Python Conditions and If statements
Python supports the usual logical conditions from mathematics:
- Equals: a == b
- Not Equals: a != b
- Less than: a < b
- Less than or equal to: a <= b
- Greater than: a > b
- Greater than or equal to: a >= b
These conditions can be used in several ways, most commonly in "if statements" and loops.
An "if statement" is written by using the if keyword.
Example
a = 33
b = 200
if b > a:
print("b is greater than a")
In this example we use two variables, a and b, which are used as part of the if statement to test whether b is greater than a. As a is 33, and b is 200, we know that 200 is greater than 33, and so we print to screen that "b is greater than a".
How If Statements Work
The if statement evaluates a condition (an expression that results in True or False). If the condition is true, the code block inside the if statement is executed. If the condition is false, the code block is skipped.
Example
number = 15
if number > 0:
print("The number is positive")
Indentation
Python relies on indentation (whitespace at the beginning of a line) to define scope in the code. Other programming languages often use curly-brackets for this purpose.
Example
a = 33
b = 200
if b > a:
print("b is greater than a")
# you will get an error
Note: You can use spaces or tabs for indentation, but you must use the same amount of indentation for all statements within the same code block.
Multiple Statements in If Block
You can have multiple statements inside an if block. All statements must be indented at the same level.
Example
age = 20
if age >= 18:
print("You are an adult")
print("You can vote")
print("You have full legal rights")
Using Variables in Conditions
Boolean variables can be used directly in if statements without comparison operators.
Example
is_logged_in = True
if is_logged_in:
print("Welcome back!")
Python can evaluate many types of values as True or False in an if statement.
Zero (0), empty strings (""), None, and empty collections are treated as False. Everything else is treated as True.
This includes positive numbers (5), negative numbers (-3), and any non-empty string (even "False" is treated as True because it's a non-empty string).
