Python "constant" redeclare in every periodic run

Question

Is there any problem, if in a periodic script I have a "constant" variable and I redeclare it in every run?

#!/usr/bin/env python
PATH_PATTERN = '/home/%s/config.xml'
PATH = None

def periodic_execution(function):
    PATH = PATH_PATTERN % get_user()

    interval_in_sec = 1000
    threading.Timer(interval_in_sec,periodic_execution,[function]).start()
    function()
    # in function i use the PATH variable

def main():

    periodic_execution(tasks)

if __name__ == '__main__':
    main()

I know that constant not will be constant...

Hi @tmsblgh, this kind of question probably is off-topic here, since your code works and the question itself is primarily opinion-based. Try http://codereview.stackexchange.com/ — Paulo Scardine, Sep 20 '16 at 07:31
@PauloScardine: code does not work because he forgot to declare the variable `global`. And I do not think that codereview fellows like primarily opinion based questions... — Serge Ballesta, Sep 20 '16 at 07:39
Global mutable variables are bad juju, why don't you just pass `path` as an argument to `function`? — Paulo Scardine, Sep 20 '16 at 07:50

score 3 · Accepted Answer · answered Sep 20 '16 at 07:38

Beware! In shown code, the PATH variable is a global one, but in periodic_execution you assign to PATH before using it and without declaring it as global => Python actually creates a local PATH variable in the function and leaves the global untouched which is certainly not what you expect.

You should write:

def periodic_execution(function):
    global PATH
    PATH = PATH_PATTERN % get_user()

to change a global variable.

But this is by no way a constant. AFAIK you cannot declare true constants in Python, even if you can build read only properties.

score 0 · Answer 2 · edited May 23 '17 at 10:27

0

Did you also look at the keyword global in Python? See this link. It is necessary since you change the variable in a function.

edited May 23 '17 at 10:27

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answered Sep 20 '16 at 07:33

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This is terrible as an answer - it is OK if you post it as a comment. – Paulo Scardine Sep 20 '16 at 07:39

Python "constant" redeclare in every periodic run

2 Answers2