using subclass attribute in the main class in python

Question

I'm trying to set the self.role under credential class to use the self.role under AXL class. the idea is to have different classes based on the role the api needs to be. if axl class is only reading data then the role would be r.

PATH = 'home_drive_'
PLATFORM = 'Linux_'
ITEM = '_PC'

class Credential:

    def __init__(self, path, platform):
        self.role = 'rx'
        self.username_file = path + platform + ('The role should be the same as AXL role: ' + self.role)

class AXL(Credential):

    def __init__(self, path, platform, item):
        super().__init__(path, platform)
        self.role = 'r'
        self.item = item

    def final(self):
        return self.username_file + self.item

reg1 = AXL(PATH, PLATFORM, ITEM)

print('AXL role:', reg1.role)
print(reg1.username_file)
print(reg1.final())

the result would be

AXL role: r 
home_drive_Linux_The role should be the same as AXL role: rx 
home_drive_Linux_The role should be the same as AXL role: rx_PC

Instead of rx, I need to see r

Here is a link to the sandbox

SuperShoot · Answer 1 · 2019-03-22T13:22:21.120

Answer by @chepner is definitely correct solution. I’ll leave this here as a homage to over complicating a simple problem.

You could make username_file a @property on Credential so it’s evaluated on access not on instance creation.

class Credential:
    def __init__(self, path, platform):
        self.role = 'rx'
        self.path = path
        self.platform = platform

    @property
    def username_file(self):
        return self.path + self.platform + self.role

If it’s an instance of AXL roll will be ‘r’ and ‘rx’ if instance of Credential.

You can also cache the result of the first property access if you want to streamline:

class Credential:
    def __init__(self, path, platform):
        self.role = 'rx'
        self.path = path
        self.platform = platform
        self._username_file = None

    @property
    def username_file(self):
        if not self._usernme_file:
            self._username_file = self.path + self.platform + self.role
        return self._username_file

score 2 · Accepted Answer · answered Mar 22 '19 at 12:48

role should be an parameter of Credential.__init__, not hard-coded, although it could have a default value for the base class. Subclasses would pass the required role directly to super().__init__. (If there is something that must have a role of 'rx', that should be a subclass of Credential as well, not Credential itself.)

class Credential:

    def __init__(self, path, platform, role='rx'):
        self.role = role
        self.username_file = path + platform + ('The role should be the same as AXL role: ' + role)


# class SomethingNeedingRoleRX(Credential):
#      def __init__(self, path, platform):
#          super().__init__(path, platform, 'rx')


class AXL(Credential):

    def __init__(self, path, platform, item):
        super().__init__(path, platform, 'r')
        self.item = item

    def final(self):
        return self.username_file + self.item

To play nicer with super, you might consider using keyword-only arguments for __init__:

class Credential:
    def __init__(self, *, path, platform, role='rx', **kwargs):
        super().__init__(**kwargs)
        self.role = role
        self.username_file = path + platform + role


class AXL(Credential):
    def __init__(self, *, item, **kwargs):
        super().__init__(role='r', **kwargs)
        self.item = item

    def final(self):
        return self.username_file + self.item


reg1 = AXL(path=PATH, platform=PLATFORM, item=ITEM)

That defines `path` et al. as keyword-only arguments. It's no longer legal to say `AXL(PATH, PLATFORM)`, since `AXL.__init__` takes no positional arguments (aside from `self`). Using keyword arguments means you don't have to worry about positional arguments conflicting with each other in later descendants of `Credential`. — chepner, Mar 22 '19 at 15:51

nickthefreak · Answer 3 · 2019-03-22T13:19:26.153

Ok, the problem here is basically the data type of username_file; its a string created in the constructor and does not update when it's components do. There are a few ways to go around this, using properties is a pretty good and clean way to go about it:

PATH = 'home_drive_'
PLATFORM = 'Linux_'
ITEM = '_PC'


class Credential:
    def __init__(self, path, platform):
        self.path = path
        self.platform = platform
        self.role = 'rx'
        self.username_file = self.path + self.platform + ('The role should be the same as AXL role: ' + self.role)

    @property
    def role(self):
        return self._role

    @role.setter
    def role(self, new_role):
        self._role = new_role
        self.username_file = self.path + self.platform + ('The role should be the same as AXL role: ' + self.role)


class AXL(Credential):
    def __init__(self, path, platform, item):
        super().__init__(path, platform)
        self.role = 'r'
        self.item = item

    def final(self):
        return self.username_file + self.item


reg1 = AXL(PATH, PLATFORM, ITEM)

print('AXL role:', reg1.role)
print(reg1.username_file)
print(reg1.final())

EDIT:

Just a short explanation, you can turn to a property pretty much any variable, the reason for selecting role is resource management. Essentially, it takes fewer resources to update username_file in the setter every time role changes (once in the program) than add strings (slow operation) every time you call the getter. Of course, considering the size of the program resource management should not be a deal breaker, but mentioning to explain the logic of the answer.

with properties you can change anytime, so either works really! you can change role anywhere in the code, not just the constructor. this will also update the username_file attribute to the appropriate role — nickthefreak, Mar 28 '19 at 14:07
@Louis-PhilippeDescamps see example [here](https://repl.it/@nickthefreak/Inheritance), you can set role anywhere to any string — nickthefreak, Mar 28 '19 at 14:54

using subclass attribute in the main class in python

3 Answers3