Should `import` statements always at the top?

why import statement always at the top of code?

should-import-statements-always-be-at-the-top-of-a-module

이 글은 스택오버플로우의 글을 가져온 것이다.

 

Question

PEP 8 states:

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

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?

Isn't this:

class SomeClass(object):

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

more efficient than this?

from datetime import datetime # here

class SomeClass(object):

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

 

Answer

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

• Putting the imports at the top of the module is fine, because it's a trivial cost that's only paid once.
• Putting the imports within a function will cause calls to that function to take longer.

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??)

---

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

• 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.

 

결론

• 최상단에 위치하면, 모듈의 전역으로 임포트되고, 임포트되는 비용을 1회만 지출할 수 있다.
• 최상단에서 임포트하지 않는 것을 lazy import 라고 부르는 모양.