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You cannot materialize an object from a non-existing reference. Python names have to exist for them to work.

Instead, give the data3 keyword a default, then when the keyword is not specified, create the instance:

_sentinel = object()

class my_data():
   def __init__(self, data1, data2, data3=_sentinel):
      if data3 is _sentinel:
          data3 = my_data(None, None, None)

      self.data1 = data1
      self.data2 = data2
      self.data3 = data3

Now you can create instances of my_data() with a default new instance for data3:

>>> d2 = my_data('a', 'b')
>>> d1 = d2.data3
>>> d1
<__main__.my_data instance at 0x10e994cf8>
>>> vars(d2), vars(d1)
({'data1': 'a', 'data3': <__main__.my_data instance at 0x10e994cf8>, 'data2': 'b'}, {'data1': None, 'data3': None, 'data2': None})

Because we used a sentinel default value, you can still set data3 to None as well:

>>> d3 = my_data('a', 'b', None)
>>> d3.data3 is None
True

You cannot materialize an object from a non-existing reference. Python names have to exist for them to work.

Instead, give the data3 keyword a default, then when the keyword is not specified, create the instance:

_sentinel = object()

class my_data():
   def __init__(self, data1, data2, data3=_sentinel):
      if data3 is _sentinel:
          data3 = my_data(None, None, None)

      self.data1 = data1
      self.data2 = data2
      self.data3 = data3

Now you can create instances of my_data() with a default new instance for data3:

>>> d2 = my_data('a', 'b')
>>> d1 = d2.data3
>>> d1
<__main__.my_data instance at 0x10e994cf8>
>>> vars(d2), vars(d1)
({'data1': 'a', 'data3': <__main__.my_data instance at 0x10e994cf8>, 'data2': 'b'}, {'data1': None, 'data3': None, 'data2': None})

Because we used a sentinel default value, you can still set data3 to None as well:

>>> d3 = my_data('a', 'b', None)
>>> d3.data3 is None
True

A mutable list on the class can hold all instances; just set the list as a class attribute and append new instances to that:

_sentinel = object()

class my_data():
   instances = []

   def __init__(self, data1, data2, data3=_sentinel):
      if data3 is _sentinel:
          data3 = my_data(None, None, None)

      self.data1 = data1
      self.data2 = data2
      self.data3 = data3

      my_data.instances.append(self)

and you can list all instances with my_data.instances.

Do note that this creates circular references, keeping instances alive even when you no longer use them anywhere else. Use the weakref module if you want to prevent this.

You cannot materialize an object from a non-existing reference. Python names have to exist for them to work.

Instead, give the data3 keyword a default, then when the keyword is not specified, create the instance:

_sentinel = object()

class my_data():
   def __init__(self,data1,data2,data3=_sentinel):
      if data3 is _sentinel:
          data3 = my_data(None, None, None)

      self.data1 = data1
      self.data2 = data2
      self.data3 = data3

Now you can create instances of my_data() with a default new instance for data3:

>>> d2 = my_data('a', 'b')
>>> d1 = d2.data3

Because we used a sentinel default value, you can still set data3 to None as well:

>>> d3 = my_data('a', 'b', None)
>>> d3.data3 is None
True

You cannot materialize an object from a non-existing reference. Python names have to exist for them to work.

Instead, give the data3 keyword a default, then when the keyword is not specified, create the instance:

_sentinel = object()

class my_data():
   def __init__(self, data1, data2, data3=_sentinel):
      if data3 is _sentinel:
          data3 = my_data(None, None, None)

      self.data1 = data1
      self.data2 = data2
      self.data3 = data3

Now you can create instances of my_data() with a default new instance for data3:

>>> d2 = my_data('a', 'b')
>>> d1 = d2.data3
>>> d1
<__main__.my_data instance at 0x10e994cf8>
>>> vars(d2), vars(d1)
({'data1': 'a', 'data3': <__main__.my_data instance at 0x10e994cf8>, 'data2': 'b'}, {'data1': None, 'data3': None, 'data2': None})

Because we used a sentinel default value, you can still set data3 to None as well:

>>> d3 = my_data('a', 'b', None)
>>> d3.data3 is None
True

You cannot materialize an object from a non-existing reference. Python names have to exist for them to work.

Instead, give the data3 keyword a default, then when the keyword is not specified, create the instance:

_sentinel = object()

class my_data():
   def __init__(self,data1,data2,data3=_sentinel):
      if data3 is _sentinel():
          data3 = my_data(None, None, None)

      self.data1 = data1
      self.data2 = data2
      self.data3 = data3

Now you can create instances of my_data() with a default new instance for data3:

>>> d2 = my_data('a', 'b')
>>> d1 = d2.data3

Because we used a sentinel default value, you can still set data3 to None as well:

>>> d3 = my_data('a', 'b', None)
>>> d3.data3 is None
True

You cannot materialize an object from a non-existing reference. Python names have to exist for them to work.

Instead, give the data3 keyword a default, then when the keyword is not specified, create the instance:

_sentinel = object()

class my_data():
   def __init__(self,data1,data2,data3=_sentinel):
      if data3 is _sentinel:
          data3 = my_data(None, None, None)

      self.data1 = data1
      self.data2 = data2
      self.data3 = data3

Now you can create instances of my_data() with a default new instance for data3:

>>> d2 = my_data('a', 'b')
>>> d1 = d2.data3

Because we used a sentinel default value, you can still set data3 to None as well:

>>> d3 = my_data('a', 'b', None)
>>> d3.data3 is None
True