Add constants.py and its test

Task: MIL-001#3

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"