Python – Is Import Statement Inside Class/Function Definition a Good Idea?

In the long run I think you'll appreciate having most of your imports at the top of the file, that way you can tell at a glance how complicated your module is by what it needs to import.

If I'm adding new code to an existing file I'll usually do the import where it's needed and then if the code stays I'll make things more permanent by moving the import line to the top of the file.

One other point, I prefer to get an ImportError exception before any code is run — as a sanity check, so that's another reason to import at the top.

You can use a linter to check for unused modules.

Well, it is not that simple. Actually, import statement in many aspects looks like direct definition of something in place of it. If you write

class test:
    from platform import system

it looks exactly like

class test:
    def system():
        # ....

and then you have following problems:

1. you can't use just system() because system is not in global scope
2. you can't use self.system() because in this form, python automatically passes self as first argument, but system() has no parameters and you'll get TypeError: system() takes no arguments (1 given)
3. you can't use test.system() because system() looks like a plain method, and you'll get TypeError: unbound method system() must be called with test instance as first argument (got nothing instead)

There are several ways around these problems:

1. place import platform at top level, and use platform.system() wherever you want, thus fixing issue #1 from prev. list
2. use staticmethod decorator, fixing issues #2 and #3 from prev. list.

like

class test:
    from platform import system
    system = staticmethod(system)

then you can use either self.system() or test.system()

Actually, you should just import everything in toplevel and forget about it. You only have to split import declarations if you need something special for running. Like

import foo
import bar

def fun(param1, param2):
    # .....

if __name__ == '__main__':
    from sys import argv
    if len(argv) > 2:
        fun(argv[1], argv[2])

In this example, moving from sys import argv is valid, because it is needed only when running a script. But when you use it as an imported module, there is no need in this import. But that is not your case, because in your case, system() is always needed for test class, so there is no reason to move this import from toplevel. Just leave it there and never mind.