feat(day 120): add pounds-to-kilograms converter with grammatical handling and float-safe comparison

Author: JohnSunny21

FreeCodeCamp/CodingQ/PoundsToKilograms.py

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+""" 
+Pounds to Kilograms
+Given a weight in pounds as a number, return the string "(lbs) pounds equals (kgs) kilograms.".
+
+Replace "(lbs)" with the input number.
+Replace "(kgs)" with the input converted to kilograms, rounded to two decimals and always include two decimal places in the value.
+1 pound equals 0.453592 kilograms.
+If the input is 1, use "pound" instead of "pounds".
+If the converted value is 1, use "kilogram" instead of "kilograms".
+"""
+
+import unittest
+
+class PoundsToKilograms(unittest.TestCase):
+
+    def test1(self):
+        self.assertEqual(convert_to_kgs(1), "1 pound equals 0.45 kilograms.")
+
+    def test2(self):
+        self.assertEqual(convert_to_kgs(0),"0 pounds equals 0.00 kilograms.")
+
+    def test3(self):
+        self.assertEqual(convert_to_kgs(100),"100 pounds equals 45.36 kilograms.")
+
+    def test4(self):
+        self.assertEqual(convert_to_kgs(2.5), "2.5 pounds equals 1.13 kilograms.")
+
+    def test5(self):
+        self.assertEqual(convert_to_kgs(2.20462),"2.20462 pounds equals 1.00 kilograms.")
+
+
+"""
+Subtle issue with this approach
+
+The condition: 
+    if kilograms == 1:
+        ddelimiter = "kilogram"
+
+is fragile because kilograms is a float.
+
+    lbs = 2.20462
+    kilograms = round(lbs * 0.453592, 2) # 1.0
+
+Here kilograms == 1 is true, so you get "1.00 kilogram". That works
+
+But if floating - point rounding gave 0.9999999 -> 1.0, it's fine.
+However, if you ever compare floats directly, it can be risky. A safer approach is to compare the formatted string.
+refining the below code.
+
+"""
+def convert_to_kgs(lbs):
+
+    kilograms = round (lbs * 0.453592,2)
+
+    pdelimiter = "pounds"
+    ddelimiter = "kilograms"
+
+    if lbs == 1:
+        pdelimiter = "pound"
+    if kilograms == 1:
+        ddelimiter = "kilogram"
+
+    return f"{lbs} {pdelimiter} equals {kilograms:.2f} {ddelimiter}"
+
+
+def convert_to_kgs_refined(lbs):
+
+    kilograms = lbs * 0.453592
+    kgs_str = f"{kilograms:.2f}"
+
+    pdelimiter = "pound" if lbs == 1 else "pounds"
+    ddelimiter = "kilogram" if kgs_str == "1.00" else "kilograms"
+
+    return f"{lbs} {pdelimiter} equals {kgs_str} {ddelimiter}."
+
+def convert_to_kgs(lbs):
+
+    kgs = lbs * 0.453592
+    kgs_str = f"{kgs:.2f}" # always 2 decimals
+
+    pound_word = "pound" if lbs == 1 else "pounds"
+    kilogram_word = "kilogram" if kgs == "1.00" else "kilograms"
+
+    return f"{lbs} {pound_word} equals {kgs_str} {kilogram_word}."
+
+
+if __name__ == "__main__":
+    print(convert_to_kgs(50))
+    unittest.main()