Python Cookbook

Defining Constants

Credit: Alex Martelli

Problem

You need to define module-level variables that client code cannot accidentally rebind (i.e., named constants).

Solution

In Python 2.1 and later, you can install any instance as if it was a module. Just put the following in const.py:

class _const:

    class ConstError(TypeError): pass

    def _ _setattr_ _(self, name, value):
        if self._ _dict_ _.has_key(name):
            raise self.ConstError, "Can't rebind const(%s)"%name
        self._ _dict_ _[name] = value

    def _ _delattr_ _(self, name):
        if self._ _dict_ _.has_key(name):
            raise self.ConstError, "Can't unbind const(%s)"%name
        raise NameError, name

import sys
sys.modules[_ _name_ _] = _const(  )

Now any client code can import const, then bind an attribute on the const module just once, as follows:

const.magic = 23

Once the attribute is bound, the program cannot accidentally rebind or unbind it:

const.magic = 88      # would raise const.ConstError
del const.magic       # would raise const.ConstError

Discussion

In Python, variables can be rebound at will, and modules don’t let you define special methods such as an instance’s _ _setattr_ _ to stop rebinding. An easy solution (in Python 2.1 and later) is to set up an instance as if it was a module.

In Python 2.1 and later, no check is made to force entries in sys.modules to be actual module objects. You can install an instance object there and take advantage of attribute-access special methods (e.g., to prevent rebinding, to synthesize attributes on the fly in _ _getattr_ _, and so ...