17:23 ./
drwxrwxr-x 3 daipeng vboxusers 4096 Jul 17 17:43 ../
-rw-r--r-- 1 daipeng vboxusers 41 Jul 17 16:59 all.snippets
drwxr-xr-x 2 daipeng vboxusers 4096 Jul 17 17:21 python/
-rw-r--r-- 1 daipeng vboxusers 775 Jul 17 17:23 python.snippets

snippets/ultisnips/python:
total 12
drwxr-xr-x 2 daipeng vboxusers 4096 Jul 17 17:21 ./
drwxr-xr-x 3 daipeng vboxusers 4096 Jul 17 17:23 ../
-rw-r--r-- 1 daipeng vboxusers 775 Jul 17 17:21 python.snippets

then this is my own snippets file:
daipeng@daipeng:~/.vim/snippets/ultisnips$ cat python.snippets
# hd
snippet hd "header for python scripts" !b
#!/user/bin/env python
# encoding: utf-8
${0}
endsnippet

# def
snippet def "def func with docstrings" !
def ${1:func}(${2:arg}):
 """ ${3:docstrings for ${1}} """
 ${0:pass}
endsnippet

snippet defs "def func(self,) with doc" !
def ${1:func}(self, ${2:argv}):
 """ ${3:docstrings for ${1}}"""
 ${0:pass}
endsnippet

snippet defc "def func(cls,) with doc" !
def ${1:func}(cls, ${2:argv}):
 """ ${3:docstrings for ${1}}"""
 ${0:pass}
endsnippet

# class
snippet class "class with docstrings" !b
class ${1:MyC}(${2:object}):
 """
 ${3:docstring for ${1}}
 """

 ${0:pass}
endsnippet

# sqlalchemy

# others
snippet init "def __init__()"
def __init__(self${1:, argv}):
 ${0:pass}
endsnippet

hope you help. thanks at first