How to create a constant in Python

Learn how to create constant, read-only variables in Python.

Learn how to create constant, read-only variables in Python.

Many other languages like C++ or Java have keywords to declare constants:

const int maxValue = 99;

public static final int MAX_VALUE = 99;

But what about a constant specifier in Python?

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Constants in Python¶

In Python, we cannot declare a variable or value as constant in Python.

But we can indicate to programmers that a variable is a constant by using an all upper case variable name.

This is the common practice to create a constant in Python:

MAX_VALUE = 99

Even though we can change the value, it is then the programmer's responsibility to leave this variable unchanged.

typing.Final in Python¶

Since Python 3.8, there is a Final specifier in the typing module.

With type hints, the code still runs fine but type checkers like mypy can report errors when they are applied to the code:

from typing import Final

MAX_VALUE: Final[int] = 99

# This executes fine, but mypy will report an error if mypy is applied:
MAX_VALUE = 100

Use a namedtuple for constant values¶

If you really want to enforce read-only variables, you can use a namedtuple. namedtuples are easy to create, lightweight object types. Since they are a subclass of regular tuples, they are immutable:

from collections import namedtuple
Constants = namedtuple('Constants', ['pi', 'max_value'])

constants = Constants(3.14, 99)
print(constants.pi) # 3.14
print(constants.max_value) # 99

constants.max_value = 100
# Traceback (most recent call last):
#   File "<stdin>", line 5, in <module>
# AttributeError: can't set attribute