Program branches

If-else branch

Which program path is selected at a branch always depends on whether a certain condition is fulfilled or not.

UML 1.x activity diagram of an if-else branch

In Python, the if keyword is followed by the condition that ends with a colon :.
If this condition is not met, i.e. the result of the check is false, then the if branch is skipped and else path is executed.

age = int(input("Enter age: "))
if age >= 18:
    print("adult")
else:
    print("child")

(In Python, the indentation indicates which commands belong to the respective program path.)

if-elif-else branch

Any number of elif branches (abbreviation for else if) with respective conditions can be inserted in between:

age = int(input("Enter age: "))
if age >= 18:
    print("adult")
elif age >= 10:
    print("teen")
else:
    print("child")

UML 1.x activity diagram of if-elif-else branch

The conditions are checked sequentially and only the program path of the first true condition is executed. The else path is only executed if no condition is true.