Why test?

4 min read

In lesson 2.6 you typed ten scores into the Input box, one run at a time, and compared each answer with a table. That works once. By the end of this lesson, Python will run those checks for you every time you change the code, and you'll know what a passing check can and can't tell you.

Every lesson in this module runs in the browser, on a phone too. Only the last section of lesson 9.5 uses a computer.

A check you write once

Lesson 5.8 turned that table into a loop: eight scores, each with the letter it should get, and a line that printed only when an answer was wrong. That loop was a testtestCode that runs your code on an input you chose and compares the result with the answer it should give.: code that runs your code on an input you chose and compares the result with the right answer.

Python has a statement for the comparing part. assert is followed by a condition: if it's true, nothing happens; if it's false, the program stops with an error. Each line is an assertionassertionA statement that something must be true, like assert grade_for(85) == 'A'. When it's false, Python raises AssertionError.. grades.py is lesson 5.8's grade_for in its own file, imported as in lesson 8.5:

from grades import grade_for

assert grade_for(90) == "A"
assert grade_for(85) == "A"
assert grade_for(49) == "F"
print("All checks passed")

def grade_for(score):
    if score >= 85:
        return "A"
    elif score >= 70:
        return "B"
    elif score >= 50:
        return "C"
    else:
        return "F"

Ctrl or Cmd + Enter runs

Three true assertions print nothing, so the last line runs. In lesson 5.8 you broke grade_for on purpose, changing >= 85 to > 85. Here's that version. Predict what the assertions show:

What will this print?

Decide before you look. Guessing wrong is how this sticks.

from grades import grade_for

assert grade_for(90) == "A"
assert grade_for(85) == "A"
print("All checks passed")

Also in the folder with main.py:

grades.py

def grade_for(score):
    if score > 85:
        return "A"
    elif score >= 70:
        return "B"
    elif score >= 50:
        return "C"
    else:
        return "F"

Your prediction

Python prints

Traceback (most recent call last):
  File "main.py", line 4, in <module>
    assert grade_for(85) == "A"
           ^^^^^^^^^^^^^^^^^^^^
AssertionError

grade_for(85) returns 'B', so the assertion on line 4 is false and Python raises AssertionError. It names the line, but not the values: nothing says 'B' or 'A'.

Read the last line first (lesson 7.1): just AssertionError. If you expected something like got B, expected A, that's the most common guess, and plain assert doesn't give it. You learn which check failed, then you'd need a print to see what grade_for(85) returned. The next lesson's tool shows both values for you.

A bug that comes back

Change > 85 back to >= 85 and the assertion passes. Now keep it. Months later you tidy grades.py, type > again by accident, and the bug is back. That has a name: a regressionregressionA bug that comes back after it was fixed, usually when other code changes..

A test you keep runs every time you run your tests, so the bug it checks can't come back quietly. Try it in the first runner: change >= 85 to > 85 in the grades.py tab and run. Line 4 stops it with AssertionError.

In lesson 7.4's six steps, a test is step 4, "check the guess", written down instead of done once. A failing test also does step 1, reproducing the bug on every run. And it does step 6 for you: the check stays in the file, so remembering is the computer's job.

One mistake makes a test that can never fail. You can add a message after a comma, assert condition, "message", and it's shown after AssertionError:. Brackets around both turn them into one tuple (lesson 4.3):

Common mistake

This breaks

def passed(score):
    return score > 50

assert (passed(50), "50 is a pass")
print("All checks passed")

Python says

SyntaxWarning: assertion is always true, perhaps remove parentheses?

The fix

def passed(score):
    return score > 50

assert passed(50), "50 is a pass"
print("All checks passed")

A tuple with something in it is true (lesson 2.4), so the first version passes and prints All checks passed while passed(50) is False. Python warns you above the output, but the program still runs. Without the brackets, the bug shows: AssertionError: 50 is a pass.

What a passing test tells you

Here's grade_for with a different bug. Every assertion passes:

from grades import grade_for

assert grade_for(90) == "A"
assert grade_for(85) == "A"
assert grade_for(75) == "B"
assert grade_for(49) == "F"
print("All checks passed")

def grade_for(score):
    if score >= 85:
        return "A"
    elif score > 70:
        return "B"
    elif score >= 50:
        return "C"
    else:
        return "F"

Ctrl or Cmd + Enter runs

A score of 70 gets a C, and no assertion tried 70. Add assert grade_for(70) == "B" above the print and run: now it fails.

Quick check

All the assertions for a function pass. What do you know?

Exercise

Test santim_to_birr

Here's santim_to_birr from lesson 5.1. Write at least three assertions above the print: a normal amount, 0 santim, and 100 santim, which should be exactly 1 birr. Then break the function (/ 10 instead of / 100) and check that an assertion stops it.

Your code

def santim_to_birr(santim):
    return santim / 100

# Your assertions here
print("All checks passed")

Ctrl or Cmd + Enter runs

Show a solution

This is one way to solve it, not the only one. If yours prints the same thing, it works.

def santim_to_birr(santim):
    return santim / 100

assert santim_to_birr(2550) == 25.5
assert santim_to_birr(0) == 0
assert santim_to_birr(100) == 1
print("All checks passed")

Key takeaways

A test runs your code on an input you chose and compares the result with the answer it should give.
assert condition does nothing when it's true and raises AssertionError when it's false, without showing the values.
Keep your tests after a fix: if the bug comes back (a regression), they fail straight away.
A passing test shows the code is right for the inputs it tried, nothing more. Test the boundaries.