6.4.1 以 * 方式加载包 Importing * From a Package

Now what happens when the user writes from Sound.Effects import *? Ideally, one would hope that this somehow goes out to the filesystem, finds which submodules are present in the package, and imports them all. Unfortunately, this operation does not work very well on Mac and Windows platforms, where the filesystem does not always have accurate information about the case of a filename! On these platforms, there is no guaranteed way to know whether a file ECHO.PY should be imported as a module echo, Echo or ECHO. (For example, Windows 95 has the annoying practice of showing all file names with a capitalized first letter.) The DOS 8+3 filename restriction adds another interesting problem for long module names.

那么当用户写下 from Sound.Effects import * 时会发生什么事？理想中，总是希望在文件系统中找出包中所有的子模块，然后导入它们。不幸的是，这个操作在Mac 和 Windows 平台上工作的并不太好，这些文件系统的文件大小写并不敏感！在这些平台上没有什么方法可以确保一个叫ECHO.PY 的文件应该导入为模块 echo 、 Echo 或 ECHO 。（例如，Windows 95有一个讨厌的习惯，它会把所有的文件名都显示为首字母大写的风格。） DOS 8+3文件名限制又给长文件名模块带来了另一个有趣的问题。

The only solution is for the package author to provide an explicit index of the package. The import statement uses the following convention: if a package's __init__.py code defines a list named __all__, it is taken to be the list of module names that should be imported when from package import * is encountered. It is up to the package author to keep this list up-to-date when a new version of the package is released. Package authors may also decide not to support it, if they don't see a use for importing * from their package. For example, the file Sounds/Effects/__init__.py could contain the following code:

对于包的作者来说唯一的解决方案就是给提供一个明确的包索引。import 语句按如下条件进行转换：执行 from package import * 时，如果包中的 __init__.py 代码定义了一个名为 __all__ 的链表，就会按照链表中给出的模块名进行导入。新版本的包发布时作者可以任意更新这个链表。如果包作者不想import * 的时候导入他们的包中所有模块，那么也可能会决定不支持它（import *）。例如， Sounds/Effects/__init__.py 这个文件可能包括如下代码：

__all__ = ["echo", "surround", "reverse"]

This would mean that from Sound.Effects import * would import the three named submodules of the Sound package.

这意味着 from Sound.Effects import * 语句会从 Sound 包中导入以上三个已命名的子模块。

If __all__ is not defined, the statement from Sound.Effects import * does not import all submodules from the package Sound.Effects into the current namespace; it only ensures that the package Sound.Effects has been imported (possibly running its initialization code, __init__.py) and then imports whatever names are defined in the package. This includes any names defined (and submodules explicitly loaded) by __init__.py. It also includes any submodules of the package that were explicitly loaded by previous import statements. Consider this code:

如果没有定义 __all__ ， from Sound.Effects import * 语句不会从 Sound.Effects 包中导入所有的子模块。Effects 导入到当前的命名空间，只能确定的是导入了 Sound.Effects 包（可能会运行 __init__.py 中的初始化代码）以及包中定义的所有命名会随之导入。这样就从 __init__.py 中导入了每一个命名（以及明确导入的子模块）。同样也包括了前述的import语句从包中明确导入的子模块，考虑以下代码：

import Sound.Effects.echo
import Sound.Effects.surround
from Sound.Effects import *

In this example, the echo and surround modules are imported in the current namespace because they are defined in the Sound.Effects package when the from...import statement is executed. (This also works when __all__ is defined.)

在这个例子中，echo和surround模块导入了当前的命名空间，这是因为执行 from...import 语句时它们已经定义在 Sound.Effects 包中了（定义了 __all__ 时也会同样工作）。

Note that in general the practice of importing * from a module or package is frowned upon, since it often causes poorly readable code. However, it is okay to use it to save typing in interactive sessions, and certain modules are designed to export only names that follow certain patterns.

需要注意的是习惯上不主张从一个包或模块中用import * 导入所有模块，因为这样的通常意味着可读性会很差。然而，在交互会话中这样做可以减少输入，相对来说确定的模块被设计成只导出确定的模式中命名的那一部分。

Remember, there is nothing wrong with using from Package import specific_submodule! In fact, this is the recommended notation unless the importing module needs to use submodules with the same name from different packages.

记住， from Package import specific_submodule 没有错误！事实上，除非导入的模块需要使用其它包中的同名子模块，否则这是受到推荐的写法。