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Can anyone explain how to make the following example work? Since several functions inside of the class will use the same function from platform I thought it would be better to import it right inside of the class, but I don't see how I can use it inside of the function (since I constantly get errors about it).

#!/usr/bin/python

class test:
   from platform import system
   is_linux(self):
      system = system()
      if system == "Linux": return True

A better example:

#!/usr/bin/python

# Add ANSI colour strings
class stdout:
    from sys import stdout
    def message(message, self):  stdout.write(message)

Note: This is just a snippet, there are some parts missing but an example of what I mean.
I know I could probably just move system = system() and use self.system but perhaps a better way?

Jordon Bedwell
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    Why do you want to import external modules as class variables? Unless you have a Very Good Reason, don't do that. – Wooble Jul 30 '11 at 13:30
  • @wooble since I am still in the process of completely understanding python I did not realise that imports were global so it was a futile attempt to design around me wanting to do my own garabage cleanup, when I delete the class instance it would delete it's instances but no go on that. So in the end I did as everyone else recommended, I imported it outside of the class and I'll just delete it when I'm done with it. Since it's a long lived process and I only need the former example at startup I did think about garbage collection of my own. – Jordon Bedwell Jul 30 '11 at 16:05
  • It's almost surely premature optimization to try to do your own garbage collection. – Wooble Jul 30 '11 at 22:14
  • @Jordon if you really want to import in a class definition, you can use `stdout = __import__('sys').stdout` then refer to it as `self.stdout`. – agf Jul 31 '11 at 01:55
  • @wobble it would be ignorant of me to know that I will not use that that piece again and not remove it from memory when I'm already in that module and can add it's removal from memory then and there. You call it premature optimisation but it's not, it's standard programming practice IMO. I'm not doing it to the entire application, just that module. The rest of the application will get tweaked after time goes on but when things are obvious (like not needing a module again in an application) I will account for that then and there and cleanup. Rather then procrastinating for "rapid development" – Jordon Bedwell Jul 31 '11 at 09:46
  • @agf awesome, that's something I didn't know thanks for teaching me that :D. I'm wondering if it wouldn't be useful to do that inside of the method itself since only that ____init____ method in server needs to populate from platform. – Jordon Bedwell Jul 31 '11 at 09:52

4 Answers4

13

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.

Alex Laskin
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2

I'm surprised no one thought of this:

class test:
    def __init__(self):
        from platform import system
        self.system = system()
    def test(self):
        return self.system()

d = test()
print d.system()
print d.test()
Logan
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1

Just import it up top, before you declare the class. That's where the interpreter expects to see it, and if you tried importing it within a class or other definition, you'd run the risk of running into major scope issues later on.

Alternatively, based on what it seems you're trying to do, you might try subclassing some of the things you're using (which, to dramatically oversimplify, is somewhat like extends in Java.)

ajcl
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  • "major scope issues" -- no, you won't! `import` works anywhere. Sometimes it's nicer not to put all the imports right at the top of the module. I think importing into a class scope is a good example of this. – Katriel Jul 30 '11 at 13:51
  • I've never found a case where it's a good idea in my own travels, but this Q&A here: http://stackoverflow.com/questions/128478/should-python-import-statements-always-be-at-the-top-of-a-module seems to suggest that there are some cases. Good learning material. :) – ajcl Jul 30 '11 at 15:26
  • What about when you want to import Gtk for dialogs in one class, but import windows dialogs in another class, using a common interface? Is it best to just set `self.Gtk = Gtk` or similar? – NoBugs Dec 09 '13 at 01:47
0

Looks like a case of premature optimization!? I think it's not possible to import something inside a class definition. Why do you want to do that? I certain cases it might make sense to import inside a method, but even those are rather rare. Why don't just put the import on top of your file where everybody can see it?

Achim
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