Checking identity (is, is not) in PyTest - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this pytest assertion?

Consider the following pytest test function. What will be the result of running this test?

PyTest

def test_identity():
    a = [1, 2, 3]
    b = a
    assert a is b

A TypeError at runtime

B Test fails with AssertionError

C SyntaxError at runtime

D Test passes

💡 Hint

Remember that 'is' checks if two variables point to the same object in memory.

Since 'b' is assigned to 'a', both variables point to the same list object. Therefore, 'a is b' is True and the assertion passes.

❓ Predict Output

intermediate

What happens when comparing two identical lists with 'is'?

What will be the result of this pytest test function?

PyTest

def test_identity_lists():
    a = [1, 2, 3]
    b = [1, 2, 3]
    assert a is not b

A SyntaxError at runtime

B Test passes

C NameError at runtime

D Test fails with AssertionError

💡 Hint

Two lists with the same content are different objects unless assigned explicitly.

Although 'a' and 'b' have the same content, they are two separate list objects in memory. So 'a is not b' is True and the assertion passes.

❓ assertion

advanced

Which assertion correctly checks that two variables do NOT refer to the same object?

You want to write a pytest assertion to confirm that variables 'x' and 'y' are different objects. Which option is correct?

A assert x is not y

B assert x != y

C assert x is y

D assert x == y

💡 Hint

Remember 'is not' checks identity, while '!=' checks value equality.

'assert x is not y' checks that 'x' and 'y' are not the same object in memory, which is what is needed.

🔧 Debug

advanced

Why does this pytest assertion fail unexpectedly?

Given the code below, why does the assertion fail?

PyTest

def test_identity_failure():
    a = 256
    b = 256
    assert a is b

def test_identity_failure2():
    x = 257
    y = 257
    assert x is y

A The assertion fails because 'is' compares values, not objects

B The assertion fails because variables 'x' and 'y' are not defined

C Python caches small integers, so 'a is b' is True for 256 but not for 257

D The assertion fails due to a syntax error in the code

💡 Hint

Python caches small integers from -5 to 256 for performance.

Python reuses objects for small integers between -5 and 256, so 'a is b' is True for 256. For 257, new objects are created, so 'x is y' is False and assertion fails.

❓ framework

expert

How to write a pytest test that checks identity and reports a clear message on failure?

You want to write a pytest test that asserts two variables 'obj1' and 'obj2' are the same object. If they are not, the test should fail with the message: "Objects are not identical". Which code snippet achieves this?

A

def test_obj_identity():
    assert obj1 is obj2, "Objects are not identical"

B

def test_obj_identity():
    assert obj1 == obj2, "Objects are not identical"

C

def test_obj_identity():
    assert obj1 is not obj2, "Objects are not identical"

D

def test_obj_identity():
    assert obj1 != obj2, "Objects are not identical"

💡 Hint

Use 'is' for identity and provide a message after the comma in assert.

Using 'assert obj1 is obj2, "Objects are not identical"' checks identity and shows the message if the assertion fails.

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