CODEWITHSUNDEEP
pythonpep8
Dimebag
Dimebag

Reputation: 843

Convention: Storing Python function output in constant?

I am aware that constants in Python are more of a convention yet I got into an argument with a fellow programmer over the following line:

FILE = open("file.txt").readlines()

With FILE later used multiple times, e.g. in a for line in FILE

We want to code as good as possible. I believe FILE should never be declared as a constant because it relies on the output of the open() and readlines() functions. Yet the counter-argument is that FILE never changes in our program, so behaves like a constant should. Which one is it?

Exceptions aside, shouldn't it be a variable, i.e. written as a variable according to the naming conventions, so file in all lowercase?

Upvotes: 0

Views: 175

Answers (1)

Barmar
Barmar

Reputation: 782427

A constant is not just something that doesn't change over the life of a program run, it's a name for something whose value is predefined, and generally independent of the dynamic state of the application or environment. Typical uses are for things like mathematical constants like PI, or application parameters like NUMBER_OF_PLAYERS.

In your example, the value of FILE is dependent on the contents of the file. It's not a predefined constant, since the file can change from one run of the program to the next.

Another way to look at it is that when you see a use of a constant in the program, you just have to find the initialization of it to know what its value is. But if you initialize the constant from a file, you can't tell what its value will be.

PEP8 doesn't define "constant", so you can do whatever you like, but I think most programmers would find your intended use of FILE to be confusing or non-idiomatic. Consider that in some programming languages, what you're attempting would not even be possible -- if they have explicit constant declarations, they require that the value be calculable at compile time.

Upvotes: 2

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