Approximate comparisons (pytest.approx) - Test Execution Trace

Test Overview

This test checks if a floating-point calculation result is approximately equal to an expected value using pytest.approx. It verifies that small differences due to floating-point precision do not cause the test to fail.

Test Code - pytest

PyTest

import pytest

def test_approximate_comparison():
    result = 0.1 + 0.2
    expected = 0.3
    assert result == pytest.approx(expected, rel=1e-9)

Execution Trace - 5 Steps

Step	Action	System State	Assertion	Result
1	Test starts	pytest test runner initialized	—	PASS
2	Calls test_approximate_comparison function	Function execution begins	—	PASS
3	Calculates result = 0.1 + 0.2	result variable holds floating-point sum 0.30000000000000004	—	PASS
4	Compares result to expected using pytest.approx with relative tolerance 1e-9	Comparison allows small floating-point difference	Assert result == pytest.approx(expected, rel=1e-9)	PASS
5	Test completes successfully	Test runner reports test passed	—	PASS

Failure Scenario

Failing Condition: The relative tolerance is set too low, causing the small floating-point difference to fail the comparison

Execution Trace Quiz - 3 Questions

What does pytest.approx help with in this test?

A Checks if two strings are exactly equal

B Skips the test if values differ

C Allows comparison of floating-point numbers with a tolerance

D Converts numbers to integers before comparing

Key Result

Use pytest.approx to compare floating-point numbers approximately, avoiding false test failures caused by tiny precision differences.

Practice

1. What is the main purpose of using pytest.approx in tests?

easy

A. To compare floating-point numbers allowing small differences

B. To check if two strings are exactly the same

C. To test if a function raises an exception

D. To verify if a list contains a specific element

Solution

1. Step 1: Understand floating-point comparison issues

   Floating-point numbers can have tiny differences due to rounding, so exact equality often fails.

2. Step 2: Role of pytest.approx

   pytest.approx allows tests to pass if numbers are close within a tolerance, avoiding false failures.

3. Final Answer:

   To compare floating-point numbers allowing small differences -> Option A

4. Quick Check:

   Approximate comparison = floating-point tolerance [OK]

Hint: Use approx for floats, not exact matches [OK]

Common Mistakes:

• Using == for floats causing flaky tests
• Confusing approx with exception testing
• Using approx for string comparisons

2. Which of the following is the correct syntax to assert that result is approximately equal to 3.14 using pytest.approx?

easy

A. assert result == pytest.approx[3.14]

B. assert result == approx(3.14)

C. assert result = pytest.approx(3.14)

D. assert result == pytest.approx(3.14)

Solution

1. Step 1: Identify correct usage of pytest.approx

   The correct syntax uses the function call form: pytest.approx(value).

2. Step 2: Check assertion syntax

   Assertions use '==' to compare, '=' is assignment and brackets [] are invalid here.

3. Final Answer:

   assert result == pytest.approx(3.14) -> Option D

4. Quick Check:

   Use '==' and parentheses with pytest.approx [OK]

Hint: Use '==' and parentheses with pytest.approx [OK]

Common Mistakes:

• Using '=' instead of '==' in assert
• Calling approx without pytest prefix if not imported
• Using square brackets instead of parentheses

3. What will be the result of this test code?

def test_value():
    value = 0.1 + 0.2
    assert value == pytest.approx(0.3)

medium

A. Test passes because 0.1 + 0.2 is exactly 0.3

B. Test fails due to floating-point precision error

C. Test passes because pytest.approx allows small differences

D. SyntaxError because pytest.approx is used incorrectly

Solution

1. Step 1: Understand floating-point addition

   0.1 + 0.2 is not exactly 0.3 due to floating-point precision limits.

2. Step 2: Effect of pytest.approx in assertion

   pytest.approx allows a small tolerance, so the assertion passes despite tiny difference.

3. Final Answer:

   Test passes because pytest.approx allows small differences -> Option C

4. Quick Check:

   Floating-point sum ≈ 0.3 passes with approx [OK]

Hint: Use approx to handle float rounding errors [OK]

Common Mistakes:

• Assuming 0.1 + 0.2 equals exactly 0.3
• Expecting test to fail without approx
• Thinking pytest.approx causes syntax errors

4. Identify the error in this test code snippet:

def test_approx():
    result = 1.0001
    assert result == pytest.approx(1.0, rel=0.00001)

medium

A. The relative tolerance is too small, causing test to fail

B. pytest.approx does not accept rel parameter

C. The assertion syntax is incorrect

D. The variable result should be an integer

Solution

1. Step 1: Check relative tolerance meaning

   rel=0.00001 means allowed difference is 0.00001 * 1.0 = 0.00001.

2. Step 2: Compare actual difference with tolerance

   Difference is 0.0001, which is greater than 0.00001, so assertion fails.

3. Final Answer:

   The relative tolerance is too small, causing test to fail -> Option A

4. Quick Check:

   Difference > rel tolerance causes failure [OK]

Hint: Check if difference fits within rel tolerance [OK]

Common Mistakes:

• Assuming rel parameter is invalid
• Ignoring difference size vs tolerance
• Confusing syntax error with logic error

5. You want to test that a function returns a value close to 100, but it may vary by up to 0.5 units. Which is the best way to write this assertion using pytest.approx?

hard

A. assert result == pytest.approx(100, rel=0.5)

B. assert result == pytest.approx(100, abs=0.5)

C. assert result == pytest.approx(100, abs=0.05)

D. assert result == pytest.approx(100, rel=0.005)

Solution

1. Step 1: Understand absolute vs relative tolerance

   Absolute tolerance (abs) allows fixed difference; relative (rel) allows percentage difference.

2. Step 2: Match tolerance to requirement

   Variation up to 0.5 units means absolute tolerance of 0.5 is correct.

3. Final Answer:

   assert result == pytest.approx(100, abs=0.5) -> Option B

4. Quick Check:

   Use abs=0.5 for fixed unit tolerance [OK]

Hint: Use abs for fixed tolerance, rel for percentage [OK]

Common Mistakes:

• Using rel when fixed tolerance needed
• Setting abs too small to cover variation
• Confusing abs and rel parameters