Controlling Flow

Python Control Flow

Boolean Logic

Operator Description
== Equal to
!= Not equal to
> Greater than
< Less than
>= Greater than or equal to
<= Less than or equal to

If, Else, and Elif

if condition:
    # code block
elif condition:
    # code block
elif condition:
    # code block
else:
    # code block

If Statements, cont.

condition = {"a": True, "b": True}
if condition["a"]:
    print("a is True")
elif not condition["b"]:
    print("b is False")
else:
    print("neither a nor b is True")
a is True

If Statements, cont.

condition = {"a": True, "b": True}
if condition["a"]:
    print("a is True")
if condition["b"]:
    print("b is True")
if not condition["b"] and not condition["a"]:
    print("neither a nor b is True")
a is True
b is True

Logical Operators

Operator Description
== Equal to
!= Not equal to
> Greater than
< Less than
>= Greater than or equal to
<= Less than or equal to

Important Resources

  • The best resource is always the Python Documentation.
  • Full explanation of Python Control Flow at the above address.

Booleans: true/false values

  • Booleans are True/False values.
  • They are used to make decisions in code.
  • They are often the result of a comparison.

Boolean Example

x = 7

my_boolean = x >= 5

print("is x greater or or equal to 5? ", my_boolean)
is x greater or or equal to 5?  True
  • my_boolean evaluates to True if x is greater than or equal to 5.
  • In other words, python stores my_boolean as a True or False value
    • not as x >= 5

if Statements

  • if statements are used to make decisions in code.
  • They are followed by a boolean expression.
  • If the boolean expression is True, the code block is executed.
  • If the boolean expression is False, the code block is skipped.

if Statement Example

x = 7

my_boolean = x >= 5

if my_boolean:
    print("of course x is greater than 5")
else:
    print("No, stupid, x is less than 5")
of course x is greater than 5

Truthy vs Falsy Values

  • The important point is that the value to the right will only be read if the value to the left evaluates to truthy
  • Falsy is any value that evaluates to False
    • An empty list [], tuple (), set set(), string "", or dictionary {}
    • Any Numeric zero: 0, 0.0, or complex 3j
    • Any constant: False (obviously), or None
  • Truthy = everything else.

Truthy / Falsy Examples

a = []
b = 3
c = 0
d = 2 - 2

if a:
    print("a is truthy")
else:
    print("a is falsy")

if b:
    print("b is truthy")
else:
    print("b is falsy")
print("d equals ", d)
print("so d evaluates to: ", bool(d))
a is falsy
b is truthy
d equals  0
so d evaluates to:  False

Operators: and

  • The and operator returns if both operands are True.
  • If the first operand is False, the second operand is not evaluated.

Operators: and, example 1

x = True
y = False
z = True

if x and y:
    print("This will not print because both x and y are not true")
else:
    print("box x and y are not true")

if x and z:
    print("But both x and z evaluate to true, so you see me")
elif z and x:
    print("This will not print because the first if is true")
else:
    print("box x and y are not true")

if x and z:
    print("But both x and z evaluate to true, so you see me")
else:
    print("You will not see this, because x and z evaluate to true")
box x and y are not true
But both x and z evaluate to true, so you see me
But both x and z evaluate to true, so you see me

Operators: or

  • The or operator evaluates to True if either of the operands is True.
x = True
y = False
z = True
a = 1
b = 0

if x or y:
    print("This first if will print because x is true")
else:
    print("box x and y are not true")

if a or b == 0:
    print("This will print because b == 0 is true")
else:
    print("You will not see this, because b is true")
This first if will print because x is true
This will print because b == 0 is true

Operators: not

  • The not operator negates the boolean value of the operand.
x = 10
y = 20
z = 10
a = "Something"
b = True
if not x == y:
    print("x is not equal to y")
else:
    print("x is equal to y")  # this will not print
if type(a) != int:
    print("a is not an integer")
else:
    print("a is an integer")  # this will not print
print(not b)
x is not equal to y
a is not an integer
False

The other not

  • != is a comparison operator that means “not equal to”
x = 10
y = 20

if x != y:
    print("x is not equal to y")
else:
    print("x is equal to y")  # this will not print
x is not equal to y

Operators: in

  • The in operator checks if a value is in a sequence.
    • A sequence can be a list, tuple, string, or dictionary.
my_list = [1, 2, 3, 4, 5]
a = 3
b = 6
if a and b in my_list:
    print("Both a and b are in my_list")
elif a in my_list:
    print("only a is in my_list")
elif b in my_list:
    print("only b is in my list")
else:
    print("neither a nor b are in my_list")
only a is in my_list

Nested If Statements

  • You can nest if statements inside other if statements.
my_list = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

if 3 in my_list:
    if 3 in my_list[:5]:
        if 3 in my_list[:3]:
            print("3 is in the first quarter of the list")
        else:
            print("3 is in the first 3 of the list")
    else:
        print("3 is in the second half of the list")
else:
    print("3 is not in the list")
3 is in the first quarter of the list