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

In python, a constant is simply a variable with a name in all capitals, with words separated by the underscore character,

e.g

DAYS_IN_WEEK = 7

The value is mutable, as in you can change it. But given the rules for the name tell you is a constant, why would you? I mean, it is your program after all!

This is the approach taken throughout python. There is no private keyword for the same reason. Prefix the name with an underscore and you know it is intended to be private. Code can break the rule....just as a programmer could remove the private keyword anyway.

Python could have added a const keyword... but a programmer could remove keyword and then change the constant if they want to, but why do that? If you want to break the rule, you could change the rule anyway. But why bother to break the rule if the name makes the intention clear?

Maybe there is some unit test where it makes sense to apply a change to value? To see what happens for an 8 day week even though in the real world the number of days in the week cannot be changed. If the language stopped you making an exception if there is just this one case you need to break the rule...you would then have to stop declaring it as a constant, even though it still is a constant in the application, and there is just this one test case that sees what happens if it is changed.

The all upper case name tells you it is intended to be a constant. That is what is important. Not a language forcing constraints on code you have the power to change anyway.

That is the philosophy of python.