关于python：”import pkg.a as a”和”from pkg import a”有什么区别？

What is the difference between “import pkg.a as a” and “from pkg import a”?

我有两个模块组成一个包下的循环导入

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/test
__init__.py
a.py
b.py

A.Py

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import test.b
def a():
print("a")

B.Py

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import test.a
def b():
print("b")

但是当我从python交互式解释器中执行"import test.a"时，它会抛出attributeError:module"test"没有属性"a"。

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>>> import test.a
Traceback (most recent call last):
File"<stdin>", line 1, in <module>
File"test/a.py", line 2, in <module>
import test.b as b
File"test/b.py", line 1, in <module>
import test.a as a
AttributeError: module 'test' has no attribute 'a'

但当我把它改成from test import a和from test import b时，它就可以工作了。

那么有什么区别呢？

我在用Python3.5

编辑1：

正如@davis herring所问，python2的行为不同。使用import test.a as a格式时，不会引发错误。

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Python 2.7.12 (default, Dec 4 2017, 14:50:18)
[GCC 5.4.0 20160609] on linux2
Type"help","copyright","credits" or"license" for more information.
>>> import test.a

但是，当使用from test import a时，它会抛出错误

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Python 2.7.12 (default, Dec 4 2017, 14:50:18)
[GCC 5.4.0 20160609] on linux2
Type"help","copyright","credits" or"license" for more information.
>>> import test.a
Traceback (most recent call last):
File"<stdin>", line 1, in <module>
File"test/a.py", line 2, in <module>
from test import b
File"test/b.py", line 1, in <module>
from test import a
ImportError: cannot import name a