Question

Why dictionary content shared between class instances, but strings not shared?

Test code:

class Config(object):
    file_names  = dict()
    file_name_1 = None
    file_name_2 = None

    def __init__(self):
        if not self.file_names:
            print "DEBUG: Config(): set initial values"
            self.file_names["0"] = None
            self.file_names["1"] = None

    def set_file_names(self, file_names):
        self.file_name_1     = file_names[0]
        self.file_names["0"] = file_names[0]
        self.file_name_2     = file_names[1]
        self.file_names["1"] = file_names[1]

class CommandLineParser(object):
    def __init__(self):
        self.cfg = Config()
        self.start()

    def start(self):
        self.cfg.set_file_names(("file_1.log", "file_2.log"))
        print self.__class__.__name__, "FileName1 (str) :", self.cfg.file_name_1
        print self.__class__.__name__, "FileName1 (dict):", self.cfg.file_names["0"]

class LogProcessor(object):
    def __init__(self):
        self.cfg   = Config()
        self.cmdln = CommandLineParser()
        print  self.__class__.__name__, "FileName1 (str) :", self.cfg.file_name_1
        print  self.__class__.__name__, "FileName1 (dict):", self.cfg.file_names["0"]

if __name__ == '__main__':
    processor = LogProcessor()

Output:

DEBUG: Config(): set initial values
CommandLineParser FileName1 (str) : file_1.log
CommandLineParser FileName1 (dict): file_1.log
LogProcessor FileName1 (str) : None
LogProcessor FileName1 (dict): file_1.log

Is it a right way to share options between instances in dictionary attribute?

Answer 1

files_names is a class attribute of your class Config so it is shared by all instances of this class. file_name_1 and file_name_2 are also initially class attributes (set to None) but you rebind them in instances inside the set_file_names method so they become instance attributes, private to each instance, at that time.

Your idea to use a class attribute which is a mutable container (dict) to contain configuration common to all instances and which can be updated dynamically is good.

If you still want to use the individual string attributes file_name_1 and file_name_2 but you want them to be shared between all instances, then you can do one of the following things:

• Rebind the class attribute instead of creating an instance attribute in set_file_names by using Config.file_name_1 = whatever instead of self.file_name_1 = whatever
• Make the set_file_names method a class method (use the @classmethod decorator) so that it will operate on the class, not on the instance.
• Do not instantiate the Config class at all, single it is meant to be a singleton anyway. Use Config instead of Config().