In Python, the fundamental abstraction of a computation is as a procedure (other books call them "functions" instead; we will end up using both values). A function that takes a number as an argument and returns the argument value plus 1 is de?ned as:
return x + 1
The indentation is important here, too. All of the instructions of the procedure have to be indented one level below the def. It is important to remind the return statement at the end, if you want your method to give a value. So, if you described f as above, then operated with it in the shell,4 you may get something like that:
If we just evaluate f, Python tells us it is a method. Then we can give it to 4 and get 5, or give it multiple times, as given.
What if we de?ne
x + 1
Now, when we play with it, we might get something like this:
Traceback (most recent call last):
File "", line 1, in ? File "", line 2, in g
TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'
What happened!! First, when we calculated g(4), we take nothing at all, because our definition of g did not give anything. Well...strictly speaking, it given a special number known as None, which the file does not bother printing out. The number None has a special type, known NoneType. So, then, when we tried to give g to the answer of g(4), it ended up trying to calculate g(None), which made it try to evaluate None + 1, which made it complain that it did not know how to add something of type NoneType and something of type int.