When is a variable considered constant in terms of PEP8 naming styles?

Question

In keeping with PEP8 conventions, in a .py I can define constants as:

NAME = "Me"
AGE = "Old"
GENER = "Male"

If a .txt contained Me Old Male on a single line, and in another .py I performed:

FILE = "C:/path/to/file.txt"  # a declared constant, easy
with open(FILE, 'r') as f:
    content = f.read().rstrip('\n').split()
    data = ','.join(content)  # returns Me,Old,Male

Question(s):

Can content and data be considered constants?

To be constant, must a variable be declared as a constant at build?

Or is constant vice variable a function of the ability to be altered by user input in runtime?

Supporting Informaton:

content is what is in the file, but it is subject to .rstrip() and .split() but it as a whole is never changed later. data is made from content, which hasn't changed and wont, and is subject to .join(). Neither values change after they are initialized.

I would view this similar to:

>>> A = 2  # a declared constant
>>> B = 2  # another declared constant
>>> TOTAL = A + B  # 'caps' per PEP8 for constant naming
4

Assuming the program has terminated and TOTAL is never altered, I would consider this value a constant. Again under the presumption that any variable that is not alterable during runtime is to be considered a constant.

Feel free to alter my notions as required to align with standards!

Solution

If you are going to treat the value as a constant in the rest of your code, by all means, use CONSTANT_CASE for those globals. It's up to you, it's merely a documentation convention.

In other words, it's a convention that aims to make it easier for future readers of your code to understand that the value of such a global is set just once and is expected not to change over the lifetime of the program.

Note that I'd generally try to avoid loading file data on module import; it makes it harder to test and impacts performance. Load that data on first use instead (using a function).

Comments

[madeslurpy]

I like to be thorough and inline with practice standards. Thanks for the clean response! To follow up, why is having a file path declared in a program bad practice? It is used in a script that is designed to have user data auto-gathered each time the run the script so they don't have to supply a path to their info each time.

[Christian Dean]

I think your missing the premise of his question(as did I). He is asking what are the general guidelines for when you should make a variable "constant" in Python. Not how constant-ness could be implemented.

[madeslurpy]

What is meant by "Load that data on first use instead (using a function)?

[Martijn Pieters]

Test if that data is read when you need access to it, and if not, then read it and cache the data. Put that logic in a function so you can test it with a unittest.

[madeslurpy]

So declaring the path as a constant is fine on load, but I don't get why I cant then just sequentially read the data into a variable? What if that is the first thing I need to do? Run .py, imports modules, then has to read that file to get data before moving on. Are you suggesting to check if data is NULL and if so, then go read?

[madeslurpy]

Appreciate the response but not in the scope of what I was asking. I've read about this by Alex Martellis AS.

[Martijn Pieters]

@madeslurpy: yes, but now importing your module incurs extra I/O on import, increasing the risk that something goes wrong at that time and making it harder to write tests for your code (certainly for the code that loads the data).