Python Module Initialization

Question

Is it bad practice to initialize the objects in the module, in the module code?

in Module.py:

class _Foo(object):
    def __init__(self):
        self.x = 'Foo'

Foo = _Foo()

Than in user code, you could:

>>> from Module import Foo
>>> print Foo.x
'Foo'
>>>

...without having to initialize the Foo class in the user code. Of course, only useful if you don't need arguments to initialize the object.

Is there a reason not to do this?

Fielding

you can use @staticmethod and class variables if you only need one instance – Flu

@Flu You can't use staticmethod and class variables, you can use classmethod and class variables. That's a poor way to create a singleton though. – Townspeople

@Townspeople I know it looks like the java's (bad) way of life... but it's not so bad. – Flu

You still mess up subclassing if you refer to the class by name from staticmethods instead of using the automatically passed reference to the class from classmethods. – Townspeople

Answer 1

Typically, you only want to run the minimum necessary to have your module usable. This will have an overall effect on performance (loading time), and can also make debugging easier.
Also, usually more than one instance will be created from any given class.

Having said that, if you have good reasons (such as only wanting one instance of a class), then certainly initialize it at load time.

Chaplet

Answer 2

I do this sometimes, when it's really convenient, but I tend to do foo = Foo(). I really dislike the idea of making the class appear private, and making the instance available as Foo. As a developer using your code I'd find that pretty disconcerting.

Genna

Even if for some reason you really do want the class to be private, you still should just lowercase for the instance name. – Townspeople

Yes, because following PEP8 makes other Python developer's brains hurt less. – Genna