Can a python class return a new instance of its class?

Question

Is the following python code valid?

class Test:
  def __init__(self):
    self.number = 5

  def returnTest(self):
    return Test()

Asked By: cytinus

||

Source

Answer 1

Yes it is valid. The class is defined by the time you create an object and call the returnTest method.

In [2]: x = Test()

In [3]: y = x.returnTest()

In [4]: y
Out[4]: <__main__.Test instance at 0x1e36ef0>

In [5]:

However, in cases where a method acts like a factory, you might want to consider using the classmethod decorator. When inheritance and other annoyances come in the way, this can help.

Answered By: Noufal Ibrahim

Answer 2

Yes, it’s valid. returnTest will not be run until it is called. It does not create an infinite loop, since the method is not called on the newly created object.

Answered By: mayhewr

Answer 3

Yes. It’s a valid python code. Many programming languages allow to return an instance of a class being defined. Think to the singleton pattern.

Answered By: user278064

Answer 4

Yes, it works but it appears that returnTest() is always the same instance of Test.

class Test:
  def __init__(self):
    self.number = 5

  def returnTest(self):
    return Test()


t = Test()
print t
print t.returnTest()
print t.returnTest()


$ python te.py
<__main__.Test instance at 0xb72bd28c>
<__main__.Test instance at 0xb72bd40c>
<__main__.Test instance at 0xb72bd40c>

This is true for Python 2.7 and 3.2. @classmethod didn’t make a difference. Interestingly enough, pypy returns a different instance each time:

$ pypy te.py
<__main__.Test instance at 0xb6dcc1dc>
<__main__.Test instance at 0xb6dcc1f0>
<__main__.Test instance at 0xb6dcc204>

Answered By: Mark Roberts

Answer 5

When working with subclasses, I suggest using something more like this:

return type(self)(*args, **kwargs)

This ensures that the returned type is the same as the class returning it.

Here’s a full example:

class Test():
    def __init__(self):
        self.number = 5

    def returnTest(self):
        return type(self)()

class SubTest(Test):
    def __init__(self):
        super().__init__()
        self.number = 6

t1 = Test()
t2 = SubTest()
print( type(t1.returnTest()), type(t2.returnTest()) )

Outputs:

<class '__main__.Test'> <class '__main__.SubTest'>

Answered By: ExperimentFailed

Can a python class return a new instance of its class?

Question:

Answers: