Reading and writing text to files in Python

Intermediate

You have written enough test_something functions to last a lifetime. Your test files are full of names like test_user_creation_when_email_is_valid_and_age_is_over_18 — forty characters of snake_case just to describe one behaviour. When a test fails, the output tells you the function name but not what the test was actually checking. You end up reading the function body to figure it out, which defeats the point of a test name entirely.

Python’s ward library takes a different approach. Instead of naming test functions, you describe them — passing a plain English string to the @test decorator. When a test fails, ward prints exactly what it was supposed to do, in the language you wrote it, not a slug-cased identifier. The library also ships with a fixture system built on function arguments, a fluent expect() assertion API, and parameterized tests that read like a table of examples. Install it with pip install ward — no extra dependencies.

This article walks you through everything you need to use ward effectively. We will cover writing your first tests with the @test decorator, organizing shared setup with @fixture, running parameterized test cases, using the expect() assertion chain, and building a real-world test suite for a small utility library. By the end, you will have a working test suite and a clear picture of where ward fits against pytest.

Writing a ward Test: Quick Example

Before diving into the details, here is a minimal ward test file you can run right now. Create a file called test_math.py and paste in the following:

# test_math.py
from ward import test

@test("adding two positive integers returns their sum")
def _():
    assert 1 + 2 == 3

@test("dividing by zero raises ZeroDivisionError")
def _():
    try:
        _ = 10 / 0
        assert False, "should have raised"
    except ZeroDivisionError:
        pass

Run it with:

ward

Output:

  PASS  test_math  adding two positive integers returns their sum
  PASS  test_math  dividing by zero raises ZeroDivisionError

2 passed in 0.02 seconds

Two things stand out immediately. First, every test function is named _ — the name is irrelevant because ward uses the string you pass to @test instead. Second, the output reads like a sentence, not a mangled identifier. Those two changes alone make failing tests much easier to diagnose at a glance.

The sections below explain each ward feature in depth and show you how to apply them to realistic code.

What Is ward and Why Use It?

Ward is a Python testing framework designed to make test code more readable and test output more useful. It was created as an alternative to pytest and unittest, borrowing the best ideas from both while rethinking the parts that have always felt awkward — especially test naming and fixture injection.

The central idea is that a test is a fact you assert about your code, and that fact deserves to be written in human language. In pytest you write def test_cart_total_is_zero_when_no_items_added():. In ward you write @test("cart total is zero when no items are added"). The string is the documentation; the function body is the proof.

Here is how ward compares to pytest on the features most Python developers care about:

Feature	pytest	ward
Test description	Function name (snake_case)	Plain English string
Fixtures	@pytest.fixture	@fixture with argument injection
Parameterization	@pytest.mark.parametrize	@using with each()
Assertions	assert (with rewriting)	assert or fluent expect()
Output format	Dots, F, E characters + tracebacks	Coloured PASS/FAIL lines with descriptions
Installation	pip install pytest	pip install ward

Ward is not trying to replace pytest in every project — it is a deliberate choice that pays off most when you want test output to function as living documentation. If you share test runs with non-developers or treat CI output as a changelog, ward’s readable output earns its place immediately.

Installing ward

Ward requires Python 3.6 or later. Install it into your project’s virtual environment:

# install.sh
pip install ward

Verify the installation:

# verify_install.py
import ward
print(ward.__version__)

Output:

0.68.0b0

Ward discovers tests automatically. By default it searches for any file matching the pattern test_*.py in the current directory and its subdirectories — the same convention used by pytest. You can override the search path:

# Run all tests in a specific directory
ward --path tests/

# Run tests matching a keyword in their description
ward --search "total"

# Run tests in a single file
ward --path tests/test_cart.py

There is no configuration file required to get started. For larger projects, ward reads from a pyproject.toml [tool.ward] section if one exists.

Writing Tests with @test

The @test decorator is the foundation of every ward test suite. It takes a single string argument that describes what the test is verifying. The decorated function’s name is ignored entirely — the convention is to name every test function _ to make that clear.

