What's the best way to initialise and use constants across Python classes?

Question

Here's how I am declaring constants and using them across different Python classes:

# project/constants.py
GOOD = 1
BAD = 2
AWFUL = 3

# project/question.py
from constants import AWFUL, BAD, GOOD

class Question:
    def __init__(self):
        ...

Is the above a good way to store and use contant values? I realise that after a while, the constants file can get pretty big and I could explicitly be importing 10+ of those constants in any given file.

Well first there is no constant variable in python , and second your global variable are they used in more than one module or just in one ? — mouad, Jun 14 '11 at 15:15

Daren Thomas · Accepted Answer · 2011-06-15T07:30:21.253

85

why not just use

import constants

def use_my_constants():
    print constants.GOOD, constants.BAD, constants.AWFUL

From the python zen:

Namespaces are good. Lets do more of those!

EDIT: Except, when you do quote, you should include a reference and check it, because as others have pointed out, it should read:

Namespaces are one honking great idea -- let's do more of those!

This time, I actually copied it from the source: PEP 20 -- The Zen of Python

edited Jun 15 '11 at 07:30

answered Jun 14 '11 at 15:16

Daren Thomas

67,947
40
154
200

1

... Which is also an infinitely more approrpiate way to put it. – Jun 14 '11 at 15:55
@Daniel Roseman: Ouch. I stand corrected. I'll edit my post accordingly! – Daren Thomas Jun 15 '11 at 07:28
1

Another thing you can do to reduce typing is to use "import constants as CN" and "print(CN.GOOD)" etc. – Jon Coombs Jan 30 '14 at 17:47
2

For a moment I thought, constants is an actual python library, how nice it would have been, LoL. I didn't read the question. :-) – Whirl Mind Aug 18 '19 at 11:49
what is I have Constance with critical static critical data and I want to read it demo ini file and assign it to variable before using it, I tried writing methods but not able to call in in definition also tried with __init__ method but failed. I'm new to python anyway. – Mayur Sep 08 '20 at 20:23

score 16 · Answer 2 · answered Jun 14 '11 at 15:19

16

You also have the option, if the constants are tied to a particular class and used privately within that class of making them specific to that class:

class Foo(object):
   GOOD = 0
   BAD = 1
   WTF = -1

   def __init__(self...

and away you go.

answered Jun 14 '11 at 15:19

wheaties

35,646
15
94
131

score 4 · Answer 3 · answered Jun 14 '11 at 15:17

You have a few options:

Do it the way you're doing right now with a file of constants
Use a flat file and parse it once, pass a dictionary / class around
Query off to a database

From an overhead point of view, 1 and 2 are about the same. As for your question about importing specific constants, it's much easier to use one of the following conventions:

import constants; some_func(constants.AWFUL)
from constants import *; some_func(AWFUL)

score 3 · Answer 4 · edited May 23 '17 at 12:17

3

Try to look at the following links(one link contains a receipt from active state of how to implement a real constants):

Importing a long list of constants to a Python file

Create constants using a "settings" module?

Can I prevent modifying an object in Python?

edited May 23 '17 at 12:17

Community

1
1

answered Jun 14 '11 at 15:20

Artsiom Rudzenka

27,895
4
34
52

score 0 · Answer 5 · answered Dec 26 '22 at 16:30

0

I use a function as immutable constant, like this:

def test_immutable_constant():
    def GOOD():
        return 1

    good_value = GOOD()
    good_value = 2

    assert GOOD() == 1
    assert good_value == 2

answered Dec 26 '22 at 16:30

Wendel

2,809
29
28

What's the best way to initialise and use constants across Python classes?

5 Answers5

Linked