Check Several Possibilities

An if/else statement chooses between two paths. Some decisions have more than two useful outcomes.

Python’s elif keyword lets one decision check several possibilities in order. The name is short for “else if.”

Add another condition with elif

Run this example:

Python checks the conditions from top to bottom:

1. temperature >= 30 is False, so Python skips the first block.

2. temperature >= 20 is True, so Python assigns "Warm".

3. The chain stops. Python does not continue to else.

Only one branch in this chain runs.

The first true condition wins

An if/elif/else chain stops as soon as it finds a true condition. This makes the order of conditions part of the program’s logic.

Consider a score of 92:

The program prints Passed. The first condition is already true for every score of 60 or higher, so Python never reaches the more specific score >= 90 condition.

Put the narrower, higher boundary first:

Now the result is Excellent. A useful habit is to test a value at every boundary and one value on each side of it.

Here is the same top-to-bottom rule with a score of 82: the 90 check says no, the 75 check says yes, and that first match skips every later branch.

You can use more than one elif

A chain can contain several elif branches:

Python checks only as far as necessary. With score = 74, the first two conditions are false and the third is true. The final else catches every score below 60.

The else branch is optional, but it is valuable when the program needs a result for every possible value.

One chain or separate decisions?

Use an if/elif/else chain when the branches are alternatives and exactly one should run.

Use separate if statements when each question is independent and more than one block may need to run. For example, a weather program might print both a heat warning and a wind warning. Those messages are not competing answers to one question.

age = 18
if age >= 21:
    print("Level C")
elif age >= 18:
    print("Level B")
elif age >= 16:
    print("Level A")
else:
    print("Not available")

Classify a temperature

The exercise asks you to assign one of four labels:

• Hot for temperatures of 30 or more;

• Warm for temperatures of at least 20 but less than 30;

• Cool for temperatures of at least 10 but less than 20;

• Cold for anything below 10.

Use an if/elif/else chain with the boundaries in a clear order. The fixed temperature is 21, so the submitted program should choose Warm. After it passes, try 30, 20, 10, and 9 to check every boundary.