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Python Control Flow

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elif Statement

The Python elif statement allows for continued checks to be performed after an initial if statement. An elif statement differs from the else statement because another expression is provided to be checked, just as with the initial if statement.

If the expression is True, the indented code following the elif is executed. If the expression evaluates to False, the code can continue to an optional else statement. Multiple elif statements can be used following an initial if to perform a series of checks. Once an elif expression evaluates to True, no further elif statements are executed.

# elif Statement
pet_type = "fish"
if pet_type == "dog":
print("You have a dog.")
elif pet_type == "cat":
print("You have a cat.")
elif pet_type == "fish":
# this is performed
print("You have a fish")
else:
print("Not sure!")

or Operator

The Python or operator combines two Boolean expressions and evaluates to True if at least one of the expressions returns True. Otherwise, if both expressions are False, then the entire expression evaluates to False.

True or True # Evaluates to True
True or False # Evaluates to True
False or False # Evaluates to False
1 < 2 or 3 < 1 # Evaluates to True
3 < 1 or 1 > 6 # Evaluates to False
1 == 1 or 1 < 2 # Evaluates to True

Equal Operator ==

The equal operator, ==, is used to compare two values, variables or expressions to determine if they are the same.

If the values being compared are the same, the operator returns True, otherwise it returns False.

The operator takes the data type into account when making the comparison, so a string value of "2" is not considered the same as a numeric value of 2.

# Equal operator
if 'Yes' == 'Yes':
# evaluates to True
print('They are equal')
if (2 > 1) == (5 < 10):
# evaluates to True
print('Both expressions give the same result')
c = '2'
d = 2
if c == d:
print('They are equal')
else:
print('They are not equal')

Not Equals Operator !=

The Python not equals operator, !=, is used to compare two values, variables or expressions to determine if they are NOT the same. If they are NOT the same, the operator returns True. If they are the same, then it returns False.

The operator takes the data type into account when making the comparison so a value of 10 would NOT be equal to the string value "10" and the operator would return True. If expressions are used, then they are evaluated to a value of True or False before the comparison is made by the operator.

# Not Equals Operator
if "Yes" != "No":
# evaluates to True
print("They are NOT equal")
val1 = 10
val2 = 20
if val1 != val2:
print("They are NOT equal")
if (10 > 1) != (10 > 1000):
# True != False
print("They are NOT equal")

Comparison Operators

In Python, relational operators compare two values or expressions. The most common ones are:

  • < less than
  • > greater than
  • <= less than or equal to
  • >= greater than or equal too

If the relation is sound, then the entire expression will evaluate to True. If not, the expression evaluates to False.

a = 2
b = 3
a < b # evaluates to True
a > b # evaluates to False
a >= b # evaluates to False
a <= b # evaluates to True
a <= a # evaluates to True

if Statement

The Python if statement is used to determine the execution of code based on the evaluation of a Boolean expression.

  • If the if statement expression evaluates to True, then the indented code following the statement is executed.
  • If the expression evaluates to False then the indented code following the if statement is skipped and the program executes the next line of code which is indented at the same level as the if statement.
# if Statement
test_value = 100
if test_value > 1:
# Expression evaluates to True
print("This code is executed!")
if test_value > 1000:
# Expression evaluates to False
print("This code is NOT executed!")
print("Program continues at this point.")

else Statement

The Python else statement provides alternate code to execute if the expression in an if statement evaluates to False.

The indented code for the if statement is executed if the expression evaluates to True. The indented code immediately following the else is executed only if the expression evaluates to False. To mark the end of the else block, the code must be unindented to the same level as the starting if line.

# else Statement
test_value = 50
if test_value < 1:
print("Value is < 1")
else:
print("Value is >= 1")
test_string = "VALID"
if test_string == "NOT_VALID":
print("String equals NOT_VALID")
else:
print("String equals something else!")

and Operator

The Python and operator performs a Boolean comparison between two Boolean values, variables, or expressions. If both sides of the operator evaluate to True then the and operator returns True. If either side (or both sides) evaluates to False, then the and operator returns False. A non-Boolean value (or variable that stores a value) will always evaluate to True when used with the and operator.

True and True # Evaluates to True
True and False # Evaluates to False
False and False # Evaluates to False
1 == 1 and 1 < 2 # Evaluates to True
1 < 2 and 3 < 1 # Evaluates to False
"Yes" and 100 # Evaluates to True

Boolean Values

Booleans are a data type in Python, much like integers, floats, and strings. However, booleans only have two values:

  • True
  • False

Specifically, these two values are of the bool type. Since booleans are a data type, creating a variable that holds a boolean value is the same as with other data types.

is_true = True
is_false = False
print(type(is_true))
# will output: <class 'bool'>

not Operator

The Python Boolean not operator is used in a Boolean expression in order to evaluate the expression to its inverse value. If the original expression was True, including the not operator would make the expression False, and vice versa.

not True # Evaluates to False
not False # Evaluates to True
1 > 2 # Evaluates to False
not 1 > 2 # Evaluates to True
1 == 1 # Evaluates to True
not 1 == 1 # Evaluates to False