Default Values for classes

You can't set a default value for classes in the __init__ function. For instance:

Wrong way

class A:
    def __init__(self,data=[]):
        self.data = data
a = A()
a.data.append(1)
b = A()
b.data == [1]

The above returns True.

The appropriate pattern is to set the default to None and adjust in the __init__ function.

Right Way

class A:
    def __init__(self,data=None):
        if data is None:
            data = []
        self.data = data
a = A()
a.data.append(1)
b = A()
b.data == [1]

This now returns False.

You could also use a ternary function.

Ternary Way

class A:
    def __init__(self,data=None):
       self.data = data if data is not None else []
a = A()
a.data.append(1)
b = A()
b.data == [1]