CODEWITHSUNDEEP
pythonpython-2.7
AlokThakur
AlokThakur

Reputation: 3741

Instance object getting deleted when tried to print it

I am trying to overload new and this is my attempt:- 

class String(object):
  def __new__(cls, *args, **kargs):
    print "in __new__"
    return super(String, cls).__new__(cls)
  def __init__(self):
    print "Initiating instance of String"
    raise Exception
  def __del__(self):
    print "Deleting instance of String"

I read at many places that actually __new__ create the instance and __init__ is just to initialize the instance. I am deliberately throwing exception in __init__ to let it fail. Here call to new returns the instance but init fails, so I am expecting an instance which won't have any attributes. But result surprised me like anything -

st = String()
in __new__
Initiating instance of String

Traceback (most recent call last):
  File "<pyshell#89>", line 1, in <module>
    st = String()
  File "<pyshell#88>", line 7, in __init__
    raise Exception

As expected it failed in __init__, Next I tried to print newly created instance 'st' and result surprised me, it deleted instance before printing. 

>>> print st
**Deleting instance of String**

Traceback (most recent call last):
  File "<pyshell#90>", line 1, in <module>
    print st
NameError: name 'st' is not defined

Please help me to understand this strange behavior.

Note - I know when we should overload __new__ and when we shouldn't.

Upvotes: 4

Views: 89

Answers (1)

Nikita
Nikita

Reputation: 6341

Exception was raised before Python managed to assign the object in memory to the the variable name. Therefore the reference counter of the object in memory is 0, so it got 'garbage collected', and you got name is not defined exception.

Update for the comment: Python runs GC when it tries to allocate memory for a new object, if the threshold is filled. In your case it's probably the threshold for generation 0 that was reached, because it's where that new 'failed' String object should have been. And the event itself that triggered GC was memory allocation to execute print.

Upvotes: 3

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