Quality Assurance

Back in Development Environment we suggested a common kind of project organization. There can be a src directory with the final application and a tests directory with the unit test cases.

This works out extremely well because a lot of Python tools expect a structure similar to this.

Because there are some variations, we need to include

export PYTHONPATH=src

Or, as an alternative, we can set this each time we execute the tests.

PYTHONPATH=src pytest

We don’t need to provide very many options to the pytest program because the default directory for tests, tests is what we set up for outselves.

In this chapter, we’ll look at several other parts of test automation:

  1. Static Type Checking with mypy.

  2. Static Code Quality Checks with pylint.

  3. Code Formatting with black.

  4. Test Coverage Analysis with coverage.

  5. Test Automation suite with tox.

The idea behind this suite of tools it to show a number of common practices that can be applied to this application.

Static Type Checking

We’ve annotated all of the descriptions of classes and methods with type hints. These are relatively new to Python. The hints have no run-time impact; they’re used for static analysis of the code to provide confidence it’s likely to work. See http://mypy-lang.org for more information on this tool.

If it’s not already installed, the mypy tool can be installed with

conda install mypy

The tool is used like this:

mypy src/blackjack.py

The output will often include numerous nuanced issues where the type hints on a parameter to a method don’t precisely match the type hints on the return type of another method.

Some Common Problems

One of the more common problems is reconciling the subclass-superclass features with methods actually used. Python duck typing rules mean any object will be searched for the expected attribute, either instance variable or method. The mypy tool looks for evidence the attribute or method is likely to be present.

Consider the following class definition

11 class Wheel_RNG:
12     def __init__(self, bins: List[Bin], rng: random.Random=None) -> None:
13         self.bins = bins
14         self.rng = rng or random.Random()
16     def choose(self) -> Bin:
17         return self.rng.choice(self.bins)

We’ve defined the second parameter, rng, to be an instance of random.Random. The default value, however, is None.

We’ll get the following kind of error:

code/wheel_examples.py:12: error: Incompatible default for argument "rng" (default has type "None", argument has type "Random")

This kind of thing requires us to be much more explicit in our statements of what the data type is.

We need to change the type hint of rng: random.Random=None. This needs to be rng: Optional[random.Random]=None to more correctly state what we expect for the parameter to this method.

Additionally, some assignment statements can be ambiguous. In this cases, we may need to add a type hint to an assignment statement.

some_dict: DefaultDict[str, int] = collections.defaultdict(int)

The collections.defaultdict() function only includes a function to create a value to be created when a key is not found. It doesn’t describe the type of the value or the type of the keys. The mypy tool can examine the int() function to determine the type of value returned. There’s no suggestion about what the keys are, and Python doesn’t really need any suggestion.

In order to be confident, we need to provide a hint, using the typing.DefaultDict type definition. This definition lets us provide a key type and a value type. With this additional information, the mypy tool can confirm that the some_dict variable is used properly.

Static Code Quality Checks

The pylint tool does some checks for “style” issues. It can also check a number of programming techniques that – while legal – are likely to be a code smell. See https://www.pylint.org for the complete list of checks that can be performed.

We install this with conda, also.

conda install pylint

We run this to get a list of possible problems with the source code of the module.

 pylint code/blackjack.py
************* Module code.blackjack
code/blackjack.py:17:0: C0115: Missing class docstring (missing-class-docstring)
code/blackjack.py:35:4: C0103: Attribute name "hardValue" doesn't conform to snake_case naming style (invalid-name)
code/blackjack.py:35:4: C0116: Missing function or method docstring (missing-function-docstring)
code/blackjack.py:39:4: C0103: Attribute name "softValue" doesn't conform to snake_case naming style (invalid-name)
code/blackjack.py:39:4: C0116: Missing function or method docstring (missing-function-docstring)
code/blackjack.py:49:4: C0116: Missing function or method docstring (missing-function-docstring)
code/blackjack.py:50:8: C0103: Variable name "s" doesn't conform to snake_case naming style (invalid-name)
code/blackjack.py:56:8: C0103: Variable name "r" doesn't conform to snake_case naming style (invalid-name)
code/blackjack.py:81:0: C0115: Missing class docstring (missing-class-docstring)
code/blackjack.py:99:0: C0115: Missing class docstring (missing-class-docstring)
code/blackjack.py:117:0: C0116: Missing function or method docstring (missing-function-docstring)

Your code has been rated at 8.38/10

This outout lists a number of problems that are either omissions of docstrings or variables with names that don’t follow the typical Python style, called “snake_case”. The names hardValue and softValue should be hard_value and soft_value.

This a pervasive change to the code examples throughout this book. We haven’t made the change, but the reader can consider revising their own code to reduce the number of pylint problems.

The rating, 8.38/10, clearly shows room for improvement. Adding the required docstrings, for example, would lead to a dramatic improvement in the quality metric.

Code Formatting

One way to achieve consistent and easy-to-read code is to use a code formatting tool. One of the more popular tools is black. This tool is described as “uncompromising.” It has relatively few options and produces consistent, easy-to-read code.

conda install black

The tool will simply reformat any valid Python to fit the recommended style. The black tool can also be used to confirm that a module is already formatted correctly. This becomes a kind of testing tool to confirm the formatting before doing a Git checkin, or starting a long integration and deployment process.

