Programming involves writing instructions for a computer to execute. However, what allows computer programs to be most useful is when they are able to decide which instructions instructions to execute based on a particular situation. This is refered to as code branching, and is used to shape the flow of control of the computer code. In this module, you will learn how to utilize conditional statements in order to include control flow in your Python scripts.

Contents

• Resources
• Booleans
  • Boolean Operators
• Conditional Statements
  • Designing Conditions
  • Modules vs. Scripts

• Conditional Execution (Downey)
• Conditionals (Severance)
• Flow Control (Sweigart) (first half)

In addition to the basic data types int, float, and str, Python supports a logical data type called a Boolean (class bool). A boolean represents "yes-or-no" data, and can be exactly one of two values: True or False (note the capitalization). Importantly, these are not the Strings "True" and "False"; boolean values are a different type!

type(True)    # <class 'bool'>
type("True")  # <class 'str'>
type(true)    # NameError: name 'true' is not defined
              # e.g., no variable called `true`!

• Fun fact: logical values are called "booleans" after mathematician and logician George Boole, who invented many of the rules and uses of this construction (called Boolean algebra).

Boolean values are most commonly the result of applying a relational operator (also called a comparison operator) to some other data type. Comparison operators are used to compare values and include: < (less than), > (greater than), <= (less-than-or-equal, written as read), >= (greater-than-or-equal, written as read), == (equal), and != (not-equal).

x = 3
y = 3.15

# compare numbers
x > y  # returns logical value False ("x is bigger than y" is a False statement)
y != x  # returns logical value True ("y is not-equal to x" is a True statement)

# compare x to pi (built-in variable)
y == math.pi  # returns logical value False

# compare strings (based on alphabetical ordering)
"cat" > "dog"  # returns False

• Important == (two equals signs) is a comparison operator, but = (one equals sign) is the assignment operator!

Note that boolean variables should be named as statements of truth. Use words such as is in the variable name:

is_early = True
is_sleeping = False
needs_coffee = True

In addition, boolean values support their own operators (called logical operators or boolean operators). These operators are applied to boolean values and produce boolean values, and allow you to make more complex boolean expressions:

• and (conjunction) produces True if both of the operands are True, and False otherwise
• or (disjunction) produces True if either of the operands are True, and False otherwise
• not (negation) is a unary operator that produces True if the operand is False, and False otherwise

x = 3.1
y = 3.2

# Assign bool values to variables
x_less_than_pi = x < math.pi  # True
y_less_than_pi = y < math.pi  # False

# boolean operators
x_less_than_pi and y_less_than_pi  # False
x_less_than_pi or y_less_than_pi  # True

# this works because Python is amazing
x < math.pi < y  # True


pet = "dog"

# it is NOT the case that pet is "cat"
not pet == "cat"  # True

# pet is "cat" OR "dog"
pet == "cat" or pet == "dog"  # True

# this doesn't work (operators are applied left-to-right; check the types!)
# see "short-circuiting" below, as well as http://stackoverflow.com/a/19213583
pet == "cat" or "dog"  # "dog"

Because boolean expressions produce more booleans, it is possible to combine these into complex logical expressions:

# given two booleans P and Q
P = True
Q = False

P and not Q  # True
not P and Q  # False
not (P and Q)  # True
(not P) or (not Q)  # True

The last two expressions in the above example are equivalent logical statements for any combination of values for P and Q, what is known as De Morgan's Laws. Indeed, many logical statements can be written in multiple equivalent ways.

Finally, note that when using an and operator, Python will short-circuit the second operand and never evaluate it if the first is found to be False (after all, if the first operand isn't True there is no way for the entire and expression to be!)

x = 2
y = 0
x == 2 and x/y > 1  # ZeroDivisionError: division by zero
x == 3 and x/y > 1  # no error (short-circuited)

# Use a "guardian expression" (make sure y is not 0)
# to avoid any errors
y != 0 and x/y > 1  # no error (short-circuited)

• The reason this works is because Python interpreter reads the expression P and Q, it produces Q if P is True, and P otherwise. This is because if P is True, then the overall truth of the expression is dependent entirely on Q (and thus that can just be returned). Similarly, if P is False, then the whole statement is false (equivalent to P!)

One of the primary uses of Boolean values (and the boolean expressions that produce them) is to control the flow of execution in a program (e.g., what lines of code get run in what order). While we can use functions are able to organization instructions, we can also have our program decide which set of instructions to execute based on a set of conditions. These deciisions are specified using conditional statements.

In an abstract sense, an conditional statement is saying:

IF something is true
  do some lines of code
OTHERWISE
  do some other lines of code

In Python, we write these conditional statements using the keywords if and else and the following syntax:

if condition:
    # lines of code to run if condition is True
else:
    # lines of code to run if condition is False

The condition can be any Boolean value (or any expression that evaluates to a boolean value). Both the if statement and else clause are followed by a colon : and a block, which is a set of indented statements to run (similar to the blocks used in functions). It is also possible to omit the else statement and its block if there are no instructions to run in the "otherwise" situation:

porridge_temp = 115  # temperature in degrees F

if porridge_temp > 120:
    print("This porridge is too hot!")
else:
    print("This porridge is NOT too hot!")

too_cold = porridge_temp < 70  # a boolean variable
if too_cold:
  print("This porridge is too cold!")

