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 __init__(): is like the constructor in Java. 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?

Another thing that makes the transition from Java to python slightly difficult is that in Java you have to define all the instance fields of the class with the type and sometimes an initial value. In python all of this just seems to disappear and developers can just define new fields on the fly.

For example I have come across a program like so:

class A(Command.UICommand):
    FIELDS = [
        Field( 'runTimeStepSummary', BOOL_TYPE)
        ]

    def __init__(self, runTimeStepSummary=False):
        self.runTimeStepSummary = runTimeStepSummary

    """Other methods"""

    def execute(self, cont, result):
        self.timeStepSummaries = {}
        """ other code"""

The thing that confuses (and slightly irritates me) is that this A class does not have a field called timeStepSummaries yet how can a developer in the middle of a method just define a new field? or is my understanding incorrect?

So to be clear, my question is in Python can we dynamically define new fields to a class during runtime like in this example or is this timeStepSummaries variable an instance of a java like private variable?

EDIT: I am using python 2.7

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__
<slot wrapper '__init__' of 'object' objects>

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 "<stdin>", line 1, in <module>
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 "<stdin>", line 1, in <module>
  File "<stdin>", 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.)

Answer 2

Attributes of Python objects are generally stored in a dictionary, just like the ones you create with {}. Since you can add new items to a dictionary at any time, you can add attributes to an object at any time. And since any type of object can be stored in a dictionary without previous declaration of type, any type of object can be stored as an attribute of an object.

In short, my_object.abc = 42 is (often) just a shorthand for my_object.__dict__["abc"] = 42.

It is possible to define objects without a __dict__ by defining the __slots__ attribute, or to override certain special methods and store attributes in some other way, though most of the time you shouldn't do that.