We can use it to reformat an entire directory tree like this:

black src

That’s all there is to it. It provides a very small summary of what it did. This is delightfully simple and creates consistent-looking source files.

Test Coverage Analysis

Generally, we want test cases to confirm the application under test will work. It’s difficult writing a good application, and it’s difficult writing good test cases. One of the key questions is how much of the software is covered by test cases. Ideally, a vague “all of the software” has been tested. The difficult problem is defining what “all” means.

There are a number of ways to measure software and decide if “all” of it has been tested.

  • Tests execute code in all of the modules.

  • Tests execute code in all of the classes.

  • Tests execute code in all of the methods of all of the classes.

  • Test execute all of the statements.

  • Tests evaluate both true and false for each condition expressed in an if, elif, or while statement; the implicit condition of each else, so each each exception in a try statement; also a test for each for statement. These are the places were there’s a choice among statements to be executed.

  • Tests exercise each possible combination of the conditional processing choices (if, elif, else, etc.)

Clearly, some of these don’t test very much, and some are quite complex to design.

It’s very easy to measure whether or not a test touches all of the statements. We’ll focus on this.

The Test Coverage Tool

Since we’re using pytest, we can install the pytest-cov plug-in. This will, in turn, install the coverage tool.

conda install pytest-cov

To use the coverage tool, we need to provide a top-level module (or package of modules) for which we’d like coverage metrics.

PYTHONPATH=src pytest --cov=src

Here’s what the output looks like:

================================= test session starts =================================
platform darwin -- Python 3.7.4, pytest-5.0.1, py-1.8.0, pluggy-0.13.0
rootdir: /Users/slott/Documents/Writing/Technical/building-skills-oo-design-book/demo
plugins: cov-2.7.1
collected 4 items

tests/test_hw_1.py ..                                                           [ 50%]
tests/test_hw_2.py ..                                                           [100%]

---------- coverage: platform darwin, python 3.7.4-final-0 -----------
Name        Stmts   Miss  Cover
src/hw.py      11      1    91%

============================== 4 passed in 0.06 seconds ===============================

The pytest tool tells us it’s running two tests, test_hw_1.py and test_hw_2.py. The final coverage report says the 91% of the code has been exercised.

What’s missing?

We an add the --cov-report=term-missing option to see which lines were not tested. Here’s the revised command.

PYTHONPATH=src pytest --cov=src --cov-report=term-missing

Here’s the revised coverage report

---------- coverage: platform darwin, python 3.7.4-final-0 -----------
Name        Stmts   Miss  Cover   Missing
src/hw.py      11      1    91%   25

Line 25 is not exercised by the test. What is this difficult-to-test line?

24 if __name__ == "__main__":
25     main()

Line 25 is very hard to test with conventional unit testing approaches. It’s not impossible, but it’s difficult and the extra effort doesn’t create a lot of value.

We can add a special # pragma: no cover comment to tell the coverage tool to apply this “fact” or “pragma” to the line in question. This improves the coverage score and adds a comment informing everyone of what line was skipped.

Test Automation

We have a large suite of testing tools to provide real confidence in the quality of the application programming. It can seem daunting to keep all of the tools straight. The tox tool, however, gives us a path forward. We can use tox to run our suite of static and dynamic tests.

We’ll use pip to install tox.

python -m pip install tox

This is required because tox is not in any of the common respositories searched by conda.

Once we have tox installed, we need to create a configuration file. This can be called pyproject.toml or tox.ini. The content of the file will look like the following example:

skipsdist = True
requires =

ignore_errors = True
deps =
setenv =
commands =
    mypy src
    pylint src
    black src
    pytest --doctest-modules src
    pytest --cov=src

The first section, [tox] is general information about tox. The skipsdist avoids building a source distribution kit. We’re not going to upload this code to the Python Package Index (https://pypi.org) sometimes called the “Cheese Shop.” Since we’re not building a source distribution, we don’t need some of the overheads associated with that step.

The main section, [testenv] is repeated for each distinct test environment. In this case, we only need one test environment, so we don’t have a complex configuration. We’ve provided four distinct configuration values:

  • The ignore_errors makes sure the testing continues even if an error in one step has been found. This lets us run the whole suite of tests, then fix all of the problems.

  • The deps defines the dependencies required to run the tests. We’ve listed specific versions of each tool.

  • THe setenv defines the environment variables to provide when running the tests. The default setup provides very few environment settings. Because our application is located in a src directory, we need this to be on the PYTHONPATH environment variable.

  • The commands is a sequence of commands to execute. There are strict limitations on these commands. This allows the tox tool to run in a wide variety of environments and remain perfectly consistent.

To run tox, we use the command


If we’re in the same working directy as the tox.ini (or pyproject.toml) file, then tox will execute each of the commands, giving us a detailed view of how well our application passes the suite of quality assurance tests.

The use of a tool like tox makes test execution simple and consistent. It gives us a lot of help in producing useful, trustworthy, reliable software.

Looking Forward

In the next chapter we’ll look at what’s required to create a final, command line interface (CLI) to run the simulation command with various options and arguments.