Deitel - Python, How To Program.pdf

pythonhtp1_02.fm Page 47 Wednesday, December 12, 2001 12:12 PM
Chapter 2 Introduction to Python Programming 47
Portability Tip 2.1
In Python version 3.0 (due to be released no sooner than 2003), the / operator can perform
only true division. After the release of version 3.0, programmers need to update applications
to compensate for the new behavior. For more information on this future change, see
python.sourceforge.net/peps/pep-0238.html 2.1
Python provides the modulus operator (%), which yields the remainder after integer
division. The expression x % y yields the remainder after x is divided by y. Thus, 7 %4
yields 3 and 17 % 5 yields 2. This operator is most commonly used with integer operands,
but also can be used with other arithmetic types. In later chapters, we discuss many interesting
applications of the modulus operator, such as determining whether one number is a
multiple of another. (A special case of this is determining whether a number is odd or even.)
[Note: The modulus operator can be used with both integer and floating-point numbers.]
Arithmetic expressions in Python must be entered into the computer in straight-line
form. Thus, expressions such as “a divided by b” must be written as a / b, so that all constants,
variables and operators appear in a straight line. The algebraic notation
--
a
b
is generally not acceptable to compilers or interpreters, although some special-purpose
software packages do exist that support more natural notation for complex mathematical
expressions.
Parentheses are used in Python expressions in much the same manner as in algebraic
expressions. For example, to multiply a times the quantity b + c, we write
a * (b + c)
Python applies the operators in arithmetic expressions in a precise sequence determined
by the following rules of operator precedence, which are generally the same as those
followed in algebra:
1. Expressions contained within pairs of parentheses are evaluated first. Thus, parentheses
may force the order of evaluation to occur in any sequence desired by the
programmer. Parentheses are said to be at the “highest level of precedence.” In
cases of nested, or embedded, parentheses, the operators in the innermost pair of
parentheses are applied first.
2. Exponentiation operations are applied next. If an expression contains several exponentiation
operations, operators are applied from right to left.
3. Multiplication, division and modulus operations are applied next. If an expression
contains several multiplication, division and modulus operations, operators are
applied from left to right. Multiplication, division and modulus are said to be on
the same level of precedence.
4. Addition and subtraction operations are applied last. If an expression contains several
addition and subtraction operations, operators are applied from left to right.
Addition and subtraction also have the same level of precedence.