Asserting warnings (pytest.warns) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assert that a warning is raised.

PyTest

import warnings
import pytest

def test_warning():
    with pytest.warns([1]):
        warnings.warn("This is a warning", UserWarning)

Drag options to blanks, or click blank then click option'

AUserWarning

BRuntimeWarning

CDeprecationWarning

DWarning

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different warning type than the one raised causes the test to fail.

Not using pytest.warns() context manager.

The pytest.warns() context manager expects the warning class to check. Here, UserWarning matches the warning raised.

2fill in blank

medium

Complete the code to capture the warning message text.

PyTest

import warnings
import pytest

def test_warning_message():
    with pytest.warns(UserWarning) as record:
        warnings.warn("Check this warning", UserWarning)
    assert record[0].message.args[[1]] == "Check this warning"

Drag options to blanks, or click blank then click option'

A2

B0

C1

Dmessage

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 1 or 2 which are out of range and cause errors.

Trying to access message attribute directly instead of args.

The warning message is stored as the first argument in args, so index 0 is correct.

3fill in blank

hard

Fix the error in the code to correctly assert a warning is raised.

PyTest

import warnings
import pytest

def test_warn():
    with pytest.warns([1]):
        warnings.warn("Deprecated", DeprecationWarning)

Drag options to blanks, or click blank then click option'

ADeprecationWarning

BUserWarning

CRuntimeWarning

DWarning

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different warning type causes the test to fail.

Not using a warning class but a string or variable.

The warning raised is DeprecationWarning, so pytest.warns() must check for the same type.

4fill in blank

hard

Fill both blanks to assert a warning and check its message.

PyTest

import warnings
import pytest

def test_warn_message():
    with pytest.warns([1]) as record:
        warnings.warn("Be careful", UserWarning)
    assert str(record[0].message) [2] "Be careful"

Drag options to blanks, or click blank then click option'

AUserWarning

BDeprecationWarning

C==

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong warning type in pytest.warns().

Using != instead of == in the assertion.

The warning type is UserWarning. The assertion checks if the message string equals "Be careful" using ==.

5fill in blank

hard

Fill all three blanks to assert a warning, capture it, and check the message content.

PyTest

import warnings
import pytest

def test_warning_content():
    with pytest.warns([1]) as record:
        warnings.warn("Use new API", [2])
    assert record[0].message.args[0] [3] "Use new API"

Drag options to blanks, or click blank then click option'

AUserWarning

BDeprecationWarning

C==

DRuntimeWarning

Attempts:

3 left

💡 Hint

Common Mistakes

Using different warning types in pytest.warns() and warnings.warn().

Using wrong comparison operators in the assertion.

The warning raised is DeprecationWarning, so both pytest.warns() and warnings.warn() use it. The assertion checks equality with ==.

Practice

1. What is the main purpose of using pytest.warns in a test?

easy

A. To skip tests that produce warnings

B. To ignore all warnings during test runs

C. To raise an error when no warnings occur

D. To check if a specific warning is raised during code execution

Solution

1. Step 1: Understand the role of pytest.warns

   pytest.warns is used to verify that a warning is raised when running a piece of code.

2. Step 2: Compare options with this purpose

   Only To check if a specific warning is raised during code execution correctly states that pytest.warns checks for specific warnings during execution.

3. Final Answer:

   To check if a specific warning is raised during code execution -> Option D

4. Quick Check:

   pytest.warns checks warnings = A [OK]

Hint: Remember: pytest.warns confirms warnings appear [OK]

Common Mistakes:

• Thinking pytest.warns ignores warnings
• Confusing pytest.warns with error raising
• Assuming pytest.warns skips tests

2. Which of the following is the correct syntax to assert a warning of type UserWarning using pytest?

easy

A. with pytest.warns(UserWarning):\n function_call()

B. pytest.warns(UserWarning, function_call())

C. assert pytest.warns(UserWarning, function_call)

D. pytest.warns(function_call(), UserWarning)

Solution

1. Step 1: Recall pytest.warns usage

   pytest.warns is used as a context manager with with to catch warnings during code execution.

