Here, the body of if has two statements. We know this because two statements (immediately after if) start with indentation.We usually use four spaces for indentation in Python, although any number of spaces work
It's akin to guard-let in Swift. Here is a somewhat more precise description. We change the grammar like so: ifStatement ::= "if" "(" expression ")" statement ( "else" statement )? | positiveIfVariable | negativeIfVariable positiveIfVariable ::= "if" "(" ifVariable positiveTest "...