Question

I want to pass a self-reference down to an instantiated class (the child should have access to the parent). It works if everything is in one file like this:

class ClassB:
  def __init__(self, name, parent):
    assert isinstance(parent, ClassA)
    self.name = name
    self.parent = parent
    print('my parent is', parent.name)

class ClassA:
  def __init__(self, name):
    self.name = name
    self.b = ClassB('child', self)

a = ClassA('parent')

output is my parent is parent as expected

The 2-file version is this:

class ClassB:
  def __init__(self, name, parent):
    from ClassA import ClassA
    assert isinstance(parent, ClassA)
    self.name = name
    self.parent = parent
    print('my parent is', parent.name)

and:

from ClassB import ClassB

class ClassA:
  def __init__(self, name):
    self.name = name
    self.b = ClassB('myName', self)

if __name__ == '__main__':
  a = ClassA('parent')

output is assert isinstance(parent, ClassA) AssertionError

Answer 1

That’s because the second passes a __main__.ClassA, whereas ClassB expects a ClassA.ClassA. Find a different way of doing this, such as putting ClassA in its own module.