Built-in fixtures in Python

tmp_path, monkeypatch and capsys cover most of what a test needs.

import os
from pathlib import Path


def count_lines(path):
    return len(Path(path).read_text().splitlines())


def report(name):
    print(f"theme is {os.environ.get('TYPESTAR_THEME', 'default')}")
    return name


def test_counts_lines(tmp_path):
    target = tmp_path / "runs.txt"
    target.write_text("one\ntwo\nthree\n")
    assert count_lines(target) == 3


def test_reads_the_environment(monkeypatch, capsys):
    monkeypatch.setenv("TYPESTAR_THEME", "monokai")
    report("x")
    assert "monokai" in capsys.readouterr().out

How it works

1. tmp_path is a fresh directory per test.
2. monkeypatch undoes its changes when the test ends.
3. capsys captures what the code printed.

Keywords and builtins used here

• assert
• count_lines
• def
• len
• print
• report
• return
• test_counts_lines
• test_reads_the_environment

The run, in numbers

Lines

23

Characters to type

500

Tokens

130

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 74 seconds.

Type this snippet

Step 2 of 2 in Fixtures, step 5 of 9 in Testing with pytest.

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