Python Create Constant Variable: A Beginner Guide

By | April 15, 2020
1 Comment

Python has not defined a constant variable data type, however, we can use other way to implement it. In this tutorial, we will discuss how to create a constant variable in python.

What is constant variable

If a variable is constant, which means the value of this variable can not be changed or edited.

How to create a constant variable in python?

We provides two methods to create python constant variable.

Method 1: Create a python class to make the property cannot be edited.

Here is an example.

class Constant(object):
    def __init__(self, val):
        super(Constant, self).__setattr__("value", val)
    def __setattr__(self, name, val):
        raise ValueError("Trying to change a constant value", self)

Then you can use this Constant class to create a python constant variable.

PI = Constant(3.14)
print(PI.value)

PI.value = 3.1415926
print(PI.value)

Run this code, you will find PI = 3.14. However, if you assign a new value to PI, it will report error.

create a python constant variable with python class

Method 2: Use python pconst library

Python pconst library can allow us to create a python constant variable.

To use it, you should install this library first.

pip install pconst

How to create a python constant variable using pconst?

Here is an example:

from pconst import const

const.PI = 3.14
print(const.PI)

const.PI = 3.1415926
print(const.PI)

We have created a python constant variable const.PI, we can not change its value.

Run this code, you will find this result.

create a python constant variable with python pconst

One thought on “Python Create Constant Variable: A Beginner Guide”

  1. Madilsupra

    Sorry Dear! First method is not perfect!

    PI = Constant(3.14)
    PI.__dict__[‘value’] = 3.1415926
    print(PI.value)
    3.1415926

    MY METHOD:

    from types import MappingProxyType
    from functools import wraps
    import sys

    class Const(object):
    class Temp:
    __attributes = MappingProxyType({})
    @staticmethod
    def __exception_handler__(func):
    @wraps(func)
    def handler(*args, **kwargs):
    try:
    func(*args, **kwargs)
    except AssertionError as error:
    errors = {
    ‘constant’ : ‘Constant variable cannot be assigned a value’,
    ‘magicMethod’ : ‘Magic method cannot be assign a method’
    }
    print(‘Error: %s’ % errors[str(error)])
    return handler
    def __init__(self, **kwargs):
    self.temp_ = Const.Temp()
    for var, value in kwargs.items():
    super(Const, self).__setattr__(var, value)
    def __concat__(self, **kwargs):
    return MappingProxyType(kwargs)
    @property
    def __dict__(self):
    return self.temp_._Temp__attributes
    @__exception_handler__
    def __setattr__(self, var, value):
    try:
    assert not(var.endswith(‘__’) and var.startswith(‘__’)), ValueError(‘magicMethod’)
    assert not self.__getattribute__(var), ValueError(‘constant’)
    except AttributeError:
    super(Const, self).__setattr__(var, value)
    self.temp_._Temp__attributes = self.__concat__(**dict(self.temp_._Temp__attributes), **{var : value})
    def __repr__(self, *args):
    return self.__getattribute__(args[0]) if args else super(Const, self).__repr__()

    ”’
    #Example

    const_var = Const(t1=’Test String 1′, t2 = 1)
    const_var.t3 = [1,2,3,4]

    print(const_var.t1)
    # Output: Test String 1
    const_var.t1 = ‘Test String 2′
    # Output: Error: Constant variable cannot be assigned a value
    print(const_var.t1)
    # Output: Test String 1

    const_var.t2 += 1
    # Output: Error: Constant variable cannot be assigned a value
    print(const_var.t2)
    # Output: 1

    print(const_var.t3)
    # Output: [1,2,3,4]
    const_var.t3.append(5)
    # Output: [1,2,3,4,5]
    # But Immutable objects cannot be changed!
    const_var.t3 = [1,2,3,4]
    # Output: Error:Constant variable cannot be assigned a value

    print(const_var.t3)
    # Output: [1,2,3,4,5]
    ”’

    ↓

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