Python - Best/Cleanest way to define constant list

Question

First time user on stack overflow and I'm excited to be here.

INTRO: I recently began the magical adventure into the world of Python programming - I love it. Now everything has gone smoothly in my awkward transition from C, but I'm having trouble creating something which would be synonymous to a HEADER file (.h).

PROBLEM: I have medium sized dictionaries and lists (about 1,000 elements), lengthy enums, and '#defines' (well not really), but I can't find a CLEAN way to organize them all. In C, I would throw them all in a header file and never think again about it, however, in Python that's not possible or so I think.

CURRENT DIRTY SOLUTION: I'm initializing all CONSTANT variables at the top of either the MODULE or FUNCTION (module if multiple functions need it).

CONCLUSION: I would be forever grateful if someone had come up with a way to CLEANLY organize constant variable's.

THANK YOU SO MUCH!

Answer 1

Usually I do this:

File: constants.py

CONSTANT1 = 'asd'
CONSTANT_FOO = 123
CONSTANT_BAR = [1, 2, 5]

File: your_script.py

from constants import CONSTANT1, CONSTANT_FOO
# or if you want *all* of them
# from constants import *

...

Now your constants are in one file and you can nicely import and use them.

Answer 2

Make a separate file constants.py, and put all globally-relevant constants in there. Then you can import constants to refer to them as constants.SPAM or do the (questionable) from constants import * to refer to them simply as SPAM or EGGS.

While we're here, note that Python doesn't support constant constants. The convention is just to name them in ALL_CAPS and promise not to mutate them.

Answer 3

If you want to mess with nested constants and don't like dicts, I came up with this fun solution:

# Recursively transform a dict to instances of the desired class

import json
from collections import namedtuple

class DictTransformer():
    @classmethod
    def constantize(self, d):
        return self.transform(d, klass=namedtuple, klassname='namedtuple')

    @classmethod
    def transform(self, d, klass, klassname):
        return self._from_json(self._to_json(d), klass=klass, klassname=klassname)

    @classmethod
    def _to_json(self, d, access_method='__dict__'):
        return json.dumps(d, default=lambda o: getattr(o, access_method, str(o)))

    @classmethod
    def _from_json(self, jsonstr, klass, klassname):
        return json.loads(jsonstr, object_hook=lambda d: klass(klassname, d.keys())(*d.values()))

Ex:

constants = {
  'A': {
    'B': {
      'C': 'D'
    }
  }
}
CONSTANTS = DictTransformer.transform(d, klass=namedtuple, klassname='namedtuple')
CONSTANTS.A.B.C == 'D'

Pros:

• handles nested dicts
• can potentially generate other types of dicts/classes
• namedtuples provide immutability for constants

Cons:

• may not respond to .keys and .values if those are not provided on your klass (though you can sometimes mimic with ._fields and list(A.B.C))

Thoughts?

h/t to @hlzr and you guys for the original class idea

Answer 4

Put your constants into their own module:

# constants.py

RED = 1
BLUE = 2
GREEN = 3

Then import that module and use the constants:

import constants

print "RED is", constants.RED

The constants can be any value you like, I've shown integers here, but lists and dicts would work just the same.