making a class return None from __init__

[Martin v. Loewis]

Rajarshi Guha <rajarshi at presidency.com> writes:

> Is there any way to make the constructor return a None object?

No. When __init__ is invoked, the object it "returns" is already
determined - it is the self object. You cannot change the identity of
this object, or the fact that it is an instance object, you can only
modify it.

Therefore, the return value of __init__ is irrelevant. Python checks
that applications do not attempt to "return" something from __init__:
If there is a non-None return value, Python raises an error.

If you want a constructor to fail, instead of completing, consider to
raise an exception. If you want Graph(foo) to return None, make Graph
a function:

class _Graph:
	
  def __init__(self,v):
      pass

def Graph(v):
  if SOME_TEST:
    return None
  return _Graph(v)

If you absolutely must have a type whose constructor returns None,
implement a type that inherits from object, and implement an __new__.

HTH,
Martin