导入语句python3的变化 [英] Changes in import statement python3

问题描述

我不明白 pep-0404 中的以下内容><块引用>

在 Python 3 中，包内的隐式相对导入不再是可用 - 只有绝对导入和显式相对导入支持的.此外，明星导入(例如 from x import *)仅在模块级代码中允许.

什么是相对导入?python2 中还有哪些地方允许导入星号?请举例说明.

解决方案

每当您导入相对于当前脚本/包的包时，就会发生相对导入.

以下面的树为例:

mypkg├── base.py└── 衍生.py

现在，您的 derived.py 需要来自 base.py 的东西.在 Python 2 中，你可以这样做(在 derived.py 中):

from base import BaseThing

Python 3 不再支持这一点，因为它并不明确您想要相对"还是绝对"base.换句话说，如果系统中安装了一个名为 base 的 Python 包，你就会得到一个错误的包.

相反，它要求您使用显式导入，它在类似路径的基础上显式指定模块的位置.你的 derived.py 看起来像:

from .base import BaseThing

前导. 表示'import base from module directory';换句话说，.base 映射到 ./base.py.

同样，有 .. 前缀，它像 ../ 一样在目录层次结构中上升(..mod 映射到 ../mod.py)，然后 ... 向上两级 (../../mod.py) 等等

但是请注意，上面列出的相对路径是相对于当前模块 (derived.py) 所在目录的，不是当前工作目录.

<小时>

@BrenBarn 已经解释了明星导入案例.为了完整起见，我将不得不说同样的;)

例如，您需要使用几个 math 函数，但您只在单个函数中使用它们.在 Python 2 中，你可以半懒惰:

def sin_degrees(x):从数学导入 *返回罪(度(x))

请注意，它已经在 Python 2 中触发了警告:

a.py:1: SyntaxWarning: import * 仅允许在模块级别def sin_degrees(x):

在现代 Python 2 代码中你应该这样做，而在 Python 3 中你必须这样做:

def sin_degrees(x):from math import sin, degree返回罪(度(x))

或:

from math import *def sin_degrees(x):返回罪(度(x))

I don't understand the following from pep-0404

In Python 3, implicit relative imports within packages are no longer available - only absolute imports and explicit relative imports are supported. In addition, star imports (e.g. from x import *) are only permitted in module level code.

What is a relative import? In what other places star import was allowed in python2? Please explain with examples.

解决方案

Relative import happens whenever you are importing a package relative to the current script/package.

Consider the following tree for example:

mypkg
├── base.py
└── derived.py

Now, your derived.py requires something from base.py. In Python 2, you could do it like this (in derived.py):

from base import BaseThing

Python 3 no longer supports that since it's not explicit whether you want the 'relative' or 'absolute' base. In other words, if there was a Python package named base installed in the system, you'd get the wrong one.

Instead it requires you to use explicit imports which explicitly specify location of a module on a path-alike basis. Your derived.py would look like:

from .base import BaseThing

The leading . says 'import base from module directory'; in other words, .base maps to ./base.py.

Similarly, there is .. prefix which goes up the directory hierarchy like ../ (with ..mod mapping to ../mod.py), and then ... which goes two levels up (../../mod.py) and so on.

Please however note that the relative paths listed above were relative to directory where current module (derived.py) resides in, not the current working directory.

---

@BrenBarn has already explained the star import case. For completeness, I will have to say the same ;).

For example, you need to use a few math functions but you use them only in a single function. In Python 2 you were permitted to be semi-lazy:

def sin_degrees(x):
    from math import *
    return sin(degrees(x))

Note that it already triggers a warning in Python 2:

a.py:1: SyntaxWarning: import * only allowed at module level
  def sin_degrees(x):

In modern Python 2 code you should and in Python 3 you have to do either:

def sin_degrees(x):
    from math import sin, degrees
    return sin(degrees(x))

or:

from math import *

def sin_degrees(x):
    return sin(degrees(x))

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