How do I declare python constants in Python?
In Java, we declare constants like this:
public static final String CONST_NAME = "Name";
How can I do something similar in Python to create a constant?
Ah, this is a classic question when transitioning to Python! Python doesn’t enforce immutability for constants like Java does. Instead, it’s all about convention. By convention, developers use uppercase variable names to signal that a value should be treated as constant.
While this doesn’t prevent someone from reassigning the variable (Python doesn’t have built-in constant types), it serves as a clear visual cue to others working on the code.
Example:
MY_CONSTANT = 10
Simple and effective!
Great point, Ian! If you’re looking for something more robust than a naming convention to mimic true immutability, you might consider using collections.namedtuple. Namedtuples are immutable by nature, so once you set their values, they can’t be altered.
Here’s how you can use it to define python constants:
from collections import namedtuple
Constants = namedtuple('Constants', ['MY_CONSTANT'])
constants = Constants(MY_CONSTANT=10)
print(constants.MY_CONSTANT)
This approach adds an extra layer of safety by making the value immutable.
These are both solid approaches, but if you’re working with a set of constants and want to make the whole structure immutable, Python has another tool: types.MappingProxyType. It’s like a read-only dictionary. You can use it to store multiple python constants in a single, immutable dictionary-like structure.
Here’s an example:
from types import MappingProxyType
constants = MappingProxyType({'MY_CONSTANT': 10})
print(constants['MY_CONSTANT'])
# Attempting to modify this will raise a TypeError:
# constants['MY_CONSTANT'] = 20
This is great when you have a group of constants and want a completely read-only view.