# test_string_utils.py
from ward import test

def shout(text):
    """Convert text to uppercase with an exclamation mark."""
    return text.upper() + "!"

@test("shout converts text to uppercase")
def _():
    assert shout("hello") == "HELLO!"

@test("shout appends an exclamation mark")
def _():
    result = shout("ward")
    assert result.endswith("!")

@test("shout works on an already uppercase string")
def _():
    assert shout("PYTHON") == "PYTHON!"

Output:

  PASS  test_string_utils  shout converts text to uppercase
  PASS  test_string_utils  shout appends an exclamation mark
  PASS  test_string_utils  shout works on an already uppercase string

3 passed in 0.01 seconds

When a test fails, ward shows you the description, the file and line number, and the values that caused the failure. You get the “what failed” and “what were the values” without reading the traceback from the bottom up. That is a significant quality-of-life improvement when you have a large test suite and a CI run with dozens of failures.

Shared Setup with @fixture

Most tests need shared setup — a database connection, a sample data structure, or a configured object. Ward handles this with the @fixture decorator, which works very similarly to pytest fixtures. You declare a fixture function, then inject it into your test by using the fixture function as a default argument.

# test_user.py
from ward import test, fixture

@fixture
def sample_user():
    return {
        "name": "Alice",
        "email": "alice@example.com",
        "age": 25,
        "active": True,
    }

@test("user name is a string")
def _(user=sample_user):
    assert isinstance(user["name"], str)

@test("user age is a positive integer")
def _(user=sample_user):
    assert user["age"] > 0

@test("inactive users cannot be created with this fixture")
def _(user=sample_user):
    assert user["active"] is True

Output:

  PASS  test_user  user name is a string
  PASS  test_user  user age is a positive integer
  PASS  test_user  inactive users cannot be created with this fixture

3 passed in 0.01 seconds

The fixture function runs fresh for each test that uses it — there is no shared state between tests unless you explicitly use a module-level or session-level scope. To add teardown logic, use yield inside the fixture:

# test_file_fixture.py
import os
import tempfile
from ward import test, fixture

@fixture
def temp_file():
    # Setup: create a temporary file
    fd, path = tempfile.mkstemp(suffix=".txt")
    os.close(fd)
    with open(path, "w") as f:
        f.write("hello ward")
    yield path
    # Teardown: delete the file after the test
    if os.path.exists(path):
        os.remove(path)

@test("temp file exists on disk")
def _(path=temp_file):
    assert os.path.isfile(path)

@test("temp file contains the expected text")
def _(path=temp_file):
    with open(path) as f:
        content = f.read()
    assert content == "hello ward"

Output:

  PASS  test_file_fixture  temp file exists on disk
  PASS  test_file_fixture  temp file contains the expected text

2 passed in 0.03 seconds

The code after yield runs automatically after each test that uses temp_file. Ward handles the teardown even if the test throws an exception, so you do not need try/finally blocks in your test functions.

Parameterized Tests with @using and each()

Parameterized tests let you run the same logic against multiple inputs without duplicating code. Ward’s approach is cleaner than pytest’s marker syntax — you use the @using decorator combined with the each() helper to define the value sets.

# test_math_ops.py
from ward import test, each, using

def clamp(value, lo, hi):
    """Clamp value to the range [lo, hi]."""
    return max(lo, min(hi, value))

@test("clamp({value}, {lo}, {hi}) returns {expected}")
@using(
    value=each(5, -3, 100, 50),
    lo=each(0, 0, 0, 0),
    hi=each(10, 10, 10, 10),
    expected=each(5, 0, 10, 10),
)
def _(value, lo, hi, expected):
    assert clamp(value, lo, hi) == expected

Output:

  PASS  test_math_ops  clamp(5, 0, 10) returns 5
  PASS  test_math_ops  clamp(-3, 0, 10) returns 0
  PASS  test_math_ops  clamp(100, 0, 10) returns 10
  PASS  test_math_ops  clamp(50, 0, 10) returns 10

4 passed in 0.01 seconds

Notice that ward interpolates the parameter values into the test description for each run. When one case fails, the output tells you exactly which input caused the problem — clamp(100, 0, 10) returns 10 is far more useful than test_clamp_params[2].

