Chapter 5. Operators and Expressions¶

Table 5.1. Operators and their usage¶

Operator Name Explanation Examples + Plus Adds the two objects 3 + 5 gives 8. ‘a’ + ‘b’ gives ‘ab’. - Minus Either gives a negative number or gives the subtraction of one number from the other -5.2 gives a negative number. 50 - 24 gives 26. * Multiply Gives the multiplication of the two numbers or returns the string repeated that many times. 2 * 3 gives 6. ‘la’ * 3 gives ‘lalala’. ** Power Returns x to the power of y 3 ** 4 gives 81 (i.e. 3 * 3 * 3 * 3) / Divide Divide x by y 4/3 gives 1 (division of integers gives an integer). 4.0/3 or 4/3.0 gives 1.3333333333333333 // Floor Division Returns the floor of the quotient 4 // 3.0 gives 1.0 % Modulo Returns the remainder of the division 8%3 gives 2. -25.5%2.25 gives 1.5 . << Left Shift Shifts the bits of the number to the left by the number of bits specified. (Each number is represented in memory by bits or binary digits i.e. 0 and 1) 2 << 2 gives 8. - 2 is represented by 10 in bits. Left shifting by 2 bits gives 1000 which represents the decimal 8. >> Right Shift Shifts the bits of the number to the right by the number of bits specified. 11 >> 1 gives 5 - 11 is represented in bits by 1011 which when right shifted by 1 bit gives 101 which is nothing but decimal 5. & Bitwise AND Bitwise AND of the numbers 5 & 3 gives 1. | Bit-wise OR Bitwise OR of the numbers 5 | 3 gives 7 ^ Bit-wise XOR 5 ^ 3 gives 6 ~ Bit-wise invert The bit-wise inversion of x is -(x+1) ~5 gives -6. < Less Than Returns whether x is less than y. All comparison operators return 1 for true and 0 for false. This is equivalent to the special variables True and False respectively. Note the capitalization of these variables’ names. 5 < 3 gives 0 (i.e. False) and 3 < 5 gives 1 (i.e. True). Comparisons can be chained arbitrarily: 3 < 5 < 7 gives True. > Greater Than Returns whether x is greater than y 5 < 3 returns True. If both operands are numbers, they are first converted to a common type. Otherwise, it always returns False. <= Less Than or Equal To Returns whether x is less than or equal to y x = 3; y = 6; x <= y returns True. >= Greater Than or Equal To Returns whether x is greater than or equal to y x = 4; y = 3; x >= 3 returns True. == Equal To Compares if the objects are equal x = 2; y = 2; x == y returns True. x = ‘str’; y = ‘stR’; x == y returns False. x = ‘str’; y = ‘str’; x == y returns True. != Not Equal To Compares if the objects are not equal x = 2; y = 3; x != y returns True. not Boolean NOT If x is True, it returns False. If x is False, it returns True. x = True; not y returns False. and Boolean AND x and y returns False if x is False, else it returns evaluation of y x = False; y = True; x and y returns False since x is False. In this case, Python will not evaluate y since it knows that the value of the expression will has to be false (since x is False). This is called short-circuit evaluation. or Boolean OR If x is True, it returns True, else it returns evaluation of y x = True; y = False; x or y returns True. Short-circuit evaluation applies here as well.

Table 5.2. Operator Precedence¶

Operator                    Description
lambda               Lambda Expression
or                   Boolean OR
and                  Boolean AND
not x                Boolean NOT
in, not in           Membership tests
is, is not           Identity tests
<, <=, >, >=, !=, == Comparisons
|                    Bitwise OR
^                    Bitwise XOR
&                    Bitwise AND
<<, >>               Shifts
+, -                 Addition and subtraction
*, /, %              Multiplication, Division and Remainder
+x, -x               Positive, Negative
~x                   Bitwise NOT
**                   Exponentiation
x.attribute          Attribute reference
x[index]             Subscription
x[index:index]       Slicing
f(arguments ...)     Function call
(expressions, ...)   Binding or tuple display
[expressions, ...]   List display
{key:datum, ...}     Dictionary display
`expressions, ...`   String conversion

The operators which we have not already come across will be explained in later chapters.

Operators with the same same precedence are listed in the same row in the above table. For example, + and - have the same precedence.

Order of Evaluation¶

By default, the operator precedence table decides which operators are evaluated before others. However, if you want to change the orer in which they are evaluated, you can use parentheses. For example, if you want addition to be evaluated before multiplication in an expression, then you can write something like (2 + 3) * 4.

Associativity¶

Operators are usually associated from left to right i.e. operators with same precedence are evaluated in a left to right manner. For example, 2 + 3 + 4 is evaluated as (2 + 3) + 4. Some operators like assignment operators have right to left associativity i.e. a = b = c is treated as a = (b = c).

Using Expressions¶

#!/usr/bin/python
# Filename: expression.py

length = 5
breadth = 2

area = length * breadth
print 'Area is', area
print 'Perimeter is', 2 * (length + breadth)

$ python expression.py
Area is 10
Perimeter is 14