Deal can automatically test your functions. First of all, your function has to be prepared:

  1. All function arguments are type-annotated.

  2. All exceptions that function can raise are specified in deal.raises.

  3. All preconditions are specified with deal.pre.

@deal.pre(lambda a, b: a >= 0 and b >= 0)
def div(a: int, b: int) -> float:
    return a / b

Then you can use deal.cases to generate test cases for the function. This is a polimorphic object that can be used in many ways.

Here is the shortest way to create a test:

test_div = deal.cases(div)

It is enough for pytest to find and run the test. Or you can run it manually by just calling it: test_div().

However, it is not scalable. What if we want to use a pytest fixture? What if we need to prepare something before running the test case? Or what if we want to check additional conditions? So, let’s make a proper test function:

# type annotations below are optional
def test_div(case: deal.TestCase) -> None:

In this example, when we (or pytest) call test_div(), deal will generate test cases (using hypothesis) and run the function body for every case. The test function itself decides when to execute the test case. Here case is an instance of deal.TestCase class. This form works as expected with pytest fixtures if the test case is the first argument and has the name case.


There is a CLI command named test. It extracts deal.pure and @deal.has() (without arguments) wrapped functions and runs deal.cases powered tests for it.

python3 -m deal test project/*.py

The command is helpful when you don’t have tests for some pure functions yet but want to get an early feedback.

For every ran function, deal calculates and shows coverage. This is a helpful indication on how good deal was at finding the correct input values.

test command output


Specify samples count (50 by default):

deal.cases(div, count=20)

Explicitly specify arguments to pass into the function:

deal.cases(div, kwargs=dict(b=3))

See deal.cases API documentation and More on testing for details.

Practical example

The best case for Contract-Driven Development is when you have a clear business requirements for part of code. Write these requirements as contracts, and then write a code that satisfy these requirements.

In this example, we will implement index_of function that returns index of the given element in the given list. Let’s think about requirements:

  1. Function accepts list of elements (let’s talk about list of integers), one element, and returns index.

  2. Result is in range from zero to the length of the list.

  3. Element by given index (result) is equal to the given element.

  4. If there are more than one matching element in the list, we’ll return the first one.

  5. If there is no matching elements, we’ll raise LookupError.

And now, let’s convert it from words into the code:

from typing import List, NoReturn
import deal

# if you have more than 2-3 contracts,
# consider moving them from decorators into separate variable
# like this:
contract_for_index_of = deal.chain(
    # result is an index of items
    deal.post(lambda result: result >= 0),
    deal.ensure(lambda items, item, result: result < len(items)),
    # element at this position matches item
        lambda items, item, result: items[result] == item,
        message='invalid match',
    # element at this position is the first match
        lambda items, item, result: not any(el == item for el in items[:result]),
        message='not the first match',
    # LookupError will be raised if no elements found
    deal.reason(LookupError, lambda items, item: item not in items),
    # no side-effects

Now, we can write a code that satisfies our requirements:

def index_of(items: List[int], item: int) -> int:
    for index, el in enumerate(items):
        if el == item:
            return index
    raise LookupError

And tests, after all, the easiest part. Let’s make it a little bit interesting and in the process show all valid samples:

# test and make examples
@deal.cases(index_of, count=1000)
def test_div(case):
    # run test case
    result = case()
    if result is not NoReturn:
        # if no exceptions was raised, print the result
        print(f"index of {case.kwargs['item']} in {case.kwargs['items']} is {result}")