Comparing objects in python

Published on: 2020-11-19

Comparison operators

Python basic comparison operators:

These operators compare the values of comparing objects. In other words we are comparing values saved inside the variable. Each int, float or string type is saved in memory as object in python. And we want to compare content of these objects with operators below.

In generally, its are operators which we’ve already known from school.

Python identical operators:

These operators compared if two objects are same objects. For better understanding. Every object in python has its own id number. We can get id number of certain object in python by method id(). So when two objects are identical than they have same id number.

See example below.

Variable a and b have same values saved in but they have different id.

>>> a = 3100
>>> b = 3100
>>> id(a)
4332864560
>>> id(b)
4332864880

Identical objects - The singletones

The singleton is one of the simplest design pattern of object in Python.

Examples of singleton are:

• None
• True
• False

What does it means ? Whenever we create one of these object above in our code always will be one same object.

It can’t be two None objects with different id number.

Although if we have two and more None in our code, always it is referenced to one object saved in memory.

>>> id(None)
4407467184

All None objects in my certain code have id number equal to 4407467184.

Difference between == and is in examples

>>> a = 3100
>>> b = 3100
>>> id_a = id(a)
>>> id_b = id(b)
>>> id_a
4332864560
>>> id_b
4332864880
>>> a is b
False
>>> a == b
True

Comparison a is b can be rewrite as id_a == id_b, so we just compare id number of each object, in other words if two id integers are same. In some cases it is much faster than compare values of objects, expecially when we compare list of values.

Comparison a == b compares values saved in each variable. In this case we compare two integers saved in 3100 == 3100.

This explanation is same for != and is not. But with vice versa logic.

Compare objects identical number in examples

None examples:

>>> def my_func():
...     return None
...
>>> oliver = {"age": 20}
>>> my_func() is None
True
>>> oliver.get("height", None) is None
True
>>> oliver.get("age", None) is None
False
>>> oliver.get("age", None) is not None
True

True examples:

>>> 3 == 3 is True
True

Python will evaluate like that:

3 == 3 = True, than id(True) is id(True) = True

Two lists:

>>> a = [1, 2, 3]
>>> b = a
>>> a is b
True
>>> a == b
True
>>> b.append(4)
>>> a
[1, 2, 3, 4]
>>> b
[1, 2, 3, 4]
>>> a is b
True
>>> a == b
True
>>> del a
>>> a
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'a' is not defined
>>> b
[1, 2, 3, 4]
>>> a = [1, 2, 3, 4]
>>> a is b
False
>>> a == b
True

Notice at the end of code I’ve created new list a which has same values as list b. But when I’ve created new list, than the new list is a new object with new id number.