2. Step 2: Check syntax correctness

   with pytest.warns(UserWarning):\n function_call() correctly uses with pytest.warns(UserWarning): followed by the code that triggers the warning.

3. Final Answer:

   with pytest.warns(UserWarning):\n function_call() -> Option A

4. Quick Check:

   Use 'with' for pytest.warns = B [OK]

Hint: Always use 'with' block for pytest.warns [OK]

Common Mistakes:

• Calling pytest.warns as a function without 'with'
• Passing function call instead of function inside 'with'
• Swapping arguments order

3. What will be the result of this pytest code snippet?

import warnings
import pytest

def func():
    warnings.warn('Deprecated feature', DeprecationWarning)

def test_func():
    with pytest.warns(DeprecationWarning) as record:
        func()
    assert len(record) == 1
    assert record[0].message.args[0] == 'Deprecated feature'

medium

A. Test fails because no warning is raised

B. Test passes because the warning is correctly caught and message matches

C. Test fails due to wrong warning type

D. Test raises an exception because record is not iterable

Solution

1. Step 1: Analyze the warning raised by func()

   The function func() raises a DeprecationWarning with message 'Deprecated feature'.

2. Step 2: Check pytest.warns usage and assertions

   The test uses pytest.warns to catch DeprecationWarning and stores it in record. It asserts one warning was caught and the message matches exactly.

3. Final Answer:

   Test passes because the warning is correctly caught and message matches -> Option B

4. Quick Check:

   Warning caught and message correct = A [OK]

Hint: Check warning type and message inside pytest.warns block [OK]

Common Mistakes:

• Assuming warnings.warn does not raise warnings
• Confusing warning types
• Not using 'as record' to capture warnings

4. Identify the error in this pytest warning assertion code:

import warnings
import pytest

def test_warning():
    with pytest.warns(UserWarning):
        warnings.warn('Be careful!')
    assert pytest.warns(UserWarning)

medium

A. The assert statement incorrectly calls pytest.warns as a function

B. The warning message is missing in pytest.warns

C. The warning type UserWarning is invalid

D. The warnings.warn call should be outside the with block

Solution

1. Step 1: Review the use of pytest.warns

   pytest.warns is a context manager used with 'with' to catch warnings, not a function to assert directly.

2. Step 2: Identify incorrect assert usage

   The assert line calls pytest.warns(UserWarning) as a function, which is invalid and causes an error.

3. Final Answer:

   The assert statement incorrectly calls pytest.warns as a function -> Option A

4. Quick Check:

   pytest.warns used only with 'with' block = C [OK]

Hint: Do not call pytest.warns as a function; use 'with' block only [OK]

Common Mistakes:

• Calling pytest.warns without 'with'
• Expecting pytest.warns to return a boolean
• Placing warnings.warn outside the 'with' block

5. You want to test a function that may raise either DeprecationWarning or UserWarning. How can you assert that exactly one warning of either type is raised during the test?

hard

A. Use two separate with blocks for each warning type and assert one warning each

B. Use a single with block with pytest.warns(Warning) and check the warning type inside

C. Use pytest.warns with a tuple of warning types and assert one warning caught

D. Use pytest.warns without specifying warning type and count warnings

Solution

1. Step 1: Understand pytest.warns supports multiple warning types

   pytest.warns can accept a tuple of warning classes to catch any of those types.

2. Step 2: Use tuple to catch either DeprecationWarning or UserWarning

   Using pytest.warns((DeprecationWarning, UserWarning)) catches either warning type in one block.

3. Step 3: Assert exactly one warning was caught

   Inside the block, assert that one warning was raised to ensure test precision.

4. Final Answer:

   Use pytest.warns with a tuple of warning types and assert one warning caught -> Option C

5. Quick Check:

   Tuple of warnings in pytest.warns = D [OK]

Hint: Pass tuple of warnings to pytest.warns to catch multiple types [OK]

Common Mistakes:

• Using multiple with blocks causing multiple warnings
• Not specifying warning types and catching unrelated warnings
• Assuming pytest.warns accepts multiple calls in one block