Deitel - Python, How To Program.pdf

pythonhtp1_02.fm Page 49 Wednesday, December 12, 2001 12:12 PM
Chapter 2 Introduction to Python Programming 49
Operator(s) Operation(s) Order of Evaluation (Precedence)
( ) Parentheses Evaluated first. If the parentheses are nested, the
expression in the innermost pair is evaluated first. If
there are several pairs of parentheses “on the same
level” (i.e., not nested), they are evaluated left to right.
** Exponentiation Evaluated second. If there are several, they are evaluated
right to left.
* / // % Multiplication
Division
Modulus
+ - Addition
Subtraction
Fig. Fig. 2.16 2.16 Precedence of arithmetic operators.
The circled numbers under the statement indicate the order in which Python applies the
operators. The multiplication, modulus and division are evaluated first, in left-to-right order
(i.e., they associate from left to right) because they have higher precedence than addition
and subtraction. The addition and subtraction are applied next. These are also
applied left to right. Once the expression has been evaluated, Python assigns the result to
variable z.
To develop a better understanding of the rules of operator precedence, consider how a
second-degree polynomial is evaluated:
y = a * x ** 2 + b * x + c
2 1 4 3 5
The circled numbers under the statement indicate the order in which Python applies the operators.
Suppose variables a, b, c and x are initialized as follows: a=2, b =3, c =7 and
x=5. Figure 2.17 illustrates the order in which the operators are applied in the preceding
second-degree polynomial.
The preceding assignment statement can be parenthesized with unnecessary parentheses,
for clarity, as
y = ( a * ( x ** 2 ) ) + ( b * x ) + c
Evaluated third. If there are several, they are evaluated
left to right. [Note: The // operator is new in version
2.2]
Evaluated last. If there are several, they are evaluated
left to right.
Good Programming Practice 2.7
As in algebra, it is acceptable to place unnecessary parentheses in an expression to make the
expression clearer. These parentheses are called redundant parentheses. Redundant parentheses
are commonly used to group subexpressions in a large expression to make that expression
clearer. Breaking a large statement into a sequence of shorter, simpler statements also
promotes clarity. 2.7