# Python / Contract-Driven Development

Let’s take for example an incredibly simple code and imagine that it’s incredibly complicated logic.

def cat(left, right):
"""Concatenate two given strings.
"""
return left + right


## Tests

How can we be sure this code works? No, it’s not obvious. Remember the rules of the game, we have an incredibly complicated realization. So, we can’t say it works or not while we haven’t tested it.

def test_cat():
result = cat(left='abc', right='def')
assert result == 'abcdef'



Now, run pytest:

pytest cat.py


It passes. So, our code works. Right?

## Table tests

Wait, but what about corner cases? What if one string is empty? What if both strings are empty? What if we have only one character in both strings? We need check more values and this is where table driven tests will save our time. In pytest, we can use @pytest.mark.parametrize to make such tables.

import pytest

@pytest.mark.parametrize('left, right, expected', [
('a', 'b', 'ab'),
('', '', ''),
('', 'b', 'b'),
('a', '', 'a'),
('text', 'check', 'textcheck'),
])
def test_cat(left, right, expected):
result = cat(left=left, right=right)
assert result == expected


## Properties

Table tests can be enormously long, and for every test case, we have to manually calculate the expected result. For complicated code, it’s a lot of work. Can we do it better and think and write less? Yes, we can instead of expected result talk about expected properties of the result. The big difference is the result is different for different input values, but properties always the same. The coolest thing is in most cases you already know result properties, it is the business requirements, and your code is no more than the implementation of these requirements.

So, what are the properties of our function?

1. The result string starts with the first given string.
2. The result string ends with the second given string.
3. Result string has the length equal to the sum of lengths of given strings.

Now, we can check these properties for the result instead of checking particular values.

@pytest.mark.parametrize('left, right', [
('a', 'b'),
('', ''),
('', 'b'),
('a', ''),
('text', 'check'),
])
def test_cat(left, right):
result = cat(left=left, right=right)
assert result.startswith(left)
assert result.endswith(right)
assert len(result) == len(left) + len(right)


## Hypothesis

We’ve tested a few corner cases but not all of them. What about Unicode strings? What if one string is Unicode, but another one isn’t? What about spaces? What if we have a string termination symbol somewhere? What if both strings contain only digits (the place where JS always surprises)? It’s so hard to find examples for all possible cases where something can go wrong. In theory, you even can’t say that it works while you haven’t checked all possible values (that impossible even for our simple function). So, instead of trying to figure out all possible nightly values we can ask the machine to do so. This is where the property-based testing comes in. In Python, we have a great tool hypothesis that can generate test examples for us:

import hypothesis
from hypothesis import strategies

@hypothesis.given(left=strategies.text(), right=strategies.text())
def test_cat(left, right):
result = cat(left=left, right=right)
assert result.startswith(left)
assert result.endswith(right)
assert len(result) == len(left) + len(right)


## Type annotations

Another one cool thing in Python we have to talk about before moving further is type annotations:

def cat(left: str, right: str) -> str:
return left + right


Type annotations aren’t perfect and can be too complicated. However, what is most important is now humans and machines know much more about your code. You can run mypy and check that you haven’t made type errors. And the thing is it’s not only about catching type errors. Now we can use hypothesis-auto wrapper around hypothesis. It will infer parameters types and explain names and types of parameters to Hypothesis. So, instead of writing hypothesis.given(left=strategies.text(), right=strategies.text()) we can just say hypothesis_auto.auto_pytest(cat).

import hypothesis_auto

@hypothesis_auto.auto_pytest(cat)
def test_cat(test_case):
result = test_case()
left = test_case.parameters.kwargs['left']
right = test_case.parameters.kwargs['right']
assert result.startswith(left)
assert result.endswith(right)
assert len(result) == len(left) + len(right)


It looks longer because now parameters are placed inside the long name test_case.parameters.kwargs but the most important thing here is we don’t talk about function inputs at all, the machine does everything. The test isn’t about any values of the function anymore but only about the function properties.

## Contracts

Can we make it even simpler? Not really. The implementation can produce some values, and the machine can infer some properties of the result. However, someone else must say which properties are good and expected, and which are not. However, there is something else about our properties that we can do better. At this stage we have type annotations and, to be honest, they are just kind of properties. Annotations say “the result is a text”, and our test properties clarify the length of the result, it’s prefix and suffix. However, the difference is type annotations are the part of the function itself. It gives some benefits:

1. The machine can check statically, without the actual running of the code.
2. The human can see types (think “possible values set”) for arguments and the result.

Package deal is the thing that can make the same for our properties.

import deal

@deal.ensure(lambda left, right, result: result.startswith(left))
@deal.ensure(lambda left, right, result: result.endswith(right))
@deal.ensure(lambda left, right, result: len(result) == len(left) + len(right))
def cat(left: str, right: str) -> str:
return left + right


Now, it’s not just properties, but contracts. They can be checked in the runtime, simplify tests, tell humans about the function behavior. And tests for this implementation are incredibly trivial:

@pytest.mark.parametrize('case', deal.cases(cat))
def test_cat(case):
case()


You can read more about writing tests for contracted code in the deal documentation.

## Contracts for machines

The most exciting thing is deal can check contracts statically, like mypy checks annotations. However, contracts can be any code while types are standardized and limited. Although the machine can’t check everything (yet), it can catch some trivial cases. For example:

@deal.post(lambda result: 0 <= result <= 1)
def sin(x):
return 2


And when we run deal linter on this code, we see contract violation error:

❯ flake8 --show-source sin.py
sin.py:6:5: DEAL011: post contract error
return 2
^