Conditionals

Testing truth value

bool

In [1]:
print('type of True and False: {}'.format(type(True)))

type of True and False: <class 'bool'>
In [2]:
print('0: {}, 1: {}'.format(bool(0), bool(1)))
print('empty list: {}, list with values: {}'.format(bool([]), bool(['woop'])))
print('empty dict: {}, dict with values: {}'.format(bool({}), bool({'Python': 'cool'})))

0: False, 1: True
empty list: False, list with values: True
empty dict: False, dict with values: True

==, !=, >, <, >=, <=

In [3]:
print('1 == 0: {}'.format(1 == 0))
print('1 != 0: {}'.format(1 != 0))
print('1 > 0: {}'.format(1 > 0))
print('1 > 1: {}'.format(1 > 1))
print('1 < 0: {}'.format(1 < 0))
print('1 < 1: {}'.format(1 < 1))
print('1 >= 0: {}'.format(1 >= 0))
print('1 >= 1: {}'.format(1 >= 1))
print('1 <= 0: {}'.format(1 <= 0))
print('1 <= 1: {}'.format(1 <= 1))

1 == 0: False
1 != 0: True
1 > 0: True
1 > 1: False
1 < 0: False
1 < 1: False
1 >= 0: True
1 >= 1: True
1 <= 0: False
1 <= 1: True

You can combine these:

In [4]:
print('1 <= 2 <= 3: {}'.format(1 <= 2 <= 3))

1 <= 2 <= 3: True

and, or, not

In [5]:
python_is_cool = True
java_is_cool = False
empty_list = []
secret_value = 3.14
In [6]:
print('Python and java are both cool: {}'.format(python_is_cool and java_is_cool))
print('secret_value and python_is_cool: {}'.format(secret_value and python_is_cool))

Python and java are both cool: False
secret_value and python_is_cool: True
In [7]:
print('Python or java is cool: {}'.format(python_is_cool or java_is_cool))
print('1 >= 1.1 or 2 < float("1.4"): {}'.format(1 >= 1.1 or 2 < float('1.4')))

Python or java is cool: True
1 >= 1.1 or 2 < float("1.4"): False
In [8]:
print('Java is not cool: {}'.format(not java_is_cool))

Java is not cool: True

You can combine multiple statements, execution order is from left to right. You can control the execution order by using brackets.

In [9]:
print(bool(not java_is_cool or secret_value and  python_is_cool or empty_list))
print(bool(not (java_is_cool or secret_value and  python_is_cool or empty_list)))

True
False

if

In [10]:
statement = True
if statement:
    print('statement is True')
    
if not statement:
    print('statement is not True')

statement is True
In [11]:
empty_list = []
# With if and elif, conversion to `bool` is implicit
if empty_list:
    print('empty list will not evaluate to True')  # this won't be executed
In [12]:
val = 3
if 0 <= val < 1 or val == 3:
    print('Value is positive and less than one or value is three')

Value is positive and less than one or value is three

if-else

In [13]:
my_dict = {}
if my_dict:
    print('there is something in my dict')
else:
    print('my dict is empty :(')

my dict is empty :(

if-elif-else

In [14]:
val = 88
if val >= 100:
    print('value is equal or greater than 100')
elif val > 10:
    print('value is greater than 10 but less than 100')
else:
    print('value is equal or less than 10')

value is greater than 10 but less than 100

You can have as many elif statements as you need. In addition, else at the end is not mandatory.

In [15]:
greeting = 'Hello fellow Pythonista!'
language = 'Italian'

if language == 'Swedish':
    greeting = 'Hejsan!'
elif language == 'Finnish':
    greeting = 'Latua perkele!'
elif language == 'Spanish':
    greeting = 'Hola!'
elif language == 'German':
    greeting = 'Guten Tag!'
    
print(greeting)

Hello fellow Pythonista!

For more detailed overview about conditionals, check this tutorial from Real Python.