Accessing a global module in python

Question

I apologize if this is a basic question but I can't seem to find the answer here or on Google. Basically I'm trying to create a single config module that would be available to all other modules imported in a python application. Of course it works if I have import config in each file but I would like to make my config dynamic based on the environment the application is running in and I'd prefer not to have to copy the logic into every file.

Here's an example:

app.py:

import config
import submodule

# do other stuff

submodule.py:

print config.some_config_variable

But python of course complains that config isn't defined.

I did find some stuff about global variables but that didn't seem to work either. Here's what I tried:

Edit I changed this to show that I'd like the actual config being imported to be dynamic. However I do currently have a static config modle for my tests just to figure out how to import globally and then worry about that logic

app.py

# logic here that defines some_dynamic_config
global config
config = __import__(some_dynamic_config)
import submodule

submodule.py

print config.some_config_variable

But config still isn't defined.

I'm aware that I could create a single config.py and place logic to set the variables but I dislike that. I prefer the config file to just configuration and not contain a bunch of logic.

@cdhowie I can and it works just that I'd like to be able to dynamically decide what "config" will be. — Cfreak
– Cfreak, Commented Jul 30, 2012 at 21:50
The proper pattern, I think, is to pass a config object when constructing objects in the submodule, or calling functions in that module. (Dependency injection, and whatnot.) Having the submodule try to find the config is a lot more complex and fragile then just passing in a config object to whatever constructor or function you need to use. — cdhowie
– cdhowie, Commented Jul 30, 2012 at 21:51

Chris Lawlor · Accepted Answer · 2012-07-30 22:20:29Z

3

You've got to put your logic somewhere. Your config.py could be a module that determines which config files to load, something like this:

#config.py

import sys

from common_config import *

if sys.platform == 'darwin':
    from mac_config import *
elif sys.platform == 'win32':
    from win32_config import *
else:
    from linux_config import *

With this approach, you can put common settings in common_settings.py, and platform-specific settings in their respective files. Since the platform-specific settings are imported after common_settings, you can also override anything in common_settings by defining it again in the platform-specific files.

This is a common pattern used in Django settings files, and works quite well there.

You could also wrap each import call with try... except ImportError: blocks if need be.

answered Jul 30, 2012 at 22:20

Chris Lawlor

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Max · Accepted Answer · 2012-07-30 21:47:34Z

1

Modules are shared, so each module can import config without issue.

config.py itself can have logic to set it's global variables howver you like. As an example:

config.py:

import sys
if sys.platform == "win32":
    temp = "c:\\temp"
else:
    temp = "/tmp"

Now import config in any module to use it:

import config
print "Using tmp dir: %s" % config.temp

If you have a module that you know will be initialized before anything else, you can create an empty config.py, and then set it externally:

import config
config.temp = "c:\\temp"

But you'll need to run this code before anything else that uses it. The empty module can be used as a singleton.

answered Jul 30, 2012 at 21:47

Max

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3 Comments

Max Over a year ago

However, program code put at the top level of config.py will not accessible during run time, only it's globals (functions are global variables internally), so there's no reason not to put your configuration logic there.

Cfreak Over a year ago

I agree I can go this route but I'm specifically trying to avoid having logic in the config file. In your example I'd rather have win32-config.py for windows specific stuff.

Max Over a year ago

As I mentioned in the second half of the post, you can use an empty module and assign whatever you like into it. You can even have config.py do from win32-config import * to copy the values from win32-config to config. (Note: import * is a bit messy and shouldn't be used in general, but this may not be a bad use for it).

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