Add constants.py and its test

Task: MIL-001#3

tests/conftest.py

@@ -0,0 +1,16 @@
+"""Shared pytest configuration for the weather data tests."""
+
+from pathlib import Path
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def isolated_working_directory(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
+    """Run every test in an empty temporary working directory.
+
+    The export has a default file name relative to the working directory. Without
+    this fixture, a test that uses the default would write into the folder pytest
+    was started from.
+    """
+    monkeypatch.chdir(tmp_path)

tests/test_constants.py

@@ -0,0 +1,36 @@
+"""Tests of the constants that the day-25 lectures fix."""
+
+from weather import constants
+
+
+def test_data_file_is_the_one_of_the_lecture() -> None:
+    assert constants.DATA_FILE_NAME == "weather_data.csv"
+
+
+def test_data_file_lies_next_to_the_modules_not_in_the_working_folder() -> None:
+    assert constants.DATA_FILE.is_absolute()
+    assert constants.DATA_FILE.parent == constants.PACKAGE_DIR
+    assert constants.DATA_FILE.name == constants.DATA_FILE_NAME
+
+
+def test_column_names_match_the_header_of_the_lecture() -> None:
+    assert constants.COLUMN_DAY == "day"
+    assert constants.COLUMN_TEMP == "temp"
+    assert constants.COLUMN_CONDITION == "condition"
+
+
+def test_temperature_is_the_second_value_of_a_row() -> None:
+    assert constants.TEMP_COLUMN_INDEX == 1
+
+
+def test_fahrenheit_factors_give_the_lecture_formula() -> None:
+    celsius = 100
+    fahrenheit = (
+        celsius * constants.FAHRENHEIT_NUMERATOR / constants.FAHRENHEIT_DENOMINATOR
+        + constants.FAHRENHEIT_OFFSET
+    )
+    assert fahrenheit == 212
+
+
+def test_exported_file_has_the_name_of_the_lecture() -> None:
+    assert constants.EXPORT_FILE_NAME == "new_data.csv"