A property that checks its value

A plain attribute like t.celsius is simple, but any line in the program can put -300 in it. @property turns access into a method call while keeping the same syntax. You read the value with t.celsius, and when you write t.celsius = 30 the value goes through a setter, marked with @celsius.setter, that checks it before storing it.

That's why in Python you don't need to write get_ and set_ methods up front. Start with a plain attribute, and when you need validation, turn it into a property: not one line changes for the code that uses the class. A property with no setter is read-only, which suits computed values like fahrenheit.

The trap is that inside the setter you have to store the value under another name, like self._celsius. Write self.celsius = value and the setter calls itself over and over until you hit RecursionError. In __init__, on the other hand, write self.celsius on purpose, so creating the object goes through the same check.