Conditionals/Making Choices Using Flow Controls

4.1. Conditionals/Making Choices Using Flow Controls#

  • So far we have only written small programs that are a sequence of instructions. Sometimes you have to alter the sequential flow of a program to suit the needs of a particular situation.

4.1.1. Boolean Type#

  • Python has a built-in Boolean type called bool. Unlike int and float, which can have almost unlimited possible values, bool has only two: True and False.

4.1.2. Boolean Operators#

  • There are three basic Boolean operators: and, or, and not. not has the highest precedence, followed by and, followed by or.

Operator

What it does?

Example

and

True if both a AND b are true (logical conjunction)

if is_teacher and is_active: print(‘You can access’)

or

True if either a OR b are true (logical disjunction)

if is_superuser or (is_teacher and is active): print(‘You can access’)

not

True if the opposite of a is true (logical negation)

if not is_superuser: print(‘You cannot access’)

 
not True

False

not False

True

True and True

True

False and False

False

True and False

False

False and True

False

True or True

True

False or False

False

True or False

True

False or True

True

cold = True
windy = False

(not cold) and windy # It is not cold and windy
not (cold and windy) #

4.1.3. Truthiness and Falsiness#

  • Things that are false on their own

    • None (basic data type)

    • False (basic data type)

    • Any empty sequence: '', [], ()

    • Any zero value: 0, 0.0

    • Anything whose len() returns 0

    • Empty objects

    • Everything else is True

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