在python的module中，可以使用 __all__ 函数来定义这个module像其他引用自己的module导入的变量：
例如：
alltag.py
__all__ = ['bar', 'baz']
waz = 5
bar = 10
def baz(): return 'baz'
alltagtest.py
from alltag import *
print bar
print baz
# The following will trigger an exception, as "waz" is not exported by the module
print waz
运行alltagtest.py结果：
虽然使用了import * 声明，但是waz并没有被导入，这是因为__all__函数发挥了作用。
而如果将__all__注释掉，那么就是对alltag的命名空间进行了全部的import，所以，__all__可以隐藏不想被import的默认值。
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refer：
it's a list of public objects of that module -- it overrides the default of hiding everything that begins with an underscore
If the __all__ above is commented out, this code will then execute to completion, as the default behaviour of import * is to import all symbols that do not begin with an underscore, from the given namespace.
Linked to, but not explicitly mentioned here, is exactly when __all__ is used. It is a list of strings defining what symbols in a module will be exported when from <module> import * is used on the module.
FROM：http://stackoverflow.com/questions/44834/can-someone-explain-all-in-python