# pytest

A Cursor rule that pins down how pytest tests get written: layout, fixtures, parametrization, mocks. Useful once more than one person touches the suite.

- rule · `type=rule` · Cursor · ★ 3579
- Author: sanjeed5
- License: CC0-1.0 · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/sanjeed5/awesome-cursor-rules-mdc/tree/HEAD/rules-mdc/pytest.mdc
- Updated: 2026-05-19
- HTML: https://affpapa.org/en/nevibecode/rules/pytest

## What it does

Drop `.cursor/rules/pytest.mdc` into a project and Cursor feeds it to the agent whenever Python files are in play. The document covers layout first: a `src/` tree, test files named `test_*.py`, `__init__.py` inside `tests/`, function names starting with `test_` and classes with `Test`.

Then the patterns. One assertion per test. Fixtures carry setup and teardown through `yield`, with `function`, `class`, `module` and `session` scopes spelled out; `autouse=True` is discouraged outside genuinely global patches. Data-driven cases go through `@pytest.mark.parametrize`, unfinished or broken tests through `skip` and `xfail`.

Mocking gets its own section: the `mocker` fixture from `pytest-mock` is preferred over raw `monkeypatch` when the mock needs assertion helpers. Coverage means adding `pytest-cov`. The globs field is `**/*.py`, so the rule applies to every Python file — if your tests sit next to the code instead of under `tests/`, the import advice about running against the installed package no longer matches your setup.

## Install

### Cursor (native platform)

**Manually: put the files in place**
- File: `.cursor/rules/pytest.mdc` — Frontmatter: description, globs, alwaysApply

### Claude Code

**Manually: put the files in place**
- File: `.claude/rules/pytest.md` — The paths frontmatter field limits the files: move globs from the source into paths

### Codex

**AGENTS.md**
- Codex reads AGENTS.md: paste the rule text there

### GitHub Copilot

**Manually: put the files in place**
- File: `.github/instructions/pytest.instructions.md` — The applyTo field sets which files it applies to: move globs from the source into applyTo

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/rules/pytest.md`

### Cline

**Manually: put the files in place**
- File: `.clinerules/pytest.md`

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/sanjeed5/awesome-cursor-rules-mdc/tree/HEAD/rules-mdc/pytest.mdc

## When to use it
- settle on names for test files and functions
- move setup and cleanup into fixtures
- run one scenario across several input sets
- mark a known-broken test as expected to fail
- swap a live API call for a mock inside a test

## Example prompts
- Write tests for `mypackage/service.py` following the pytest rule
- Rewrite this test with `@pytest.mark.parametrize`
- Replace monkeypatch with the `mocker` fixture in `tests/test_service.py`

## Source text · pytest.mdc · CC0-1.0

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: d48578427d9e913a.

BEGIN THIRD-PARTY FILE d48578427d9e913a
````text
---
description: This guide defines our team's definitive best practices for writing fast, reliable, and maintainable tests using pytest, ensuring consistency and high code quality across projects.
globs: **/*.py
---
# pytest Best Practices

This document outlines our team's standard for writing `pytest` tests. Adhering to these guidelines ensures our test suite is fast, reliable, and easy to maintain, integrating seamlessly into our CI/CD pipelines.

## 1. Code Organization & Structure

### 1.1 Project Layout
Always use the `src/` layout for your project. This prevents common import pitfalls and ensures tests run against the installed package, not local source.

**✅ GOOD:**
```
.
├── pyproject.toml
├── src/
│   └── mypackage/
│       ├── __init__.py
│       └── module.py
└── tests/
    ├── __init__.py
    └── test_module.py
```

**❌ BAD:**
```
.
├── pyproject.toml
├── mypackage/ # Top-level package
│   ├── __init__.py
│   └── module.py
└── tests/
    └── test_module.py
```

### 1.2 Test File Naming
Name test files `test_*.py` or `*_test.py`. We prefer `test_*.py`.
Place `__init__.py` files in `tests/` and its subdirectories to treat them as Python packages, enabling structured imports and avoiding name collisions.

**✅ GOOD:**
```python
# tests/test_my_feature.py
from mypackage.module import my_function

def test_my_function_returns_expected_value():
    assert my_function() == "expected"
```

### 1.3 Test Naming Conventions
Prefix test functions and methods with `test_`. For classes, prefix with `Test`. Use descriptive names that read like sentences.

**✅ GOOD:**
```python
def test_user_can_be_created_with_valid_email():
    # ...
    pass

class TestUserService:
    def test_get_user_by_id_returns_correct_user():
        # ...
        pass
```

**❌ BAD:**
```python
def create_user(): # Not discovered by pytest
    pass

def test_create_user_and_email(): # Tests multiple things
    pass

class UserServiceTest: # Incorrect class prefix
    pass
```

## 2. Common Patterns & Anti-patterns

### 2.1 Single Assert Per Test
Each test should verify one specific behavior. This improves readability and simplifies debugging.

**✅ GOOD:**
```python
def test_user_creation_sets_email():
    user = create_user("test@example.com")
    assert user.email == "test@example.com"

def test_user_creation_sends_welcome_email():
    user = create_user("test@example.com")
    assert email_service.last_sent_email == "test@example.com"
```

**❌ BAD:**
```python
def test_user_creation_and_email_sending():
    user = create_user("test@example.com")
    assert user.email == "test@example.com" # First assert
    send_welcome_email(user)
    assert email_service.last_sent_email == "test@example.com" # Second assert
```

### 2.2 Fixtures for Setup/Teardown & Dependency Injection
Use fixtures to provide isolated, reusable resources and manage setup/teardown.

*   **Scope**:
    *   `function` (default): Run once per test function. Use for most tests.
    *   `class`: Run once per test class.
    *   `module`: Run once per test module. Use for expensive setups shared across module tests.
    *   `session`: Run once per test session. Use for global, very expensive resources (e.g., database connection pool).
*   **Teardown**: Use `yield` in fixtures for automatic teardown.
*   **Modularity**: Compose fixtures by injecting them into others.
*   **Avoid `autouse=True`**: Only use for truly global, non-interfering setups (e.g., patching a common library function for all tests).

**✅ GOOD:**
```python
# conftest.py
import pytest

@pytest.fixture(scope="function")
def db_session():
````
END THIRD-PARTY FILE d48578427d9e913a

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