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()
4 Answers
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)
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.
1 Comment
xiaohan2012
Quite comprehensive and illustrative.
You're making an infinitely recursing call — MyClass is creating another MyClass during initialization, and thus it recurses infinitely.
You may want to do something like:
class MyClass:
def create_child(self):
self.child=MyClass()
mc=MyClass()
mc.create_child()
If you're feeling particularly naughty, you could try:
class MyClass(object):
@property
def child(self):
if self._child is None: self._child = MyClass()
return self._child
def __init__(self):
self._child=None
mc=MyClass()
4 Comments
xiaohan2012
MyClass is creating another MyClass. I do not quite understand it. Can you explain it more? The code you provide is useful, thanks.:-)xiaohan2012
Oh, it makes sense to me now. :-)
xiaohan2012
Asides, can you tell what the usage of @property is called? Thanks. It is new to me.
Don
It is a 'decorator': ibm.com/developerworks/linux/library/l-cpdecor/index.html
What you did there is actualy recursive, the new isntance of MyClass will create a new instance that will in turn create a new one, etc ... Soo I supose that is why your code fails, I can't tell for sure since you didn't post the error message.
Comments
I suggest to define two classes:
class MyClass(object):
def __init__(self):
self.child = MyChildClass()
...many other methods...
class MyChildClass(MyClass):
def __init__(self):
pass
I think that if two classes must behave in two different ways, they must be different (although one can subclass the other)
1 Comment
Mike Graham
This seems precarious and unnecessary to me.
self.child = Noneand only assign self.child to something when they should have children.objectrather than nothing (class MyClass(object):...) so that you're using new-style classes.