python - 为什么要在 python 包的 __init__ 中使用 __all__？

Question

我已经读过 Can someone explain __all__ in Python?我知道它只影响 from ... import * 语句，但我想不出一个真正的用例。当我可以简单地避免在 __init__ 命名空间中导入这些名称时，为什么我应该在 __all__ 中重复导出的名称(DRY!)？

例子:

mypackage/__init__.py

from a import A

mypackage/a.py

A = "A"
A1 = "A1"

mypackage/b.py

B = "B"    

然后在 python 中:

>>> from mypackage import *
>>> A
'A'
>>> 
>>> A1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'A1' is not defined
>>> b
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'b' is not defined

如您所见，A 在命名空间中，但 A1 和 b 不在。为什么我必须定义 __all__ = ["A"]？

Answer 1

唯一您想要在包的__init__.py 中定义__all__ 的时间是列出您“导出”的成员的名称想要在用户执行时导出:

from package import *

这记录在 6.4.1. Importing * From a Package 中

注意:如果您不在包中定义__all__，则默认行为如下(来自文档):

If __all__ is not defined, the statement from sound.effects import *does not import all submodules from the package sound.effects into thecurrent namespace; it only ensures that the package sound.effects hasbeen imported (possibly running any initialization code in__init__.py) and then imports whatever names are defined in the package. This includes any names defined (and submodules explicitlyloaded) by __init__.py. It also includes any submodules of the packagethat were explicitly loaded by previous import statements. Considerthis code:

对此的“天真”解释可以是:

If you don't define __all__; a from package import * will bring in everything from that package and anything imported in that pacakge's __init__.py.