Checking identity (is, is not) in PyTest - Build an Automation Script

Verify identity comparison using 'is' and 'is not' operators

Preconditions (2)

Step 1: Create two variables a and b referencing the same list object

Step 2: Create a third variable c referencing a different list object with the same content

Step 3: Use 'is' operator to check if a and b are the same object

Step 4: Use 'is not' operator to check if a and c are not the same object

Step 5: Assert that a is b returns True

Step 6: Assert that a is not c returns True

✅ Expected Result: The test passes confirming that 'is' correctly identifies identical objects and 'is not' correctly identifies different objects

Automation Requirements - pytest

Assertions Needed:

Assert that 'a is b' is True

Assert that 'a is not c' is True

Best Practices:

Use clear variable names

Use pytest assert statements

Keep test functions small and focused

Add comments explaining identity checks

Automated Solution

PyTest

import pytest

def test_identity_operators():
    # Two variables referencing the same list object
    a = [1, 2, 3]
    b = a
    # A different list object with the same content
    c = [1, 2, 3]

    # Assert that a and b are the same object
    assert a is b, "a and b should be the same object"
    # Assert that a and c are not the same object
    assert a is not c, "a and c should not be the same object"

This test defines three variables: a and b point to the same list object, while c points to a different list with the same content.

Using is checks if two variables point to the exact same object in memory. The assertion assert a is b confirms this.

Using is not checks if two variables do not point to the same object. The assertion assert a is not c confirms that although a and c have the same content, they are different objects.

Comments explain each step clearly for beginners.

Common Mistakes - 3 Pitfalls

{'mistake': "Using '==' instead of 'is' to check identity", 'why_bad': "'==' checks value equality, not if two variables point to the same object", 'correct_approach': "Use 'is' to check if two variables reference the same object"}

{'mistake': "Comparing mutable objects with 'is' expecting value equality", 'why_bad': "'is' only checks object identity, not content equality, which can cause confusion", 'correct_approach': "Use '==' for content equality and 'is' for identity checks"}

{'mistake': 'Not assigning variables properly leading to unexpected identity results', 'why_bad': "If variables do not reference the same object, 'is' will fail even if contents match", 'correct_approach': 'Explicitly assign variables to the same object when testing identity'}

Bonus Challenge

Now add data-driven testing with 3 different pairs of variables to check identity and non-identity

Show Hint

Practice

1. What does the is operator check in Python when used in pytest assertions?

easy

A. If two variables have the same value but different objects

B. If two variables point to the exact same object in memory

C. If two variables are equal using the == operator

D. If two variables are both integers

Solution

1. Step 1: Understand the is operator meaning

   The is operator checks if two variables refer to the same object in memory, not just equal values.

2. Step 2: Differentiate from ==

   The == operator checks if values are equal, but is checks identity (same object).

3. Final Answer:

   If two variables point to the exact same object in memory -> Option B

4. Quick Check:

   is checks identity [OK]

Hint: Remember: is means same object, not just equal value [OK]

Common Mistakes:

• Confusing is with ==
• Thinking is checks value equality
• Using is for comparing numbers or strings incorrectly

2. Which of the following is the correct syntax to assert two variables a and b are not the same object in pytest?

easy

A. assert a is not b

B. assert a !== b

C. assert a is! b

D. assert a != b

Solution

1. Step 1: Recall correct syntax for identity negation

   The correct syntax to check two variables are not the same object is is not, so assert a is not b.

2. Step 2: Identify invalid syntax options

   Options B and C use invalid Python syntax. assert a != b uses != which checks value inequality, not identity.

3. Final Answer:

   assert a is not b -> Option A

4. Quick Check:

   is not syntax correct [OK]

Hint: Use is not to check different objects, not != [OK]

Common Mistakes:

• Using != instead of is not
• Writing is! which is invalid
• Using JavaScript style !== in Python

3. Given the code:

obj1 = None
obj2 = None

assert obj1 is obj2

What will happen when this pytest assertion runs?

medium

A. SyntaxError due to wrong assertion

B. The assertion fails because None is not an object

C. The assertion passes because both are the same None object

D. The assertion fails because obj1 and obj2 are different None instances

Solution

1. Step 1: Understand None is a singleton

   In Python, None is a singleton object, so all variables assigned None point to the same object.

2. Step 2: Check identity assertion

   The assertion assert obj1 is obj2 checks if both variables point to the same object, which they do.

3. Final Answer:

   The assertion passes because both are the same None object -> Option C

4. Quick Check:

   None is None always True [OK]

Hint: Remember: None is always the same object in Python [OK]

Common Mistakes:

• Thinking None creates new objects each time
• Confusing identity with equality
• Expecting assertion failure for None identity

4. Identify the error in this pytest assertion:

list1 = [1, 2, 3]
list2 = [1, 2, 3]
assert list1 is list2

medium

A. RuntimeError because lists cannot be compared with is

B. No error; assertion passes because lists have same values

C. SyntaxError due to wrong use of is

D. Assertion fails because lists are different objects despite same values

Solution

1. Step 1: Understand list object identity

   Two lists with same values are different objects in memory unless explicitly assigned.

2. Step 2: Check assertion behavior

   The assertion assert list1 is list2 fails because is checks identity, not equality.

3. Final Answer:

   Assertion fails because lists are different objects despite same values -> Option D

4. Quick Check:

   Same values ≠ same object [OK]

Hint: Use == for value equality, is for identity [OK]

Common Mistakes:

• Expecting is to check value equality
• Confusing object identity with content equality
• Assuming lists with same content share identity

5. You want to write a pytest test to confirm that a function returns the exact same singleton object config every time it is called. Which assertion is best?

hard

A. assert get_config() is config

B. assert get_config() == config

C. assert get_config() is not config

D. assert get_config() != config

Solution

1. Step 1: Understand singleton identity check

   To confirm the function returns the exact same object, use is to check identity.

2. Step 2: Evaluate options

   assert get_config() == config checks value equality, which may be true for different objects. assert get_config() is config correctly asserts identity. Options C and D check for difference, which is not desired.

3. Final Answer:

   assert get_config() is config -> Option A

4. Quick Check:

   is for singleton identity [OK]

Hint: Use is to confirm same singleton object returned [OK]

Common Mistakes:

• Using == which checks value, not identity
• Using is not by mistake
• Not understanding singleton pattern in tests