9. Notes about booleans and logical operators

9.1. The boolean type

A boolean expression (or logical expression) evaluates to one of two states true or false. Python provides the boolean type that can be either set to False or True. Many functions and operations returns boolean objects.

The not keyword can also be used to inverse a boolean type.

>>> not True
False

9.2. What is False ?

Every object has a boolean value. The following elements are false:

• None
• False
• 0 (whatever type from integer, float to complex)
• Empty collections: “”, (), [], {}
• Objects from classes that have the special method __nonzero__
• Objects from classes that implements __len__ to return False or zero

Example of a class with type set to False:

>>> class A():
...     def __len__(self):
...         return 0
...
>>> class B():
...     def __nonzero__(self):
...         return 0
...
>>> a = A()
>>> bool(a)
False
>>> b = B()
>>> bool(b)
False

All other objects are True.

9.3. Comparison operators

The <, <=, >, >=, ==, != operators compare the values of 2 objects and returns True or False. Comparison depends on the type of the objects. See the Classes to see how to refedine the comparison operator of a type.

10 == 10
10 <= 10.

9.4. Chaining comparison operators

Comparison operators can be chained. Consider the following examples:

>>> x = 2
>>> 1 < x < 3
True
>>> 10 < x < 20
False
>>> 3 > x <= 2
True
>>> 2 == x < 4
True

The comparison is performed between each pair of terms to be evaluated. For instance in the first example, 1<x is evaluated to True AND x<2 is evaluated. It is not as if 1<x is evaluated to True and then True<3 is evaluated to True !!! Each term is evaluated once.

9.5. Evaluation of logical and comparison operators and membership operators

The evaluation using the and and or operators follow these rules:

• and and or evaluates expression from left to right.
• with and, if all values are True, returns the last evaluated value. If any value is false, returns the first one.
• or returns the first True value. If all are False, returns the last value

operators	descriptions
not x	Returns True if x is True, False otherwise
x and y	Returns x if x is False, y otherwise
x or y	Returns y if x is False, x otherwise

See Examples for more details.

9.6. Membership operators

• in evaluates to True if it finds a variable in a specified sequence and false otherwise.
• not in evaluates to False if it finds a variable in a sequence, True otherwise.

>>> 'good' in 'this is a great example'
False
>>> 'good' not in 'this is a great example'
True

9.7. Identity operators

• is evaluates to True if the variables on either side of the operator point to the same object and False otherwise
• is not evaluates to False if the variables on either side of the operator point to the same object and True otherwise

>>> p = 'hello'
>>> ps = p
>>> ps is p
True

9.8. Bitwise operators

Bitwise operators are used to compare integers in their binary formats.

When performing a binary operations between 2 integers, there are first converted into binary numbers.

Let us show a few examples to explain the bitwise operations. The and operation between 2 the values 5 and 4 is actually the and operations between 1011 and 1001 binaries. It is therefore equal to 1001:

>>> cmp(4, 5)
-1

bitwise operators	descriptions
>>	bitwise left shift
<<	bitwise rightshift
&	bitwise and
^	bitwise xor
	bitwise or
~	bitwise not

The left and right shifts can divide or multiply by power of 2 easily (integer conversion is made):

>>> 25 >> 2
6
>>> 25 << 2
100

Warning

There is no overflow check. so if a right shift exceeds 2^31, the operation deletes extra bits and flips the sign.

The not operator works as follows:

>>> ~38
-39
>>> ~-38
37

9.9. Order of evaluation

The order of evaluation from highest to order is as shown in this table:

operators	descriptions
(), [], {}, ‘’	tuple, list, dictionnary, string
x.attr, x[], x[i:j], f()	attribute, index, slide, function call
+x, -x, ~x	unary negation, bitwise invert
**	exponent
*, /, %	multiplication, division, modulo
+, -	addition, substraction
<<, >>	bitwise shifts
&	bitwise and
^	bitwise xor
	bitwise or
<, <=, >=, >	comparison operators
==, !=, is, is not, in,	comparison operators (continue)
not in	comparison operators (continue)
not	boolean NOT
and	boolean AND
or	boolean OR
lambda	lamnda expression

Here is the precedence table for the boolean operators only

operator
==
!=
and
or

9.10. Boolean Examples

9.10.1. Short-circuit evaluations

To speed up boolean evaluations, Python uses short-circuit evaluations. It means that boolean evaluation may stop if one of its expression is False. For instance the following expression is always False

False and X

and X is never evaluated.

9.10.2. Returned values

In logical test, the returned value is the one that has been evaluated last. Consider these examples:

>>> print (True and "OK" or "KO")
OK
>>> print (False and "OK" or "KO")
KO

Explanation:in the first statement, True and "OK" is True. There is not need to test or "KO", so this is the end of the logical test, and the returned value is the one that has been evaluated last in True and "OK". In the second statement, False and "OK" is False. So, or "KO" must be evaluated. So, the last evaluated expression is "KO", hence the returned value.

9.10.3. precedence

Consider:

 >>> True and False or True
True

Since and has higer priority, the first evaluation is “True and False”, which is always False. The second evaluation becomes therefore “False or True”, which is True.

It maybe easier sometines to use brackets to make sure that what you wrote is what you meant:

>>> (True and False) or True
True