Is it good practice to use `import main`?

Question

I’m working on a relatively large Python application, and there are several resources that I would like to keep as global variables accessible throughout several different modules. These values are things like the version number, version date, the global configuration, and some static paths to resources. I’ve also included a DEBUG flag that gets set by a command line option so that I can run my application in a debug mode without needing the full environment.

The values I’m importing I’ve been careful to ensure are ones that do not change over the course of running the program, and I’ve documented them as global constant variables that should not be touched. My code looks essentially like

# Main.py
import wx
from gui import Gui

DEBUG = False
GLOBAL_CONFIG = None
VERSION = '1.0'
ICON_PATH = 'some/path/to/the/app.ico'

def main():
    global DEBUG, GLOBAL_CONFIG

    # Simplified
    import sys
    DEBUG = '--debug' in sys.argv

    GLOBAL_CONFIG = load_global_config()
    # Other set-up for the application, e.g. setting up logging, configs, etc

    app = wx.App()
    gui = Gui()
    app.MainLoop()

if __name__ == '__main__':
    main()

# gui.py
import wx
from __main__ import DEBUG, GLOBAL_CONFIG, ICON_PATH

import controller


class Gui(wx.Frame):
    def __init__(self):
        wx.Frame.__init__(self, None)

        icon = wx.Icon(ICON_PATH, wx.BITMAP_TYPE_ICO)
        self.SetIcon(icon)

        # Always make a copy so we don't accidentally modify it
        conf = GLOBAL_CONFIG.copy()
        self.controller = controller.Controller(conf)

        # More setup, building the layout, etc

# controller.py
from __main__ import DEBUG

import logging
log = logging.getLogger('controller')

class Controller(object):
    def __init__(self, conf):
        if DEBUG:
            log.info("Initializing controller in DEBUG mode")
        self.conf = conf
        # Other setup ...

This is obviously far stripped down from what my application actually is, and neglects error handling, documentation, and basically all implementation details.

Now, I’ve seen it said that this is a bad idea, but without explanation for why. Since most results when googling for variants of “python import __main__” are questions about what if __name__ == '__main__' is, it’s hard to find some solid information on this topic. So far I’ve had no problems with it, and it’s actually been really convenient.

So is this considered good Python practice, or is there a reason I should avoid this design?

Asked By: bheklilr

||

Source

Answer 1

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.

Answered By: Henry Keiter

Answer 2

Doing so requires violating PEP8, which specifies

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

In order for gui.py to successfully import __main__.DEBUG, you would have to set the value of DEBUG before import gui.

Answered By: chepner

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