The each() values are matched positionally — the first value from every each() forms the first test case, the second values form the second, and so on. All each() calls in one @using decorator must have the same length.

Fluent Assertions with expect()

Ward provides an optional fluent assertion API called expect(). Instead of writing bare assert statements, you chain methods that read like English and produce more specific error messages when they fail. Use whichever style you prefer — both work the same way under the hood.

# test_expect.py
from ward import test, expect

@test("expect: integer is within range")
def _():
    expect(42).to_be_greater_than(40)
    expect(42).to_be_less_than(50)

@test("expect: string contains a substring")
def _():
    expect("pythonhowtoprogram").to_contain("python")

@test("expect: list has expected length")
def _():
    items = ["a", "b", "c"]
    expect(items).has_length(3)

@test("expect: dictionary contains a key")
def _():
    config = {"debug": True, "timeout": 30}
    expect(config).to_contain_key("debug")

@test("expect: value is an instance of a type")
def _():
    expect(3.14).to_be_instance_of(float)

Output:

  PASS  test_expect  expect: integer is within range
  PASS  test_expect  expect: string contains a substring
  PASS  test_expect  expect: list has expected length
  PASS  test_expect  expect: dictionary contains a key
  PASS  test_expect  expect: value is an instance of a type

5 passed in 0.02 seconds

When an expect() assertion fails, ward reports both the expected condition and the actual value on separate lines. For example, if expect(15).to_be_greater_than(40) fails, the output reads Expected 15 to be greater than 40 — no need to open the test file and read the assert statement.

The expect() API supports chaining, so you can write several assertions about the same value in sequence. This is useful when a single test function validates multiple properties of one object, and you want each failure to be described independently.

Real-Life Example: Testing a Shopping Cart

Let us build a test suite for a simple shopping cart module using everything covered above: fixtures for shared state, parameterized tests for pricing logic, and expect() for readable assertions.

First, the module under test:

# cart.py
class Cart:
    """A simple shopping cart with item management and total calculation."""

    def __init__(self):
        self._items = {}  # name -> {"price": float, "qty": int}

    def add(self, name, price, qty=1):
        if name in self._items:
            self._items[name]["qty"] += qty
        else:
            self._items[name] = {"price": price, "qty": qty}

    def remove(self, name):
        self._items.pop(name, None)

    def total(self, tax_rate=0.0):
        subtotal = sum(v["price"] * v["qty"] for v in self._items.values())
        return round(subtotal * (1 + tax_rate), 2)

    def item_count(self):
        return sum(v["qty"] for v in self._items.values())

    def is_empty(self):
        return len(self._items) == 0

Now the ward test suite:

# test_cart.py
from ward import test, fixture, expect, each, using
from cart import Cart

# ----- Fixtures -----

@fixture
def empty_cart():
    return Cart()

@fixture
def stocked_cart():
    c = Cart()
    c.add("apple", price=0.99, qty=3)
    c.add("bread", price=2.49)
    c.add("milk", price=1.75, qty=2)
    return c

# ----- Basic behaviour -----

@test("a new cart is empty")
def _(cart=empty_cart):
    expect(cart.is_empty()).to_be_truthy()
    expect(cart.item_count()).equals(0)

@test("adding an item increases item count")
def _(cart=empty_cart):
    cart.add("apple", price=0.99, qty=4)
    expect(cart.item_count()).equals(4)

@test("adding the same item twice increases quantity not entry count")
def _(cart=empty_cart):
    cart.add("apple", 0.99, qty=1)
    cart.add("apple", 0.99, qty=2)
    expect(cart.item_count()).equals(3)

@test("removing an item reduces total")
def _(cart=stocked_cart):
    before = cart.total()
    cart.remove("bread")
    expect(cart.total()).to_be_less_than(before)

# ----- Total calculation (parameterized) -----

@test("total with {rate*100:.0f}% tax is {expected}")
@using(
    rate=each(0.0, 0.1, 0.2),
    expected=each(9.46, 10.41, 11.35),
)
def _(cart=stocked_cart, rate=None, expected=None):
    # stocked: 3*0.99 + 1*2.49 + 2*1.75 = 9.46 subtotal
    expect(cart.total(tax_rate=rate)).equals(expected)

