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

在函数中导入变量/局部作用域可以提高性能。这取决于函数中导入对象的使用情况。如果你多次循环并访问一个模块全局对象,将它导入为本地会有帮助。

test.py

X=10
Y=11
Z=12
def add(i):
  i = i + 10

runlocal.py

from test import add, X, Y, Z

    def callme():
      x=X
      y=Y
      z=Z
      ladd=add 
      for i  in range(100000000):
        ladd(i)
        x+y+z

    callme()

run.py

from test import add, X, Y, Z

def callme():
  for i in range(100000000):
    add(i)
    X+Y+Z

callme()

在Linux上的时间显示了一个小的增益

/usr/bin/time -f "\t%E real,\t%U user,\t%S sys" python run.py 
    0:17.80 real,   17.77 user, 0.01 sys
/tmp/test$ /usr/bin/time -f "\t%E real,\t%U user,\t%S sys" python runlocal.py 
    0:14.23 real,   14.22 user, 0.01 sys

真实的是挂钟。用户是程序中的时间。Sys是系统调用的时间。

https://docs.python.org/3.5/reference/executionmodel.html#resolution-of-names

其他回答

我不太担心预先加载模块的效率。模块占用的内存不会很大(假设它足够模块化),启动成本可以忽略不计。

在大多数情况下,您希望在源文件的顶部加载模块。对于阅读代码的人来说,它可以更容易地区分哪个函数或对象来自哪个模块。

在代码的其他地方导入模块的一个很好的理由是,如果它在调试语句中使用。

例如:

do_something_with_x(x)

我可以用:

from pprint import pprint
pprint(x)
do_something_with_x(x)

当然,在代码的其他地方导入模块的另一个原因是,如果您需要动态导入它们。这是因为你几乎没有任何选择。

我不太担心预先加载模块的效率。模块占用的内存不会很大(假设它足够模块化),启动成本可以忽略不计。

大多数情况下,这对于清晰和明智的做法是有用的,但并不总是如此。下面是模块导入可能存在于其他地方的两个例子。

首先,你可以有一个这样的单元测试模块:

if __name__ == '__main__':
    import foo
    aa = foo.xyz()         # initiate something for the test

其次,您可能需要在运行时有条件地导入一些不同的模块。

if [condition]:
    import foo as plugin_api
else:
    import bar as plugin_api
xx = plugin_api.Plugin()
[...]

在其他情况下,您可能会在代码的其他部分导入。

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。

Module initialization only occurs once - on the first import. If the module in question is from the standard library, then you will likely import it from other modules in your program as well. For a module as prevalent as datetime, it is also likely a dependency for a slew of other standard libraries. The import statement would cost very little then since the module intialization would have happened already. All it is doing at this point is binding the existing module object to the local scope.

将这些信息与可读性参数结合起来,我会说import语句最好在模块范围内。

I do not aspire to provide complete answer, because others have already done this very well. I just want to mention one use case when I find especially useful to import modules inside functions. My application uses python packages and modules stored in certain location as plugins. During application startup, the application walks through all the modules in the location and imports them, then it looks inside the modules and if it finds some mounting points for the plugins (in my case it is a subclass of a certain base class having a unique ID) it registers them. The number of plugins is large (now dozens, but maybe hundreds in the future) and each of them is used quite rarely. Having imports of third party libraries at the top of my plugin modules was a bit penalty during application startup. Especially some thirdparty libraries are heavy to import (e.g. import of plotly even tries to connect to internet and download something which was adding about one second to startup). By optimizing imports (calling them only in the functions where they are used) in the plugins I managed to shrink the startup from 10 seconds to some 2 seconds. That is a big difference for my users.

所以我的答案是否定的,不要总是把导入放在模块的顶部。