Conditional Statements (if, elif, else)

Every program makes decisions. When you check your email, the program asks: Is there a new message? If yes, it shows a notification. If no, it stays silent. This decision-making is the job of conditional statements. A conditional statement checks whether something is true or false, then runs different code depending on the answer. In Python, we use if, elif, and else to build these decision points into our programs.

The if statement is the foundation of all conditionals. It checks a condition and runs a block of statements if that condition is true. The block of code that runs is called the if-block. If the condition is false, the if-block is skipped entirely, and the program continues with whatever comes after the if statement.

Often you want to run one block of code if a condition is true and a different block if it is false. This is where the else clause comes in. The else clause is optional—you can write an if statement without it. But when you include else, it provides a fallback: if the condition is false, the else-block runs instead. Only one of the two blocks will ever execute.

Sometimes you need to check more than two possibilities. For example, you might want to assign a letter grade based on a test score: A for 90 and above, B for 80 to 89, C for 70 to 79, and so on. You could nest multiple if-else statements, but that creates deep indentation and is hard to read. Instead, use elif (short for else-if). The elif clause lets you chain multiple conditions together in a single, readable structure. You can have as many elif clauses as you need, and they are checked in order from top to bottom. The first condition that is true wins—its block runs, and all the others are skipped. If none of the conditions are true, an optional else clause runs as a final fallback.

To understand how elif chains work, it helps to trace through the execution step by step. Let's walk through a complete example that shows how the program evaluates each condition and decides which block to run.

Python uses indentation (spaces or tabs at the start of a line) to group statements into blocks. This is different from many other languages, which use curly braces or keywords. In conditional statements, every line that belongs to an if-block, elif-block, or else-block must be indented by the same amount. The if, elif, and else keywords themselves are not indented—they sit at the same level as each other. Each of these keywords must be followed by a colon (:) at the end of the line.

A minimal valid if statement requires only the if keyword, a condition, a colon, and an indented block. The elif and else parts are optional. You can write an if statement by itself, and if the condition is false, the program simply skips the block and continues. This is useful when you only need to do something if a condition is true, and there is no alternative action needed.

In this example, if username were 'bob' instead, the if-block would be skipped, and only 'Login complete.' would print. No error occurs—the program runs fine without an else clause.

Conditional statements are the decision-makers of your programs. The if statement checks a condition and runs a block of code if it is true. The optional else clause provides a fallback block if the condition is false. The elif clause lets you chain multiple conditions together, checking them in order until one is true. Exactly one block in an if-elif-else chain will run (or none, if there is no else and all conditions are false). Always remember the colon after if, elif, and else, and always indent the statements that belong to each block. With these tools, you can write programs that respond differently to different inputs and make intelligent decisions.