PEP 8规定:

导入总是放在文件的顶部,就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。

然而,如果我导入的类/方法/函数只在很少的情况下使用,那么在需要时进行导入肯定会更有效吗?

这不是:

class SomeClass(object):

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

比这更有效率?

from datetime import datetime

class SomeClass(object):

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

当前回答

以下是对这个问题的最新答案总结 而且 相关的 的问题。

PEP 8 recommends putting imports at the top. It's often more convenient to get ImportErrors when you first run your program rather than when your program first calls your function. Putting imports in the function scope can help avoid issues with circular imports. Putting imports in the function scope helps keep maintain a clean module namespace, so that it does not appear among tab-completion suggestions. Start-up time: imports in a function won't run until (if) that function is called. Might get significant with heavy-weight libraries. Even though import statements are super fast on subsequent runs, they still incur a speed penalty which can be significant if the function is trivial but frequently in use. Imports under the __name__ == "__main__" guard seem very reasonable. Refactoring might be easier if the imports are located in the function where they're used (facilitates moving it to another module). It can also be argued that this is good for readability. However, most would argue the contrary, i.e. Imports at the top enhance readability, since you can see all your dependencies at a glance. It seems unclear if dynamic or conditional imports favour one style over another.

其他回答

这就像许多其他优化一样——你牺牲了一些可读性来换取速度。正如John提到的,如果您已经完成了分析作业,并且发现这是一个非常有用的更改,并且您需要额外的速度,那么就去做吧。最好把所有其他的导入都放在一起:

from foo import bar
from baz import qux
# Note: datetime is imported in SomeClass below

下面是一个示例,其中所有导入都位于最顶部(这是我唯一一次需要这样做)。我希望能够在Un*x和Windows上终止子进程。

import os
# ...
try:
    kill = os.kill  # will raise AttributeError on Windows
    from signal import SIGTERM
    def terminate(process):
        kill(process.pid, SIGTERM)
except (AttributeError, ImportError):
    try:
        from win32api import TerminateProcess  # use win32api if available
        def terminate(process):
            TerminateProcess(int(process._handle), -1)
    except ImportError:
        def terminate(process):
            raise NotImplementedError  # define a dummy function

(回顾:约翰·米利金所说。)

为了完成老谋子的回答和最初的问题:

当我们不得不处理循环依赖关系时,我们可以做一些“技巧”。假设我们正在处理模块a.py和b.py,它们分别包含x()和b.y()。然后:

我们可以移动模块底部的from导入之一。 我们可以将其中一个from导入移动到实际需要导入的函数或方法中(这并不总是可行的,因为您可能从多个地方使用它)。 我们可以把其中一个import改成import,就像import a

总结一下。如果您没有处理循环依赖关系,也没有使用某种技巧来避免它们,那么最好将所有导入放在顶部,因为原因已经在这个问题的其他答案中解释过了。请在做这些“技巧”时附上评论,这总是受欢迎的!:)

Putting the import statement inside of a function can prevent circular dependencies. For example, if you have 2 modules, X.py and Y.py, and they both need to import each other, this will cause a circular dependency when you import one of the modules causing an infinite loop. If you move the import statement in one of the modules then it won't try to import the other module till the function is called, and that module will already be imported, so no infinite loop. Read here for more - effbot.org/zone/import-confusion.htm

这是一个引人入胜的讨论。和许多人一样,我从未考虑过这个话题。我不得不在函数中导入,因为我想在我的一个库中使用Django ORM。在导入我的模型类之前,我不得不调用django.setup(),因为这是在文件的顶部,它被拖到完全非django库代码中,因为IoC注入器结构。

我稍微改了一下,最后把django.setup()放在了单例构造函数中,并把相关的导入放在了每个类方法的顶部。现在这运行得很好,但让我感到不安,因为导入不在顶部,而且我开始担心导入的额外时间。然后我来到这里,怀着极大的兴趣阅读了大家对此的看法。

我有很长的c++背景,现在使用Python/Cython。我对此的看法是,为什么不把导入放在函数中,除非它会导致一个概要瓶颈。这就像在你需要变量之前为它们声明空间一样。问题是我有数千行代码,所有的导入都在顶部!所以我想从现在开始,当我有时间的时候,我会在这里和那里改变奇怪的文件。