python-tdd-with-uv

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Translated

Test-driven development in Python using uv as the package manager. Covers the red-green-refactor cycle, vertical slicing, and uv project setup.

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Sourcespencerpauly/awesome-cursor-skills

Added on2026-09-05

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Python TDD with uv

Write Python code test-first using

uv

for fast dependency and environment management.

Setting Up the Project

Check if
```
uv
```
is installed:
```
uv --version
```
If the project doesn't have a
```
pyproject.toml
```
, initialize:
bash
```
uv init
```
Add pytest as a dev dependency:
bash
```
uv add --dev pytest pytest-cov
```
Confirm the test runner works:
bash
```
uv run pytest --co
```

TDD Workflow — Vertical Slicing

Work in small cycles. Never write more than one failing test at a time.

Planning Phase

Before writing code, answer:

What interface changes are needed? (functions, classes, APIs)
Which behaviors matter most? (prioritize critical paths)
Can we design for testability? (inject dependencies, avoid global state)

The Cycle

RED   → Write ONE failing test for the next behavior
GREEN → Write the MINIMUM code to make it pass
REFACTOR → Clean up without changing behavior
REPEAT

Rules:

Never write implementation before a failing test exists
Never write more than one failing test at a time
Run
```
uv run pytest
```
after every change
Tests must assert observable behavior, not implementation details
Mocks should only be used at system boundaries (I/O, network, clock)

Test File Structure

python

# tests/test_<module>.py

class TestFeatureName:
    """Group related behaviors."""

    def test_does_expected_thing_when_given_input(self):
        result = function_under_test(input_value)
        assert result == expected

    def test_raises_when_given_invalid_input(self):
        with pytest.raises(ValueError):
            function_under_test(bad_input)

Running Tests

bash

uv run pytest                      # all tests
uv run pytest tests/test_foo.py    # single file
uv run pytest -k "test_name"       # by name pattern
uv run pytest --cov=src            # with coverage
uv run pytest -x                   # stop on first failure

uv Essentials