conditions.md

Conditions

Conditions are our method to run code only based on certain criteria. For that, we use the if statement. Inside the if statement, we have an expression that will always be evaluated to either True or False. If the expression is True, the code inside the if statement will run, otherwise it will be skipped.

if True:
    print("This will always run")

if False:
    print("This will never run")

We can also use variables inside the if statement:

x = True
if x:
    print("This will always run")

But it's much more useful to use expressions that are not always True or False. For example, we can use the in keyword to check if a value exists in a collection:

x = 1
if x in (1, 2, 3):
    print("This will run if x is 1, 2 or 3")

if x in (4, 5, 6):
    print("This will run if x is 4, 5 or 6")

else

Optionally, we might run some code only if the expression is False. For that, we use the else keyword.

x = 1
if x in (1, 2, 3):
    print("This will run if x is 1, 2 or 3")
else:
    print("This will run if x is not 1, 2 or 3")

print("This will always run, because it's not part of the if code block or the else code block")

Chaining conditions

We can also chain conditions together using the and and or keywords. For example:

x = 1
y = 2
my_tuple = (1, 2, 3)
if x in my_tuple and y in my_tuple:
    print("This will run if x and y are both 1, 2 or 3")
else:
    print("This will run if x or y is not 1, 2 or 3")
    print("NOTE: x and y might still be 1, 2 or 3, but we only care if they are ***both*** 1, 2 or 3, because we used the `and` keyword")

if x in my_tuple or y in my_tuple:
    print("This will run if x or y is 1, 2 or 3")
    print("NOTE: because we used `or`, we don't know which of x or y is 1, 2 or 3, or if both are 1, 2 or 3")
else:
    print("This will run if x and y is not 1, 2 or 3")

Nested conditions

Inside code blocks for if and else, we can use any valid Python code, including other if statements. For example:

x = 1
y = 2
my_tuple = (1, 2, 3)
if x in my_tuple:
    print("This will run if x is 1, 2 or 3")
    if y in my_tuple:
        print("This will run if x ***and*** y are both 1, 2 or 3")
    else:
        print("This will run if x is 1, 2 or 3, but y is not 1, 2 or 3")
else:
    print("This will run if x is not 1, 2 or 3, but it doesn't even check what y is")

Comparison operators

We can use the following comparison operators to create expressions that evaluate to True or False:

x = 1
y = 2
print(x == y)  # Equal to
print(x != y)  # Not equal to
print(x < y)  # Less than
print(x <= y)  # Less than or equal to
print(x > y)  # Greater than
print(x >= y)  # Greater than or equal to

Because these expressions evaluate to True or False, we can use them inside if statements:

x = 1
y = 2
z = 3
if x < y:
    print("This will run if x is less than y")

if x < y and z == 3:
    print("This will run if x is less than y ***and*** z is equal to 3")

elif

We will cover this in more detail later, but we can also use the elif keyword to check for more conditions, in that case, the else block will run only if none of the conditions are True. For example:

x = 1
y = 2
if x < y:
    print("This will run if x is less than y")
elif x > y:
    print("This will run if x is greater than y")
else:
    print("This will run if x is not less than y or greater than y (they are probably equal)")