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.