More on testing

This section assumes that you’re familiar with basic testing and describes how you can get more from deal testing mechanisms.

Finding memory leaks

Sometimes, when a function is completed, it leaves in memory other objects except result. For example:

cache = {}
User = dict

def get_user(name: str) -> User:
    if name not in cache:
        cache[name] = User(name=name)
    return cache[name]

Here, get_user creates a User object and stores it in a global cache. In this case, this “leak” is a desired behavior and we don’t want to fight it. This is why we can’t a tool (or something right in the Python interpreter) that catches and reports such behavior, it would have too many false-positives.

However, things are different with pure functions. A pure function can’t store anything on a side because it is a side effect. The result of a pure function is only what it returns.

The command memtest uses this idea to find memory leaks in pure functions. How it works:

  1. It finds all pure functions (as test does).

  2. For every function:

    1. It makes memory snapshot before running the function.

    2. It runs the function with different autogenerated input arguments (as test command does) without running contracts and checking the return value type (to avoid side-effects from deal itself).

    3. It makes memory snapshot after running the function.

    4. Snapshots “before” and “after” are compared. If there is a difference it will be printed.

The return code is equal to the amount of functions with memory leaks.

If the function fails, the command will ignore it and still test the function for leaks. Side-effects shouldn’t happen unconditionally, even if the function fails. If you want to find unexpected failures, use test command instead.

Constant value for arguments

The deal.cases constructor accepts kwargs argument where you can specify constant values for the function arguments. For example:

def div(a: int, b: int) -> float:
    assert a == 1
    return a / b

# Every test case calls `div` function with `a=1`.
# So, random values are generated only for `b`.
@deal.cases(div, kwargs=dict(a=1))
def test_div(case):

Custom strategies

Under the hood, deal.cases uses hypothesis testing framework to generate test cases. The trick is that kwargs argument of deal.cases can contain hypothesis strategies:

import hypothesis.strategies as st

def div(a: int, b: int) -> float:
    assert a >= 10
    return a / b

cases = deal.cases(
for case in cases:

Reproducible failures

Argument seed of deal.cases is a random seed. That means, for the same seed value you always get the same test cases. It is a must-have thing for CI. There are a few important things:

  • If the seed is different for different pipelines, it will run a bit different test cases every time which increases your chances to find a tricky corner case.

  • If the seed is the same when you re-run the same job, it will help you to identify flaky tests. In other words, if a CI job fails, you re-run it, and it passes, it was a flaky test which happens because of tricky side-effects (database connection failed, another test changed a state etc.) and it will be hard to reproduce. However, if re-run fails with the same error, most probably it is a failure that you can easily reproduce and debug locally, knowing the seed.

  • The seed should be shown in the CI job to make it possible to use it locally to reproduce a failure. It is either printed in the job output or is something known like the pipeline run number.

There is an example for GitLab CI:

import os
import deal

seed = None
if os.environ.get('CI_PIPELINE_ID'):
    seed = int(os.environ['CI_PIPELINE_ID'])

@deal.cases(div, seed=seed)
def test_div(case):


Fuzzer is when an external tool or library (fuzzer) generates a bunch of random data in hope to break your program. That means, fuzzing requires a lot of resources and is performance-critical. This is why most of the fuzzers are written on C. However, there are few Python wrappers for existing fuzzers to simplify fuzzing for Python functions:

The deal.cases object can be used as a target for any fuzzer.


import atheris

test = deal.cases(div)
atheris.Setup([], test)


from pythonfuzz.main import PythonFuzz

test = deal.cases(div)

See fuzzing_atheris and fuzzing_pythonfuzz examples for the full code.

Iteration over cases

The deal.cases object can be used not only as a function or decorator but also as an iterable. On iteration, it emits the test cases, so you can have more control over what and when to run:

for case in deal.cases(div):

However, in this case deal doesn’t know which cases have failed and can’t provide that information back into hypothesis for shrinking (finding the smallest example to reproduce a failure). So while it is the same as decorator when everything is fine, it will provide a bit uglier report on failure.

Mixing with Hypothesis

Under the hood, deal uses hypothesis to generate test cases. So, we can mix deal.cases with hypothesis decorators. The only exception is hypothesis.settings which should be passed into deal.cases as settings argument because hypothesis doesn’t support application of settings twice but deal applies its own default settings.

See using_hypothesis example.