Add constants.py and its test

Task: MIL-001#3

src/weather/__init__.py

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+## @package weather
+#  @brief Reading CSV weather data, from Udemy's 100 Days of Code (day 25).
+#
+#  @mainpage Weather Data
+#
+#  The day-25 lectures read the weather data of one week from the file
+#  weather_data.csv: first with Python's built-in csv library, then with the
+#  pandas library, which makes the same work a one-liner. The package keeps the
+#  two ways side by side: csv_reader.py uses the standard library only, and
+#  analysis.py and export.py need the optional pandas extra.
+#
+#  The constants of the package are in constants.py.
+
+"""Reading CSV weather data, from Udemy's 100 Days of Code (day 25)."""

src/weather/constants.py

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+## @file constants.py
+#  @brief Constants of the weather data project.
+#
+#  Every value that the day-25 lectures fix lives here, so that no other module
+#  contains a magic string or number: the name of the data file, the names of
+#  its columns, the factors of the Fahrenheit conversion and the name of the
+#  exported file.
+
+"""Constants of the weather data project, as fixed by the day-25 lectures."""
+
+from pathlib import Path
+from typing import Final
+
+## @brief Folder of this package; the data file lies next to the modules.
+PACKAGE_DIR: Final[Path] = Path(__file__).resolve().parent
+
+## @brief Name of the weather data file of the lecture.
+DATA_FILE_NAME: Final[str] = "weather_data.csv"
+
+## @brief Full path of the weather data file, independent of the working folder.
+DATA_FILE: Final[Path] = PACKAGE_DIR / DATA_FILE_NAME
+
+## @brief Name of the file that the exported table is written to by default.
+EXPORT_FILE_NAME: Final[str] = "new_data.csv"
+
+## @brief Encoding of the CSV files that are read and written.
+FILE_ENCODING: Final[str] = "utf-8"
+
+## @brief Name of the column that holds the day of the week.
+COLUMN_DAY: Final[str] = "day"
+
+## @brief Name of the column that holds the temperature in degrees Celsius.
+COLUMN_TEMP: Final[str] = "temp"
+
+## @brief Name of the column that holds the weather condition, such as `Sunny`.
+COLUMN_CONDITION: Final[str] = "condition"
+
+## @brief Position of the temperature in a row that `csv.reader` returns.
+TEMP_COLUMN_INDEX: Final[int] = 1
+
+## @brief Numerator of the Celsius to Fahrenheit factor: `temp * 9 / 5 + 32`.
+FAHRENHEIT_NUMERATOR: Final[int] = 9
+
+## @brief Denominator of the Celsius to Fahrenheit factor: `temp * 9 / 5 + 32`.
+FAHRENHEIT_DENOMINATOR: Final[int] = 5
+
+## @brief Offset of the Celsius to Fahrenheit conversion: `temp * 9 / 5 + 32`.
+FAHRENHEIT_OFFSET: Final[int] = 32

tests/conftest.py

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+"""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

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+"""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"