导入语句是否应该始终位于模块的顶部? [英] Should import statements always be at the top of a module?

问题描述

PEP 08 状态:

导入总是放在文件的顶部，在任何模块注释和文档字符串之后，在模块全局变量和常量之前.

Imports are always put at the top of the file, just after any module comments and docstrings, and before module globals and constants.

但是，如果我要导入的类/方法/函数仅在极少数情况下使用，那么确定在需要时进行导入肯定会更有效吗?

However if the class/method/function that I am importing is only used in rare cases, surely it is more efficient to do the import when it is needed?

不是吗?

class SomeClass(object):

    def not_often_called(self)
        from datetime import datetime
        self.datetime = datetime.now()

比这更有效率吗?

from datetime import datetime

class SomeClass(object):

    def not_often_called(self)
        self.datetime = datetime.now()

推荐答案

模块导入相当快，但不是即时的.这意味着:

Module importing is quite fast, but not instant. This means that:

• 将导入放在模块顶部很好，因为这是微不足道的成本，只需要支付一次即可.
• 将导入内容放入函数中会导致对该函数的调用花费更长时间.

因此，如果您关心效率，则将进口放在首位.仅在您的配置文件显示有帮助的情况下才将它们移入功能(您进行配置文件以查看最能改善性能的地方，对吧?)

So if you care about efficiency, put the imports at the top. Only move them into a function if your profiling shows that would help (you did profile to see where best to improve performance, right??)

我看到执行惰性导入的最佳原因是:

The best reasons I've seen to perform lazy imports are:

• 可选的库支持.如果您的代码具有使用不同库的多个路径，则在未安装可选库的情况下不要中断.
• 在插件的__init__.py中，可能已导入但未实际使用.例如Bazaar插件，它使用bzrlib的延迟加载框架.

• Optional library support. If your code has multiple paths that use different libraries, don't break if an optional library is not installed.
• In the __init__.py of a plugin, which might be imported but not actually used. Examples are Bazaar plugins, which use bzrlib's lazy-loading framework.

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