"import *"的确切含义是什么?进口? [英] What exactly does "import *" import?

问题描述

在Python中，import *到底要导入什么?它会导入在包含文件夹中找到的__init__.py吗?

In Python, what exactly does import * import? Does it import __init__.py found in the containing folder?

例如，是否需要声明from project.model import __init__，或者from project.model import *是否足够?

For example, is it necessary to declare from project.model import __init__, or is from project.model import * sufficient?

推荐答案

from xyz import *的优势"与其他形式的导入相反，是它可以导入一切(嗯，差不多. .[请参阅下面的(a)]所有内容)，位于当前模块下的指定模块中.这样就可以使用导入的模块中的各种对象(变量，类，方法...)，而无需在其前面加上模块名称.例如

The "advantage" of from xyz import * as opposed to other forms of import is that it imports everything (well, almost... [see (a) below] everything) from the designated module under the current module. This allows using the various objects (variables, classes, methods...) from the imported module without prefixing them with the module's name. For example

>>> from math import *
>>>pi
3.141592653589793
>>>sin(pi/2)
>>>1.0

不鼓励这种做法(将*导入当前名称空间)，因为

• 为命名空间冲突提供了机会(例如，如果在导入之前您有一个变量名pi)
如果导入的对象数量很大，
• 可能效率不高
• 没有明确记录变量/方法/类的起源(很高兴拥有该程序的自述文件"，以便将来访问代码)

• provides the opportunity for namespace collisions (say if you had a variable name pi prior to the import)
• may be inefficient, if the number of objects imported is big
• doesn't explicitly document the origin of the variable/method/class (it is nice to have this "self documentation" of the program for future visit into the code)

因此，通常，我们将这种导入*做法限制为即席测试等.正如@Denilson-Sá-Maia指出的那样，某些库(例如pygame)具有一个子模块，其中定义了所有最常用的常量和函数，并且有效地对其进行了设计设计.用import *导入.除了使用这些特殊的子模块外，其他情况下最好使用...:

Typically we therefore limit this import * practice to ad-hoc tests and the like. As pointed out by @Denilson-Sá-Maia, some libraries such as (e.g. pygame) have a sub-module where all the most commonly used constants and functions are defined and such sub-modules are effectively designed to be imported with import *. Other than with these special sub-modules, it is otherwise preferable to ...:

仅明确导入一些对象

>>>from math import pi
>>>pi
>>>3.141592653589793
>>> sin(pi/2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'sin' is not defined

或在其自己的名称空间下导入模块(或其别名，特别是如果它是长名称，并且程序多次引用其对象)

or import the module under its own namespace (or an alias thereof, in particular if this is a long name, and the program references its objects many times)

  >>>import math
  >>>math.pi
  >>>3.141592653589793
  etc..


  >>>import math as m  #bad example math being so short and standard...
  >>>m.pi
  >>>3.141592653589793
  etc..

请参见 关于此主题的Python文档

(a)具体来说，from xyz import *导入了什么?
如果xyz模块定义了__all__变量，它将导入此序列中定义的所有名称，否则它将导入所有名称(除了以下划线开头的名称除外).

(a) Specifically, what gets imported with from xyz import * ?
if xyz module defines an __all__ variable, it will import all the names defined in this sequence, otherwise it will import all names, except these which start with an underscore.

注意:许多库都具有 子模块 .例如，标准库urllib包含诸如urllib.request，urllib.errors，urllib.response等子模块.常见的混淆点是

Note Many libraries have sub-modules. For example the standard library urllib includes sub-modules like urllib.request, urllib.errors, urllib.response etc. A common point of confusion is that

from urllib import *

将导入所有这些子模块. 事实并非如此:一个人需要分别用from urllib.request import *等明确地导入这些内容.顺便说一下，这并非特定于import *，普通的import也不会导入子模块(但是，当然，*通常是一切" 的简写形式，可能会误导人们认为所有子模块以及其他所有内容都将被导入).

would import all these sub-modules. That is NOT the case: one needs to explicitly imports these separately with, say, from urllib.request import * etc. This incidentally is not specific to import *, plain import will not import sub-modules either (but of course, the * which is often a shorthand for "everything" may mislead people in thinking that all sub-modules and everything else would be imported).

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