[Python] Modifying class variable before __new__

I'd like to modify the class variable bar of a class B so that the creation of the class A object will be performed with bar2 instead of bar1 (where bar1 is the default). This makes it easier for the user to select if they want to create class B with parameter bar1 or bar2 when using the class. I'm not opposed to other ways (eg. method) to modify it. I just want the user to be able to do it before the class is instantiated.

Class A:

class A(object):
def __new__(cls, foo):
obj = super(A, cls).__new__(cls)
return obj


I don't have access to Class A. Class A is coming from a third party package that doesn't have an __init__ but it takes foo as an argument of the __new__ function.

Class B:

class B(A):

bar = "bar1"

def __new__(cls, foo=bar):
print("actual",foo)
rm = super().__new__(cls,foo=foo)
return rm


Class B is a derivative class that inherits from class A. Since I want to modify the foo parameter in class A, I need to overwrite the __new__ class.

One other piece of the puzzle is that on top of B there is a long stack of child classes that inherit from B and I don't want to modify all of their init to pass along the foo variable until class A.

I want to run the following code (because it's easy from the user perspective to choose the value of 'bar' and because the modifications to the code are very little.

if __name__ == '__main__':

B.bar = 'bar2'
b = B()
print(b.bar)


I get:

actual bar1
bar2


whereas I'm expecting:

actual bar2
bar2


I modified my example to make it work but it's not pretty and I'm seeking for a more pythonic solution. This uses bar class variable as an array and uses class methods to modify it

class A(object):

def __new__(cls, foo):
obj = super(A, cls).__new__(cls)
return obj

class B(A):

_bar_array = ["bar1"]

@classmethod
def set_bar(cls, value):
cls._bar_array[0] = value

@classmethod
def get_bar(cls):
return cls._bar_array[0]

def __new__(cls, foo=_bar_array):
print("actual",foo[0])
rm = super().__new__(cls,foo=foo[0])
return rm

if __name__ == '__main__':

B.set_bar("bar2")
b = B()
print(b.get_bar())
print(B.get_bar())

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