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Hands-on Python Tutorial, Release 1.0 for Python Version 3.1+
1.4 Integer Arithmetic
1.4.1 Addition and Subtraction
We start with the integers and integer arithmetic, not because arithmetic is exciting, but because the symbolism should
be mostly familiar. Of course arithmetic is important in many cases, but Python is probably more often used to
manipulate text and other sorts of data, as in the sample program in ref:Running-A-Sample.
Python understands numbers and standard arithmetic. For the whole section on integer arithmetic, where you see a
set-off line in typewriter font, type individual lines at the >>> prompt in the Python Shell. Press Enter after each
line to get Python to respond:
77
2 + 3
5 - 7
Python should evaluate and print back the value of each expression. Of course the first one does not require any
calculation. It appears that the shell just echoes back what you printed.
The Python Shell is an interactive interpreter. As you can see, after you press Enter, it is evaluating the expression
you typed in, and then printing the result automatically. This is a very handy environment to check out simple Python
syntax and get instant feedback. For more elaborate programs that you want to save, we will switch to an Editor
Window later.
1.4.2 Multiplication, Parentheses, and Precedence
Try in the Shell:
2 x 3
You should get your first syntax error. The x should have become highlighted, indicating the location where the Python
interpreter discovered that it cannot understand you: Python does not use x for multiplication as you may have done
in grade school. The x can be confused with the use of x as a variable (more on that later).
Instead the symbol for multiplication is an asterisk
*
. Enter each of the following. You may include spaces or not.
The Python interpreter can figure out what you mean either way. Try in the Shell:
2
*
5
2 + 3
*
4
If you expected the last answer to be 20, think again: Python uses the normal precedence of arithmetic operations:
Multiplications and divisions are done before addition and subtraction, unless there are parentheses. Try
(2+3)
*
4
2
*
(4 - 1)
Now try the following in the Shell, exactly as written, followed by Enter, with no closing parenthesis:
5
*
(2 + 3
Look carefully. There is no answer given at the left margin of the next line and no prompt >>> to start a new expression.
If you are using Idle, the cursor has gone to the next line and has only indented slightly. Python is waiting for you to
finish your expression. It is smart enough to know that opening parentheses are always followed by the same number
of closing parentheses. The cursor is on a continuation line. Type just the matching close-parenthesis and Enter,
)
14 Chapter 1. Beginning With Python