Approximate comparisons (pytest.approx) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is pytest.approx used for in testing?

pytest.approx is used to compare floating-point numbers approximately, allowing small differences due to rounding errors.

beginner

How do you write an assertion to check if result is approximately equal to 3.14 using pytest.approx?

Use assert result == pytest.approx(3.14) to allow a small tolerance in the comparison.

intermediate

What are the two main ways to control tolerance in pytest.approx?

You can control tolerance by setting rel (relative tolerance) and abs (absolute tolerance) parameters.

beginner

Why is approximate comparison important when testing floating-point numbers?

Because floating-point arithmetic can introduce tiny rounding errors, exact equality checks often fail. Approximate comparison avoids false test failures.

intermediate

Can pytest.approx be used to compare lists or arrays of numbers?

Yes, pytest.approx supports comparing lists or arrays element-wise approximately.

What does pytest.approx(1.0, rel=1e-3) mean?

A Accept values within 1% relative difference of 1.0

B Accept values within 0.001 absolute difference of 1.0

C Accept values within 0.1% relative difference of 1.0

D Accept values exactly equal to 1.0

The rel=1e-3 means relative tolerance of 0.001, or 0.1% difference allowed.

Which of these is a correct way to assert approximate equality in pytest?

A assert approx(x) == y

B assert x == pytest.approx(y)

C assert x.approx(y)

D assert x.equals(y)

The correct syntax is assert x == pytest.approx(y).

If you want to allow a fixed small difference regardless of value size, which parameter do you use?

A abs

B rel

C tol

D delta

abs sets absolute tolerance, a fixed allowed difference.

What happens if you compare two floats with assert a == b and they differ by a tiny rounding error?

A Test raises an exception

B Test passes

C Test is skipped

D Test fails

Exact equality fails if floats differ even slightly, so test fails.

Can pytest.approx be used to compare nested lists of floats?

A No, only single floats

B Only flat lists, not nested

C Yes, it compares element-wise recursively

D Only numpy arrays

pytest.approx supports nested lists and compares elements recursively.

Explain why approximate comparisons are needed when testing floating-point numbers.

Think about how computers store decimal numbers.

Describe how to use pytest.approx to check if a value is close to 10 with a relative tolerance of 0.01.

Use the rel parameter for relative tolerance.