Python has been the programming language I used most in the past decade. In this series, I explore its more advanced features.

It is often said that in Python, everything is an object: builtins, functions, classes, instances, etc. Thus, improving our understanding of objects will is key to mastering Python. In this first post I explore some general concepts related to objects.

Properties of an object

Simply put, an object is a data structure with an internal state (a set of variables) and a behaviour (a set of functions). The “class” is the template to create new objects or instances. New objects are defined using the class operator, and instantiated using the class name.

Every object has, at least, three properties: a reference, a class, and a refcount.


A reference is a pointer, a way to access the memory address that stores the object. It can be associated to a name, or an element in a collection:

# create a new integer object, and 
# copy its reference to the name "a"
a = 1

# create a new integer object, and
# append its reference to the list "x"
x = list()

We can retrieve the memory address using id(), represented as an integer:


Note that the assignment operator (=) never makes a copy of the value being assigned, it just copies the reference. Similarly, the del operator never deletes an object, just a reference to it. We can check if two names point to the same memory location using is:

x = [1, 2, 3]
y = [1, 2, 3]
z = x

# same reference?
assert x is z
assert id(x) == id(z)
assert x is not y

# same value?
assert x == y


A class is the type of the object (e.g., a float, or a str). Each object contains a pointer to its class, as we will see below. We can know an object’s class using the type operator:

<class 'int'>
<class 'str'>
<class 'float'>

Similarly, we can check if an object is an instance of a given class:

assert isinstance(1, int)
assert isinstance("1", str)
assert isinstance(1., float)


The refcount is a counter that keeps track of how many references point to an object. Its value gets increased by 1 when, for instance, an object gets assigned to a new name. It gets decreased by 1 when a name goes out of scope or is explicitly deleted (del). When the refcount reaches 0, its object’s memory will be reclaimed by the garbage collector.

In principle, we can access the refcounts of a variable using sys.getrefcount:

x = []

Note that its output of getrefcount is always increased by 1, as the function itself contains a reference for the variable. Let’s see another example:


I expected that a newly created integer would have a refcount of 1. However, the actual number is much higher. The explanation might involve optimizations happening under the hood. Accordingly, the refcount of more and more unique integers gets smaller:

assert sys.refcount(2) < sys.getrefcount(123456)

Other properties

On top of these three properties, objects have additional properties and methods that encode their state and behaviors. For instance, the float class has an additional property that stores the numerical value, as well as multiple methods that enable algebraic operations. A user-defined object will have an arbitrary number of attributes and methods.

Notably, the None object has no other properties. It is a singleton: only one such object exists:

a = None
b = None

# same object, despite independent assignments
assert a is b

Objects are first-class citizens

Objects are first-class citizens in Python. In other words, they can:

  • Be assigned a name:
def pretty_print(x: str):
    print(x.title() + ".")

pp = pretty_print
pp("hey there")
Hey there.
  • Be passed as arguments:
from typing import Callable

def format(x: str, formatter: Callable[[str], None]):

format("hey there", pretty_print)
Hey there.
  • Be returned by other functions:
def formatter_factory():
    return pretty_print

formatter_factory()("hey there")
Hey there.

Copying objects

As mentioned above, = does not copy objects, only references. If we need to copy an object, we need to use the copy module. There are two kinds of copies:

  • Shallow copy: copy.copy copies the object, but any reference it stores will just get copied, i.e., not the whole referenced object.
  • Deep copy: copy.deepcopy recursively copies the object, all the objects it references to, and so on.
from copy import copy

x = [1, 2, [3, 4]]
# copies the two first integers, but only 
# the reference to the 3rd element
y = copy(x)

[3, 4, 5]
from copy import deepcopy

x = [1, 2, [3, 4]]
# copies the two first integers as 
# well as the list
y = deepcopy(x)

[3, 4]

Defining our own objects

Python allows us to define our own classes:

class Animal:

    phylum = "metazoan"

    def __init__(self, name, weight): = name
        self.weight = weight
        self.__favorite = True

    def eat(self):
        self.weight += 1

Below I zoom in on some interesting features.

Private and protected attributes

In other languages, a class’ attributes can be set as protected (only accessible within the class and subclasses) or as private (only accessible within the class). While you can always modify attributes from the outside in Python, the language emulates protected and private attributes by prepending one or two underscores respectively:

whale = Animal("whale", 100000)
whale.__favorite # a private attribute
AttributeError: 'Animal' object has no attribute '__favorite'

If we want to access that attribute, we need to put some extra effort:


However, and rather confusingly, this is valid:

whale.__favorite = False

The two dictionaries underlying an object

Two dictionaries underlie each object, and are accessible using instance.__dict__ and Class.__dict__. The first one is the instance-specific dictionary, unique to that instance and containing its writable attributes:

whale = Animal("whale", 100000)

{'name': 'whale', 'weight': 100000, '_Animal__favorite': True}

Note that private attributes like __favorite appear with an altered name of the form _{class name}{attribute}.

Similarly, each class has its own dictionary, containing the data and functions used by all instances (class’ methods, the attributes defined at the class level, etc.):

mappingproxy({'__module__': '__main__', 'phylum': 'metazoan', '__init__': <function Animal.__init__ at 0x103236b90>, 'eat': <function at 0x103236c20>, '__dict__': <attribute '__dict__' of 'Animal' objects>, '__weakref__': <attribute '__weakref__' of 'Animal' objects>, '__doc__': None})

For instance, this is where the method lives. This dictionary is shared by all the instances, which is why every non-static method requires the instance to be passed as the first argument. Under the hood, when we call an instance’s method, Python finds the method in the class dictionary and passes the instance as first argument. But we can also do it explicitly:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: missing 1 required positional argument: 'self'

Both dictionaries are linked by instance.__class__, which is assigned to the class object:

assert whale.__class__.__dict__ == Animal.__dict__

As we saw, an attribute might exist in either dictionary. To find an attribute at runtime, Python will first search instance.__dict__, and then Class.__dict__ if unsuccessful.

__slots__ helps with memory optimization

The instance’s dictionary keeps the class flexible, since we can add new attributes at any time:

whale.medium = "water"
{'name': 'whale', 'weight': 100000, '_Animal__favorite': True, 'medium': 'water'}

__slots__ allows us to fix the possible attributes a priori, allowing Python to reserve the exact amoung of memory needed and to bypass the creation of the dictionary:

class EfficientAnimal:

    __slots__ = ["name", "weight", "__favorite"]
    phylum = "metazoan"

    def __init__(self, name, weight): = name
        self.weight = weight
        self.__favorite = True

dog = EfficientAnimal("dog", 10)
AttributeError: 'EfficientAnimal' object has no attribute '__dict__'. Did you mean: '__dir__'?

In addition to the memory optimizations, this approach also helps to prevent bugs caused by typos in variable names:

dog.namme = "puppy"
AttributeError: 'EfficientAnimal' object has no attribute 'namme'

Further reading