Using else to Handle the Default Case

One Decision, Several Outcomes

A decision does not always have only two possible outcomes. Sometimes a program must choose among several results. Python uses elif, short for else if, to connect multiple conditions in one decision structure. An elif chain checks its conditions in order and selects the first branch whose condition is true.

Following the Chain

Python evaluates a chained conditional from top to bottom. It checks the if condition first. If that condition is false, Python checks the next elif condition. This continues until a true condition is found. As soon as one condition is true, its branch runs and the rest of the chain is skipped.

The First True Branch

Exactly one branch executes in an if-elif-else chain. This remains true even when more than one condition would be true. Python does not continue checking after it finds the first true condition. The later conditions are skipped, so the chain produces one selected outcome rather than several.

A Complete Chain

x = 15 if x < 10: print("less than 10") elif x < 20: print("between 10 and 20") else: print("20 or greater")

Ordering the Conditions

Because conditions are checked from top to bottom, their order affects the result. Put a more specific condition before a more general condition. If a general condition appears first and is true, Python stops there and never reaches a later condition that might describe the case more precisely.

The Default Branch

The else clause represents the remaining case. It has no condition of its own. Python reaches it only when the if condition and every elif condition before it are false. If an else clause is included, one branch of the chain will execute. Without else, it is possible for no branch to run.

Mistakes Beginners Make

• Writing several independent if statements when only one outcome should be selected.

  A value such as 15 satisfies all three conditions, so all three branches can execute.

  Fix: Use an if-elif chain when the branches represent one choice and only one branch should run.

• Putting a general condition before a more specific condition.

  A value less than 10 also satisfies x < 20, so the chain stops before the specific condition is checked.

  Fix: Place the more specific condition first.

• Expecting else to test a condition.

  Else has no condition. It handles the remaining cases after all earlier conditions are false.

  Fix: Use elif when another specific condition must be tested, and reserve else for the final default case.

• Placing else before the end of the chain.

  An else clause must come at the end of the if-elif-else structure.

  Fix: Put every elif before the final else.

Practice the Trace

Working Rules

1. Use elif to connect multiple possible outcomes in one decision structure. Python checks the conditions from top to bottom and executes only the first true branch. Put more specific conditions before general ones so the intended case is reached first. Place else at the end when every unmatched case should receive a default action. If there is no else and every condition is false, no branch runs.