The basics of syntax
INPUT · Slides
01 / 06
As you write programs you often want to say "this has not been decided yet".
You could make do with 0 or "" (empty text), but "costs nothing" and "price not decided" are two different things. To say which is which, Python gives you a special value: None.
price = Noneprint(price)Result
None
02 / 06
None is not 0, not False and not empty text. It is a value in its own right, with a type of its own.
It starts with a capital, like True and False. Write none in small letters and you get a NameError.
print(type(None))print(None == 0)Result
<class 'NoneType'> False
03 / 06
To ask whether something is None, the Python habit is to write is None rather than ==.
== None runs perfectly well, but in real Python code you will see is None almost every time. Make sure you can read it.
price = Noneif price is None: print("not decided yet")Result
not decided yet
04 / 06
if will actually take any value, not only the result of a comparison. Python reads it as True or False for you.
The ones treated as False are the empty and the zero things.
0"" (empty text)NoneFalseEverything else counts as True.
number = 0if number: print("something")else: print("nothing")Result
nothing
05 / 06
Because of that, instead of if name != "": you can write if name:.
It is short and it reads well, so you will see it constantly in Python. Read it as "if there is something in it".
name = "Aoi"if name: print(f"hello, {name}")Result
hello, Aoi
06 / 06
Careful, though. 0 and None are both treated as False.
When you need to tell "no value" apart from "the value is zero", if price: is not enough. You have to spell it out with if price is None:.
This pit gets dug into in real work all the time. Being able to write it short and writing it correctly are two different things.
price = 0if price is None: print("not decided")else: print("free")Result
free