Dynamically defining instance fields in Python classes

Question

I am new to Python having come from mainly Java programming.

I am currently pondering over how classes in Python are instantiated.

I understand that __ini

Answer 1

I understand that __init__(): is like the constructor in Java.

To be more precise, in Python __new__ is the constructor method, __init__ is the initializer. When you do SomeClass('foo', bar='baz'), the type.__call__ method basically does:

def __call__(cls, *args, **kwargs):
    instance = cls.__new__(*args, **kwargs)
    instance.__init__(*args, **kwargs)
    return instance

Generally, most classes will define an __init__ if necessary, while __new__ is more commonly used for immutable objects.

However, sometimes python classes do not have an init() method which in this case I assume there is a default constructor just like in Java?

I'm not sure about old-style classes, but this is the case for new-style ones:

>>>> object.__init__

If no explicit __init__ is defined, the default will be called.

So to be clear, my question is in Python can we dynamically define new fields to a class during runtime like in this example

Yes.

>>> class A(object):
...     def __init__(self):
...         self.one_attribute = 'one'
...     def add_attr(self):
...         self.new_attribute = 'new'
...

>>> a = A()
>>> a.one_attribute
'one'
>>> a.new_attribute
Traceback (most recent call last):
  File "", line 1, in 
AttributeError: 'A' object has no attribute 'new_attribute'
>>> a.add_attr()
>>> a.new_attribute
'new'

Attributes can be added to an instance at any time:

>>> a.third_attribute = 'three'
>>> a.third_attribute
'three'

However, it's possible to restrict the instance attributes that can be added through the class attribute __slots__:

>>> class B(object):
...     __slots__ = ['only_one_attribute']
...     def __init__(self):
...         self.only_one_attribute = 'one'
...     def add_attr(self):
...         self.another_attribute = 'two'
...

>>> b = B()
>>> b.add_attr()
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 6, in add_attr
AttributeError: 'B' object has no attribute 'another_attribute'

(It's probably important to note that __slots__ is primarily intended as a memory optimization - by not requiring an object have a dictionary for storing attributes - rather than as a form of run-time modification prevention.)