Now that we know that functions are a way of taking a number of inputs and producing an output, we should look again at what happens when we write:
x = 2 + 3
print(x)
5
This is just a pretty way of calling an “add” function. Things would be more symmetrical if add were actually written
x = +(2, 3)
Where ‘+’ is just the name of the name of the adding function.
In python, these functions do exist, but they’re actually methods of the first input: they’re the mysterious __ functions we saw earlier (Two underscores.)
x.__add__(7)
12
We call these symbols, +, - etc, “operators”.
The meaning of an operator varies for different types:
"Hello" + "Goodbye"
‘HelloGoodbye’
[2, 3, 4] + [5, 6]
[2, 3, 4, 5, 6]
Sometimes we get an error when a type doesn’t have an operator:
7 - 2
5
For example:
[2, 3, 4] - [5, 6]
—————————————————————————
TypeError Traceback (most recent call last)
<ipython-input-38-4627195e7799> in <module>()
—-> 1 [2, 3, 4] - [5, 6]
TypeError: unsupported operand type(s) for -: ‘list’ and ‘list’
The word “operand” means “thing that an operator operates on”!
Or when two types can’t work together with an operator:
[2, 3, 4] + 5
—————————————————————————
TypeError Traceback (most recent call last)
<ipython-input-39-84117f41979f> in <module>()
—-> 1 [2, 3, 4] + 5
TypeError: can only concatenate list (not “int”) to list
Just as in Mathematics, operators have a built-in precedence, with brackets used to force an order of operations:
print(2 + 3 * 4)
14
print((2 + 3) * 4)
20
Supplementary material: http://www.mathcs.emory.edu/~valerie/courses/fall10/155/resources/op_precedence.html