Question

I am trying to create a new MyClass instance in MyClass’s definition.

Why does this code fail and how can achieve it?

class MyClass:
        def __init__(self):
                self.child=MyClass()

mc=MyClass()

Answer 1

Well, it fails because it has infinite recursion. Think about it, if every MyClass has a child which is a MyClass, it will go on for infinity!

You can resolve this a couple of ways. First, you can have a parameter to the constructor:

class MyClass:
   def __init__(self, create = True):
      if create:
         self.child = MyClass(False)

mc = MyClass()

Or, you can have another, external method:

class MyClass:
    def set_child(self,child = None):
        # I prefer to make child optional for ease of use.
        child = MyClass() if child is None else child
        self.child=child

mc=MyClass()
mc.set_child()

I personally prefer the first solution as it means that outside objects don’t need to know anything about the class. Of course, you could combine the two:

class MyClass:
    def __init__(self, create):
        if create:
            self.set_child(create=False)

    def set_child(self,child = None, create = True):
        child = MyClass(create) if child is None else child
        self.child=child

mc=MyClass()

This way mc has a child by default and you have the option of setting the child whenever you like.

Then there is also the “let’s create a certain number” approach:

class MyClass:
    def __init__(self, count = 10):
        count -= 1
        if count:
           # the first child gets the value 9.
           # the second gets 8.
           # when the count gets to 0, stop!
           self.child = MyClass(count)

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Aside: If you want to get an object’s class, you can use the value obj.__class__. That will output MyClass in all of the examples above.