Output:

  PASS  test_cart  a new cart is empty
  PASS  test_cart  adding an item increases item count
  PASS  test_cart  adding the same item twice increases quantity not entry count
  PASS  test_cart  removing an item reduces total
  PASS  test_cart  total with 0% tax is 9.46
  PASS  test_cart  total with 10% tax is 10.41
  PASS  test_cart  total with 20% tax is 11.35

7 passed in 0.03 seconds

The test suite demonstrates three things at once: the stocked_cart fixture gives each test a fresh, pre-populated cart without any shared mutation; the parameterized tax tests cover three pricing scenarios without duplicated code; and the expect() calls make each assertion readable without a comment explaining what is being checked. To extend the example, try adding tests for negative quantities, zero-price items, or a discount() method — the fixture and parameterization patterns scale without any structural changes.

Frequently Asked Questions

Can ward tests coexist with pytest tests in the same project?

Yes, but they run in separate commands. Ward discovers test_*.py files using the same convention as pytest, but the two runners are independent — running ward only executes tests decorated with @test, and running pytest only executes functions prefixed with test_. If you are migrating gradually, you can keep both frameworks active during the transition. Ward can also use pytest fixtures defined in a conftest.py file, which eases migration considerably.

How do fixture scopes work in ward?

Ward fixtures support four scopes: Scope.Test (default, reruns for every test), Scope.Module (reruns once per test file), Scope.Global (reruns once per entire test run), and Scope.Call (same as Test). Set the scope with @fixture(scope=Scope.Module). Use module or global scope for expensive operations like database connections or API clients that you do not want to recreate thousands of times.

Should I use expect() or plain assert statements?

Both work correctly and produce useful output. The expect() API gives you method names that double as documentation — expect(value).to_be_greater_than(10) is self-explanatory even in a code review. Plain assert is faster to type and feels more natural if you are coming from pytest. A common middle ground: use assert for straightforward equality checks and expect() when comparing ranges, types, or collection membership where the error message matters most.

How do I run a single test or a subset of tests?

Use the --search flag to filter by description substring: ward --search "cart total" will run every test whose description contains “cart total”. You can also use --path tests/test_cart.py to restrict the discovery scope to a single file. For more precise control, ward supports tags via @test("...", tags=["slow"]) combined with ward --tags slow on the command line.

Does ward support async tests?

Yes. Ward automatically detects async def test functions and fixture functions and runs them using asyncio.run(). You do not need any plugin or marker — just write async def _(): and ward handles the event loop. This works the same way for fixtures, so an async fixture can await a database connection and yield it to an async test without any additional setup.

How do I get code coverage with ward?

Run ward under coverage using the standard coverage.py wrapper: coverage run -m ward. After the run completes, generate the report with coverage report or coverage html for a browsable HTML breakdown. Ward does not bundle its own coverage tool — it relies on coverage.py, which is the same approach pytest recommends and integrates cleanly with CI pipelines and codecov.io.

Conclusion

Python ward replaces the test-naming ceremony of traditional frameworks with something more direct: a plain English description that appears in every test report, every CI log, and every team standup where someone asks “what broke?” The @test decorator, @fixture injection, @using parameterization, and the expect() assertion chain all work together to make tests read like specifications and failures read like bug reports.

The shopping cart example in this article covers the core patterns you will reach for in a real project. Try extending it — add a coupon fixture that returns a discount function, write parameterized tests for boundary cases like zero-quantity items, or switch the Cart storage to a database and test the async fixture pattern. Each of those extensions fits naturally into the structure already in place.

For the full API reference, see the official ward documentation at ward.readthedocs.io. The project is also actively maintained on GitHub at github.com/darrenburns/ward, where you can find the changelog and open issues.

Related Articles

• How To Use Python factory-boy for Test Fixture Creation
• How To Use Python nox for Automated Test Sessions
• How To Use Python memray for Memory Profiling