Every value in Python is stored as an object in memory. Each object has:

• A value
• A data type
• A unique identity

The identity of an object is a unique number that remains constant during the object’s lifetime. In CPython, this identity is usually the memory address of the object.

Python provides the built-in id() function to view the identity of an object.

Understanding object identity helps explain how variables work internally and why assigning one variable to another does not always create a new object.

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Syntax #

id(object)

Parameter #

Parameter	Description
object	The object whose identity is to be obtained.

Return Value #

The id() function returns a unique integer representing the identity of the object.

Note: The exact number returned by id() varies from one program execution to another. Therefore, your output may be different from the examples shown here.

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Example 1: Getting the Identity of an Object #

main.py

number = 100

print(id(number))

Sample Output #

140654845202736

Explanation #

• The variable number refers to an integer object.
• The id() function returns the identity of that object.
• The value shown is unique for that object during the program’s execution.
• The exact number will differ on your computer.

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Example 2: Identity of Different Objects #

main.py

a = 100
b = 200

print(id(a))
print(id(b))

Sample Output #

140654845202736
140654845205936

Explanation #

• a and b refer to different integer objects.
• Since they are different objects, their identities are different.
• Each object has its own unique identity.

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Example 3: Two Variables Referring to the Same Object #

main.py

name1 = "Python"
name2 = name1

print(id(name1))
print(id(name2))

Sample Output #

140654846012944
140654846012944

Explanation #

• name1 refers to the string "Python".
• name2 = name1 does not create another string.
• Both variables refer to the same object.
• Therefore, both variables have the same identity.

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Example 4: Checking Identity Using is #

main.py

a = "Python"
b = a

print(a is b)

Output #

True

Explanation #

• The is operator checks whether two variables refer to the same object.
• Since both variables refer to the same object, the result is True.
• The is operator compares identities, not values.

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Example 5: Equality vs Identity #

main.py

a = [10, 20, 30]
b = [10, 20, 30]

print(a == b)
print(a is b)

Output #

True
False

Explanation #

• a == b compares the values stored in both lists.
• Since both lists contain the same elements, the result is True.
• a is b compares their identities.
• The two lists are different objects, so the result is False.

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Example 6: Assignment Does Not Create a New Object #

main.py

numbers1 = [10, 20, 30]

numbers2 = numbers1

print(numbers1 is numbers2)

Output #

True

Explanation #

• numbers2 is assigned the reference stored in numbers1.
• Both variables refer to the same list object.
• No new list is created during assignment.

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Example 7: Creating a New Object #

main.py

numbers1 = [10, 20, 30]

numbers2 = numbers1.copy()

print(numbers1 == numbers2)
print(numbers1 is numbers2)

Output #

True
False

Explanation #

• copy() creates a new list.
• Both lists contain the same values.
• However, they are separate objects in memory.
• Therefore, is returns False.

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Example 8: Identity After Reassignment #

main.py

value = 100

print(id(value))

value = 200

print(id(value))

Sample Output #

140654845202736
140654845205936

Explanation #

• Initially, value refers to the integer object 100.
• After reassignment, it refers to another integer object 200.
• The identity changes because the variable now refers to a different object.

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Example 9: Using id() with Different Data Types #

main.py

print(id(10))
print(id(3.14))
print(id("Python"))

Sample Output #

140654845201456
140654846314832
140654846012944

Explanation #

• Every Python object has an identity.
• Integers, floating-point numbers, strings, lists, and all other objects have unique identities.
• The exact values printed will vary between program executions.

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Example 10: Variables Are References #

main.py

a = [1, 2, 3]

b = a

b.append(4)

print(a)
print(b)

Output #

[1, 2, 3, 4]
[1, 2, 3, 4]

Explanation #

• Both a and b refer to the same list object.
• append(4) modifies that shared list.
• Since both variables reference the same object, the change is visible through both variables.
• This demonstrates that variables store references to objects, not the objects themselves.

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Common Mistakes #

1. Confusing == with is #

Incorrect

a = [1, 2]

b = [1, 2]

print(a is b)

Incorrect Expectation #

True

Actual Output #

False

Reason

is compares object identities, while == compares values.

Correct

print(a == b)

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2. Assuming Assignment Creates a Copy #

Incorrect Assumption

list1 = [10, 20]

list2 = list1

Reason

Both variables refer to the same object.

To create a separate list:

list2 = list1.copy()

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3. Relying on Exact id() Values #

Incorrect Assumption

print(id(100))

will always produce the same number.

Reason

The value returned by id() depends on the Python implementation and changes between program executions.

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Best Practices #

• Use == to compare values.
• Use is only when checking whether two variables refer to the same object.
• Use id() mainly for learning and debugging.
• Use .copy() when you need an independent copy of a mutable object such as a list.

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Key Points to Remember #

• Every Python value is an object.
• Every object has a unique identity.
• The id() function returns an object’s identity.
• Variables store references to objects.
• The is operator compares object identities.
• The == operator compares object values.
• Assignment creates another reference to the same object; it does not automatically create a copy.
• The numeric value returned by id() is implementation-dependent and may differ each time the program runs.