Python Enum or Constants Class

By David Updated 4 months ago (06 Jul 2008 at 07:04 PM) history recent activity

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In Brief: This class will create write-once, read-many-times 'Constants' in Python.... more

Language Python

# 's

 1class Constants:
 2
 3    # A constant variable
 4    foo = 1337
 5
 6    def __setattr__(self, attr, value):
 7        if hasattr(self, attr):
 8            raise ValueError, 'Attribute %s already has a value and so cannot be written to' % attr
 9
10        self.__dict__[attr] = value

This class will create write-once, read-many-times 'Constants' in Python.

It can be used like this:

>>> const = Constants()
>>> const.test1 = 42
>>> const.test1
42
>>> const.test1 = 43
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in __setattr__
ValueError: Attribute test1 already has a value and so cannot be written to
>>> const.test1
42

To have a completely read-only class, that behaves like an enum data type, remove the if statement and have __setattr__ always raise an error. Obviously in this case you would need to completely initialize all the variables inside the class definition.