Python Constants

Question

I have done searches on Google and here at Stackoverflow but can not find what I am looking for.

I am relatively new to Python. I looking to create a "settings" module where various application specific constants will be stored.

Here is how I am wanting to setup my code

settings.py

CONSTANT = 'value'

script.py

import settings

def func():
    var = CONSTANT
    --- do some more coding ---
    return var

I am getting a Python error stating: "global name 'CONSTANT' is not defined.

I have noticed on Django's source code their settings.py file has constants named just like I do. I am confused on how they can be imported to a script and referenced through the application.

EDIT

Thank you for all your answers! I tried the following:

import settings

print settings.CONSTANT

I get the same error: ImportError: cannot import name CONSTANT

Answer 1

Leave your settings.py exactly as it is, then you can use it dust as Django does:

import settings

def func():
    var = settings.CONSTANT

Answer 2

The easiest way to do this is to just have settings be a module.

(settings.py)

CONSTANT1 = "value1"
CONSTANT2 = "value2"

(consumer.py)

import settings

print settings.CONSTANT1
print settings.CONSTANT2

When you import a python module, you have to prefix the the variables that you pull from it with the module name. If you know exactly what values you want to use from it in a given file and you are not worried about them changing during execution, then you can do

from settings import CONSTANT1, CONSTANT2

print CONSTANT1
print CONSTANT2

but I wouldn't get carried away with that last one. It makes it difficult for people reading your code to tell where values are coming from. and precludes those values being updated if another client module changes them. One final way to do it is

import settings as s

print s.CONSTANT1
print s.CONSTANT2

This saves you typing, will propagate updates and only requires readers to remember that anything after s is from the settings module.

Answer 3

When you import settings, a module object called settings is placed in the global namespace - and this object carries has that was in settings.py as attributes. I.e. outside of settings.py, you refer to CONSTANT as settings.CONSTANT.

Answer 4

...Or, if you really want all the constants from settings.py to be imported into the global namespace, you can run

from settings import *

...but otherwise using settings.CONSTANT, as everyone else has mentioned here, is quite right.

Answer 5

Try this:

In settings.py:

CONSTANT = 5

In your main file:

from settings import CONSTANT

class A:
    b = CONSTANT

    def printb(self):
         print self.b

I think your above error is coming from the settings file being imported too late. Make sure it's at the top of the file.

Answer 6

See the answer I posted to Can I prevent modifying an object in Python? which does what you want (as well as force the use of UPPERCASE identifiers). It might actually be a better answer for this question than it was for the the other.