Why Checking identity (is, is not) in PyTest? - Purpose & Use Cases

The Big Idea

What if your test says two objects are equal but they are actually different in memory?

The Scenario

Imagine you have a list of user objects and you want to check if two variables point to the exact same user in memory. Doing this by manually comparing every attribute can be confusing and slow.

The Problem

Manually comparing objects attribute by attribute is slow and error-prone. You might miss subtle differences or mistakenly think two objects are the same when they just look alike. This wastes time and causes bugs.

The Solution

Using identity checks like is and is not in tests lets you quickly and clearly verify if two variables refer to the exact same object. This is faster, simpler, and less error-prone.

Before vs After

✗ Before

assert user1.id == user2.id and user1.name == user2.name

✓ After

assert user1 is user2

What It Enables

This lets you confidently test if two variables point to the same object, catching subtle bugs and improving test clarity.

Real Life Example

When testing a cache system, you want to confirm that fetching the same user twice returns the exact same object, not just a copy. Identity checks make this easy.

Key Takeaways

Manual attribute checks are slow and risky.

is and is not check if two variables point to the same object.

Using identity checks makes tests clearer and more reliable.

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