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Boolean (True/False) Values

Introduction: Booleans (True/False) Values

Every object can be converted to boolean (True/False): an object is True if it is "non-zero".

counter = 5
if counter:                                            # True
    print('this int is not zero')
else:
    print('this int is zero')


mystr = ''                                             # empty string

if mystr:                                              # False
    print('this string has characters')
else:
    print('this string is empty')


var = ['a', 'b', 'c']
if var:                                                # True
    print('this container has elements')
else:
    print('this container is empty')

Summary for Object: Boolean

A boolean object can be True or False. All objects can be converted to boolean, meaning that all objects can be seen as True or False in a boolean context.

print(type(True))                                      # <type 'bool'>
print(type(False))                                     # <type 'bool'>

bool() converts objects to boolean.

print(bool(['a', 'b', 'c']))                           # True
print(bool([]))                                        # False

if and while induce the bool() conversion

So when we say if var: or while var:, we're testing if bool(var) == True, i.e., we're checking to see if the value is a True value

print(bool(5))                                         # True
print(bool(0))                                         # False

if and while induce the boolean conversion -- an object is evaluated in boolean context

mylist = [0, 0]

if mylist:                         # not empty, so True
                                   # in boolean context
    print('that list is not empty')


yourlist = [1, 2, 3]

while yourlist:                    # True as long as yourlist has elements
    x = yourlist.pop()             # remove element from the end of yourlist
    print(x)

Boolean quiz

Quiz yourself: look at the below examples and say whether the value will test as True or False in a boolean expression. Beware of tricks! Remember the rule: if it represents a 0 or empty value, it is False. Otherwise, it is True.

var   = 5
var2  = 0
var3  = -1
var4  = 0.0000000000000001
varx  = '0'
var5  = 'hello'
var6  = ""
var7  = '    '
var8  = [   ]
var9  = ['hello', 'world']
var10 = [0]
var11 = {0:0}
var12 = {}

Booleans: quiz answers

Here are the answers to the quiz.

var   = 5                       # bool(var):   True
var2  = 0                       # bool(var2):  False
var3  = -1                      # bool(var3):  True (not 0)
var4  = 0.0000000000000001      # bool(var4):  True
varx =  '0'                     # bool(varx):  True (not empty string)
var5  = 'hello'                 # bool(var5):  True
ver6  = ""                      # bool(var6):  False
var7  = '    '                  # bool(var7):  True (not empty)
var8  = [   ]                   # bool(var8):  False
var9  = ['hello', 'world']      # bool(var9):  True
var10 = [0]                     # bool(var10): True (has an element)
var11 = {0:0}                   # bool(var11): True (has a pair)
var12 = {}                      # bool(var12): False

any(), all() and "in"

any() with a sequence checks to see if any elements are True; all() asks whether they are all True; in can be used to see if a value exists in a sequence.

any(): are any of these True?

mylist = [0, 0, 5, 0]

if any(mylist):
    print('at least one item is True')

all(): are all of these True?

mylist = [5, 5, 'hello', 5.0]

if all(mylist):
    print('all items are True')

in with a list: is this value anywhere within this list?

mylist = ['highest', 'lowest']

user_choice = 'lowest'

if user_choice not in mylist:
    print('please enter "higest" or "lowest"')

[pr]