# This line is outside the block, so is not part of the conditional
# (it will always be executed)
print("Time for a nap!")

Blocks can themselves contain nested conditional statements, allowing for more complex decision making. Nested if statements are indented twice (8 spaces or 2 tabs). There is no limit to how many "levels" you can nest; just increase the indentation each time.

# nesting example
if outside_temperature < 60:
    print("Wear a jacket")
else:
    if outside_temperature > 90:
        print("Wear sunscreen")
    else:
        if outside_temperature == 72:
            print("Perfect weather!")
        else:
            print("Wear a t-shirt")

Note that this form is nesting is also how we you can use conditionals inside of functions:

def flip(coin_is_heads):
    if coin_is_heads:
        print("Heads you win!")
    else:
        print("Tails you lose")

If you consider the above nesting example's logic carefully, you'll notice that many of the "branches" are mutually exclusive: that is, the code will choose only 1 of 4 different clothing suggestions to print. This can be written in a cleaner format by using an elif ("else if") clause:

if outside_temperature < 60:
    print("Wear a jacket")
elif outside_temperature > 90:
    print("Wear sunscreen")
elif outside_temperature == 72:
    print("Perfect weather!")
else:
    print("Wear a t-shirt")

In this situation, the Python interpreter will perform the following logic:

1. It first checks the if statement's condition. If that is True, then that branch is executed and the rest of the clauses are skipped.
2. It then checks each elif clause's condition in order. If one of them is True, then that branch is executed and the rest of the clauses are skipped.
3. If none of the elif clauses are True, then (and only then!) the else block is executed.

This ordering is important, particularly if the conditions are not in fact mutually exclusive:

if porridge_temp < 120:
    print("This porridge is not too hot!")
elif porridge_temp < 70:
  # unreachable statement! the condition will never be both checked and True
    print("This porridge is too cold!")

# contrast with:
if porridge_temp < 120:
    print("This porridge is not too hot!")
if porridge_temp < 70:  # a second if statement, unrelated to the first
    print("This porridge is too cold!")
# both print statements will execute for `porridge_temp = 50`

See also the resources listed at the top of the module for explanations with logical diagrams and flowcharts.

Relational operators all have logical opposites (e.g., == is the opposite of !=; <= is the opposite of >), and boolean expressions can include negation. This means that there are many different ways to write conditional statements that are logically equivalent:

# these two statements are equivalent
if x > 3:
    print("big X!")

if 3 < x:
    print("big X!")

• The second example is known as a Yoda condition; in Python you should use the former.

Thus you should follow the below guidelines when writing conditionals. These produce more "idiomatic" code which is cleaner and easier to read, as well as less likely to cause errors.

• Avoid checks for mutually exclusive conditions with an if and elif. Use the else clause instead!

  # Do not do this!
  if temperature < 50:
      print("It is cold")
  elif temperature >= 50:  # unnecessary condition
      print("It is not cold")
  
  # Do this instead!
  if temperature < 50:
      print("It is cold")
  else:
      print("It is not cold")

• Avoid creating redundant boolean expressions by comparing boolean values to True. Instead, use an effectively named variable.

  # Do not do this!
  if is_raining == True:  # unnecessary comparison
      print("It is raining!")
  
  # Do this instead!
  if is_raining:  # condition is already a boolean!
      print("It is raining!")

  Note that this gets trickier when trying to check for False values. Consider the following equivalent conditions:

  # I believe this is the cleanest option, as it reads closet to English
  if not is_raining:
      print("It is not raining!")
  
  # This is an acceptable equivalent, but prefer the first option
  if is_raining == False
      print("It is not raining!")
  
  # Use one of the above options instead
  if is_raining != True
      print("It is not raining!")
  
  # This can be confusing unless your logic is explicitly based around the
  # ABSENCE of some condition
  if is_not_raining:
      print("It is not raining!")

Overall, try to develop the simplest, most straightforward conditions you can. This will make sure that you are able to think clearly about the program's control flow, and help to clarify your own thinking about the program's logic.

As discussed in module5, it is possible to define Python variables and functoins in .py files, which can be run as stand-alone scripts. However, these files can also be imported as modules, allowing you to access their variables and functions from another script. Thus all Python scripts have two "modes": they can be used as executable scripts (run with the python command at the command-line), or they can be imported as modules (libraries of variables and functions, using the import keyword in the script).

When the .py script is run as an executable top-level script, we often want to perform special instructions (e.g., call specific functions, prompt for user input, etc). We can use a special environmental variable called __name__ to determine whether this is the "main" script that is being run, and if so execute those instructions:

if __name__ == "__main__":
    # execute only if run as a script
    print("This is run as a script, not a module!")

• The __ in the variable names and values are a special naming convention used to indicate to the human that these values are internal to the Python language and so have special meaning. They are otherwise normal variables names