Writing Robust Code and Unit Testing

Defensive Programming

We made several mistakes while writing the programs in our first few lessons:

• Are there still errors lurking in the code we have?
• How can we guard against introducing new errors in code as we modify it?

The first step is to use defensive programming, i.e., to assume that mistakes will happen and to guard against them.

One way to do this is to add assertions to our code so that it checks itself as it runs. An assertion is simply a statement that something must be true at a certain point in a program. When Python sees one, it checks the assertion’s condition:

• If true, Python does nothing
• If false, Python halts the program immediately and prints the error message provided

Let’s look at an example. As before, let’s start the Python interpreter first (e.g.)…

$ python

Python 3.4.3 |Anaconda 2.3.0 (x86_64)| (default, Mar  6 2015, 12:07:41) 
[GCC 4.2.1 (Apple Inc. build 5577)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 

This piece of code halts as soon as the loop encounters a value that isn’t positive:

numbers = [1.5, 2.3, 0.7, -0.001, 4.4]
total = 0.0
for n in numbers:
    assert n >= 0.0, 'Data should only contain positive values'
    total += n

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
AssertionError: Data should only contain positive values

Programs like the Firefox browser are littered with assertions: 10-20% of the code they contain are there to check that the other 80-90% are working correctly.

Broadly speaking, assertions fall into three categories:

• A precondition is something that must be true at the start of a function in order for it to work correctly.
• A postcondition is something that the function guarantees is true when it finishes.
• An invariant something that is always true at a particular point inside a piece of code.

For example, let’s look at rectangle1.py (in the code subdirectory). Suppose we are representing rectangles using a tuple of four coordinates (x0, y0, x1, y1), representing the lower left and upper right corners of the rectangle. In order to do some calculations, we need to normalize the rectangle so that the lower left corner is at the origin and 1.0 units long on its longest axis. The function does that, but checks that its input is correctly formatted and that its result makes sense:

def normalize_rectangle(rect):
    ' ' ' Normalizes a rectangle so that it is at the origin and 1.0 units long on its longest axis. ' ' '
    assert len(rect) == 4, 'Rectangles must contain 4 coordinates'
    x0, y0, x1, y1 = rect
    assert x0 < x1, 'Invalid X coordinates'
    assert y0 < y1, 'Invalid Y coordinates'

    dx = x1 - x0
    dy = y1 - y0
    if dx > dy:
        scaled = float(dx) / dy
        upper_x, upper_y = 1.0, scaled
    else:
        scaled = float(dx) / dy
        upper_x, upper_y = scaled, 1.0

    assert 0 < upper_x <= 1.0, 'Calculated upper X coordinate invalid'
    assert 0 < upper_y <= 1.0, 'Calculated upper Y coordinate invalid'

    return (0, 0, upper_x, upper_y)

The preconditions on lines 2, 4, and 5 catch invalid inputs. Before you try the next bit, make sure you cd to the code subdirectory first!

from rectangle1 import normalize_rectangle
print(normalize_rectangle( (0.0, 1.0, 2.0) )) # missing the fourth coordinate

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "rectangle1.py", line 2, in normalize_rectangle
    assert len(rect) == 4, 'Rectangles must contain 4 coordinates'
AssertionError: Rectangles must contain 4 coordinates

print(normalize_rectangle( (4.0, 2.0, 1.0, 5.0) )) # X axis inverted

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "rectangle1.py", line 4, in normalize_rectangle
    assert x0 < x1, 'Invalid X coordinates'
AssertionError: Invalid X coordinates

The post-conditions help us catch bugs by telling us when our calculations cannot have been correct. For example, if we normalize a rectangle that is taller than it is wide everything seems OK:

print(normalize_rectangle( (0.0, 0.0, 1.0, 5.0) ))

(0, 0, 0.2, 1.0)

but if we normalize one that’s wider than it is tall, the assertion is triggered:

print(normalize_rectangle( (0.0, 0.0, 5.0, 1.0) ))

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "rectangle1.py", line 17, in normalize_rectangle
    assert 0 < upper_y <= 1.0, 'Calculated upper Y coordinate invalid'
AssertionError: Calculated upper Y coordinate invalid

Re-reading our function, we realize that line 10 should divide dy by dx rather than dx by dy. If we had left out the assertion at the end of the function, we would have created and returned something that looked like a valid answer, but wasn’t; detecting and debugging that would almost certainly have taken more time in the long run than writing the assertion.

But assertions aren’t just about catching errors: they also help people understand programs. Each assertion gives the person reading the program a chance to check (consciously or otherwise) that their understanding matches what the code is doing.

Most good programmers follow two rules when adding assertions to their code.

• The first is, “fail early, fail often”. The greater the distance between when and where an error occurs and when it’s noticed, the harder the error will be to debug, so good code catches mistakes as early as possible.

• The second rule is, “turns bugs into assertions or tests”. If you made a mistake in a piece of code, the odds are good that you have made other mistakes nearby, or will make the same mistake (or a related one) the next time you change it. Writing assertions to check that you haven’t regressed (i.e., haven’t re-introduced an old problem) can save a lot of time in the long run, and helps to warn people who are reading the code (including your future self) that this bit is tricky.