Is it good practice to use `import __main__`?

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I think there are two main (ha ha) reasons one might prescribe an avoidance of this pattern.

  • It obfuscates the origin of the variables you’re importing.
  • It breaks (or at least it’s tough to maintain) if your program has multiple entry points. Imagine if someone, very possibly you, wanted to extract some subset of your functionality into a standalone library–they’d have to delete or redefine every one of those orphaned references to make the thing usable outside of your application.

If you have total control over the application and there will never be another entry point or another use for your features, and you’re sure you don’t mind the ambiguity, I don’t think there’s any objective reason why the from __main__ import foo pattern is bad. I don’t like it personally, but again, it’s basically for the two reasons above.

I think a more robust/developer-friendly solution may be something like this, creating a special module specifically for holding these super-global variables. You can then import the module and refer to module.VAR anytime you need the setting. Essentially, just creating a special module namespace in which to store super-global runtime configuration. 

# conf.py (for example)
# This module holds all the "super-global" stuff.
def init(args):
    global DEBUG
    DEBUG = '--debug' in args
    # set up other global vars here.

You would then use it more like this:

# main.py
import conf
import app

if __name__ == '__main__':
    import sys
    conf.init(sys.argv[1:])

    app.run()

# app.py
import conf

def run():
    if conf.DEBUG:
        print('debug is on')

Note the use of conf.DEBUG rather than from conf import DEBUG. This construction means that you can alter the variable during the life of the program, and have that change reflected elsewhere (assuming a single thread/process, obviously).

Another upside is that this is a fairly common pattern, so other developers will readily recognize it. It’s easily comparable to the settings.py file used by various popular apps (e.g. django), though I avoided that particular name because settings.py is conventionally a bunch of static objects, not a namespace for runtime parameters. Other good names for the configuration namespace module described above might be runtime or params, for example.

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