Tests were modifying repository files instead of operating in isolated temporary directories. Added documentation clarifying that tests must use tmp_path fixture and the change_cwd context manager.

Changes

CONTRIBUTING.md

• Added "Test Side-Effects" subsection explaining:
  • Tests must not modify/create/delete files in the repository directory
  • Use tmp_path fixture for all file operations
  • Use usethis._test.change_cwd context manager for CWD-dependent operations
  • Included example demonstrating correct pattern

.github/copilot-instructions.md

• Added "No side-effects" requirement to Test Requirements
• Quick reference for tmp_path and change_cwd usage

Example

from usethis._test import change_cwd

def test_example(tmp_path: Path):
    # Arrange - create files in temporary directory
    (tmp_path / "example.txt").write_text("test")
    
    # Act - change to temp directory for operations
    with change_cwd(tmp_path):
        do_something()
    
    # Assert
    assert (tmp_path / "output.txt").exists()

If tests modify repo files, it indicates missing change_cwd context manager.

• Fixes Document that tests should not have side-effects in the repo #1095

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