How do I create a constant in Python?

Question

Is there a way to declare a constant in Python? In Java we can create constant values in this manner:

public static          


        

Answer 1

We can create a descriptor object.

class Constant:
  def __init__(self,value=None):
    self.value = value
  def __get__(self,instance,owner):
    return self.value
  def __set__(self,instance,value):
    raise ValueError("You can't change a constant")

1) If we wanted to work with constants at the instance level then:

class A:
  NULL = Constant()
  NUM = Constant(0xFF)

class B:
  NAME = Constant('bar')
  LISTA = Constant([0,1,'INFINITY'])

>>> obj=A()
>>> print(obj.NUM)  #=> 255
>>> obj.NUM =100

Traceback (most recent call last):
File "", line 1, in 
ValueError: You can't change a constant

2) if we wanted to create constants only at the class level, we could use a metaclass that serves as a container for our constants (our descriptor objects); all the classes that descend will inherit our constants (our descriptor objects) without any risk that can be modified.

# metaclass of my class Foo
class FooMeta(type): pass

# class Foo
class Foo(metaclass=FooMeta): pass

# I create constants in my metaclass
FooMeta.NUM = Constant(0xff)
FooMeta.NAME = Constant('FOO')

>>> Foo.NUM   #=> 255
>>> Foo.NAME  #=> 'FOO'
>>> Foo.NUM = 0 #=> ValueError: You can't change a constant

If I create a subclass of Foo, this class will inherit the constant without the possibility of modifying them

class Bar(Foo): pass

>>> Bar.NUM  #=> 255
>>> Bar.NUM = 0  #=> ValueError: You can't change a constant