Basic

What to assert on

I asserted the whole returned object and the test broke on every cosmetic change. Assert on the thing the promise is actually about, not on everything that happens to be in the answer.

What you will see

collected 2 items test_props.py .. ============================== 2 passed in 0.01s ===============================

About 2 min of reading

Quick check

Before this new lesson, one small question about the lesson you just finished. Recall it, do not look it up.

Try It

Write tests that assert on properties and on a required error, not on whole shapes.

A test that asserts a property (length) rather than the whole list.

def test_three_words(): assert len("a b c".split()) == 3

Before you run it, say out loud what it will print. Then open the answer.

What it prints

>>>

A test that requires an error to be raised.

import pytest def test_divide_by_zero(): with pytest.raises(ZeroDivisionError): 1 / 0

Before you run it, say out loud what it will print. Then open the answer.

What it prints

>>>

Run both. The raises test passes because the error really happens.

pytest test_props.py

Before you run it, say out loud what it will print. Then open the answer.

What it prints

collected 2 items test_props.py .. ============================== 2 passed in 0.01s ===============================

Put it in order

Before you write anything, put these lines back in the right order. Click a line to move it up one step. This support step is easier than writing code from scratch, and it is here to make the last example stick.

Data Flow

You pick the narrowest fact that states the promise, assert that fact, and the test fails only when that behaviour breaks.

1 State the promise

↓

2 Pick the narrowest fact

↓

3 Assert that fact

↓

4 Fail only on real break

Why this works

Assert the narrowest fact

A test that checks the whole object is glued to the object's shape. Change a field name and the test breaks even though the behaviour is fine. Assert the specific value or property the promise is about, so the test fails only when the behaviour actually breaks.

Equality is not the only assert

Beyond == you have in, <, >, is None, and pytest.raises. Pick the one that states the promise. For a list you might assert its length or that an item is in it, not that it equals a literal, unless the exact order is the point.

Examples

Assert the result

assert title_case("hello") == "Hello"

Assert on the value the function promises. This says one thing: the output is Hello. If the function returns that, the promise holds.

Assert a property

assert len(words("a b c")) == 3

You do not need the whole list, you need its length. Asserting the smallest fact that proves the point keeps the test about behaviour, not shape.

Assert an error

with pytest.raises(ZeroDivisionError):
    divide(1, 0)

Some promises are about failing. This one says divide must raise when the divisor is zero. A test can require a crash.

Words this lesson uses

Each one is defined here on this site. Open the entry for the full definition and the other words it points at.

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